I mtm : : ^)I Vv -v-'- ; te; THE ETHEL CARR PEACOCK MEMORIAL COLLECTION Matris amort monumentum TRINITY COLLEGE LIBRARY DURHAM, N. C. 1903 Gift of Dr. and Mrs. Dred Peacock Digitized by the Internet Archive in 2016 with funding from Duke University Libraries https://archive.org/details/elementsofnatura1879houst 'L\£‘2^ THE ELEMENTS OF Natural Philosophy. tf{^ of ^ofjaob mttl BY EDWIN J. HOUSTON, A.M., PROFESSOR OF PHYSICAL GEOGRAPHY AND NATURAL PHILOSOPHY IN THE CENTRAL HIGH SCHOOL OF PHILADELPHIA; AUTHOR OF "ELEMENTS OF PHYSICAL GEOGRAPHY." PHILADELPHIA : ELDREDGE & BROTHER, No. 17 North Seventh Street. 1879 . A SERIES OF TEXT-BOOKS ON THE NATURAL SCIENCES. By Prof. E. J. HOUSTON. 1. Easy Lessons in Natural Philosophy. 2. Elements of Natural Philosophy. 3. Elements of Physical Geography. Entered, according to Act of Congress, in the year 1879, by ELDREDGE & BROTHER, in the OflBce of the Librarian of Congress, at Washington. 'T J. FAGAN i BON 1^ ELECTROTYPERS, PHILAP’A. S' 30 . 2 . P TN the “ Elements of Natural Philosophy ” will be found the more important principles of the science. The great variety of subjects embraced by Natural Philosophy, makes it impracticable, in the limits of an elementaiy work, to give more than the mere outlines of the science. These the author has endeavored to state in a concise form and in logical sequence, so that the book, though small, shall form a simple system of Natural Philosophy, and not a mere col- lection of disconnected facts. The remarkable progress made in this department of natural science within the past few years, has rendered it advisable, in the opinion of the author, to depart somewhat widely from the methods of arrangement usually adopted in elementary text-books. This is more particularly the case in the treatment of electricity, where magnetism, in- stead of preceding the general subject, is made to occupy a subordinate place as one of the effects of an electrical current. The general division into electrical charge and current electricity will also, it is believed, aid the student in obtaining clear conceptions of the principles of elec- tricity. Although the importance of the study of Natural Philos- ophy is now almost universally acknowledged, yet the ex- pense of cabinets of philosophical apparatus is so great. IV PREFACE. that a more general introduction of the study has been pre- vented. The author has introduced into “ The Elements of Natural Philosophy,” a feature which he trusts will to a great extent remove this objection. Throughout the text will be found descriptions of simple experiments that can be made with apparatus so easily contrived, as not only to render it possible for the teacher to illustrate the subject inexpensively, but also to permit the experiments to he made by the students themselves, thus enabling them to acquire that intimate knowledge of the subject that can come only by self-conducted experiments. While the book is thus suited to the use of such schools as are \inable to provide costly cabinets of ajsparatus, the standard pieces found in well selected cabinets are care- fully described and figured, thus adapting the book to the use of schools well provided with apparatus. A carefully prepared syllabus, and series of questions for review, will be found following each chapter. An examination of the book will show that no expense has been spared to bring it up to the highest standard as regards illustrations, typography, paper, and printing. The author desires to acknowledge his indebtedness to his friend Prof. Elihu Thomson for critical examination of the proof-sheets. E. J. H. Central High School, Philadelphia, May, 1879. PART 1. MATTER AND FORCE. CHAPTER I Matter Syllabus Questions foe Review CHAPTER II. General Properties of Matter 16 Syllabus 26 Questions for Review 27 CHAPTER III. The Three Conditions of Matter .29 Syllabus 34 Questions for Review 35 CHAPTER IV. Force and Motion 37 Syllabus 49 Questions for Review 51 CHAPTER V. The Mechanical Powers 52 Syllabus 62 Questions for Review 64 1 * PAGE . 9 14 . 15 V VI CONTENTS. CHAPTER VI. PAGE Geavitatiok 65 Syllabus 78 Questions for Review 80 CHAPTER VII. Cohesion and Adhesion, and Properties Peculiar to Solids 82 Syllabus 95 Questions fob Review 97 PART II. FLUIDS. CHAPTER I. Hydrostatics 98 Syllabus 112 Questions for Review Ill CHAPTER II. Hydraulics 115 Syllabus 123 Questions foe Review 125 CHAPTER III. Pneumatics . 126 Syllabus 137 Questions foe Review 139 PART III. SOUND AND HEAT. CHAPTER I. The Cause, Transmission, Reflection, and Refraction of Sound 140 Syllabus 152 Questions for Reyhew 153 CONTENTS. vu CHAPTES II. p^sE The Chaeacteeistics of Musical Souhd. — Musical Insteumehts 155 Syllabus 167 Questions foe Review 169 CHAPTER III. The Natuee of Heat. — Theemohetees and Expansion . 170 Syllabus 177 Questions foe Review 178 CHAPTER IV. The CoiiMUNicATioN of Heat. — The Sueface Action of Bodies 180 Syllabus 190 Questions foe Review 191 CHAPTER V. Change of State. — Latent and Specific Heat. — Mechan- ical Equivalent of Heat 193 Syllabus 207 Questions foe Review 208 PART IV. LIGHT AND ELECTRICITY. CHAPTER I. Light : Its Natuee and Soueces. — Action of Mattee on Light 210 Syllabus 227 Questions foe Review 229 CHAPTER II. Lenses. — Optical Insteuments and Vision .... 231 Syllabus 245 Questions foe Review 247 CHAPTER III. Electeicity. — Electeical Chaege, oe Electeicity of High Tension 249 Syllabus 268 Questions foe Review 269 VI II CONTENTS. CHAPTER IV. PAGE CuRREKT Electricity 271 Syllabus 279 Questions for Review 280 CHAPTER V. Properties of an Electrical Current 281 Syllabus 286 Questions for Review 287 CHAPTER VI. Magnetism 288 Syllabus 295 Questions for Review 296 CHAPTER VII. Magneto-Electric Currents. — Apparatus Dependent on Electro-Magnets 297 Syllabus 310 Questions for Review 311 Natural Philosophy. Part I. Matter and Force. CHAPTER I. MATTER. 1. Matter is anything which occupies space, and prevents other things from occupying the same space. If the thing merely occupies space, but does not prevent other things from occupying the same space, then it is not matter. Both iron and gold are kinds of matter, since they occupy space, and prevent other pieces of matter from being placed where they are. The shadow of an object, however, does not prevent other things from being placed in it, and is not, therefore, a kind of matter. Both air aud water are kinds of matter. When we move other bodies through them, these bodies do not occupy the same place the air or the water does, but merely push the air or the water out of the way, and then occupy the place they have thus cleared for themselves. If a vessel be filled to the brim with water, a stone dropped into it will cause as much water to run out of the vessel as will just fill the space the stone occupies. The same vessel, however, can be placed 9 10 NATURAL PHILOSOPHY. where it will be filled with sunlight, without any of the water run- ning out. 2. The Senses. — We acquire knowledge bj means of our senses, and by them become aware of the ex- istence of matter, which we can see, feel, taste, or smell. Our senses form the avenues or channels through which impres- sions are received from things outside ; thus, light entering the eye enables us to see the peculiarities of color and form of the object from which it came. By means of the touch, we learn to distinguish the nature of the surface and the texture ; by the taste or smell, we are enabled to select the pleasant and wholesome from the noxious ; and, finally, by our hearing, we are enabled to understand the thoughts of others when expressed in language. 3. Substances. Elements. — The different kind.s of matter are called substances. Iron, wood, water, milk, air, and steam are substances. Substances are either elementary or compound. Ele- mentary substances.! or elements, are those which have never been resolved into more than one kind of matter. Compound stibstances are those which are formed by the union of tw'o or more elemexttary substances. Gold is an elementary substance. We cannot, by any known means, break it up or separate it into anjffhing but gold. Brass is a compound substance, since we can separate it into copper and zinc ; or, by melting copper and zinc together in the right propor- tions, Ave can produce brass. All the compound substances in the Avorld are formed by various combinations of about seA'entA' elementary substances. Any definite piece of matter is called a body. Bodies may be either large or small ; thus, both the MA TTER. 11 eartli and a grain of sand are bodies, since they both are pieces of matter. 4. Changes of Matter. — All kinds of matter have different peculiarities or properties, by means of which we are enabled to recognize them. Thus, gold can be readily distinguished from marble, since its color and weight are different ; and then, too, it can be drawn out into wire, or beaten into thin sheets, while marble cannot. They differ also in other respects. Matter is constantly undergoing change. This change is of two distinct kinds, viz. : 1st. Physical change, or that which may occur without the loss of those peculiarities or proper- ties by which we recognize the substance ; and, 2d. Chemical change, or that which cannot occur without the loss of such peculiarities or properties. As an example of a physical change, we may take a piece of steel, such as a common pen, and after examining it carefully, so as to note its peculiarities, ruh it once or twice against a magnet. The pen will now have acquired a property it did not possess before ; it will attract iron filings to it ; but if we again examine the pen carefully, we can- not see that it has lost any of the properties it previously possessed. If, however, we expose the pen for some time to damp air, it will become covered with a thick, brown rust, which is formed by oxygen, a substance in the air, combining with the iron of the pen. Now the rust so formed in no way resembles either of the things out of which it was formed, since one of them was the pen itself, and the other an invisible gas. This change, then, is a chemical change, since both bodies have lost the properties or peculiarities by which they are gen- erally recognized. 5. Physics or Natural Philosophy, and Chemistry. — Natural Philosophy or Physics is that study tvbicb considers the causes and effects of the physical changes to which matter is subject. Chemistry is that study which considers the causes 12 NATURAL PHILOSOPHY. and effects of tlie chemical changes to which matter is subject. 6. Phenomena. — Anything which happens in the ordinary course of nature is called a phenomenon. As the word is commonly used, it means something unusual or strange ; as used in science, it means any- thing which, happens naturally. The fall of a leaf, the shining of the sun, the fall of a rain-drop, or the growth of a plant, are all natural phenomena. 7. Cause and Effect. — Nothing happens of itself. All natural phenomena are produced by certain causes ; for example, unsupported bodies fall to the earth : here all we see is the effect, namely, the motion. The cause of the motion is the attraction which the earth has for the body. An effect may itself be the cause of some succeed- ing effect ; thus, the body, in falling to the earth, may give some of its motion to another body which it strikes, and this effect may in turn be the cause of some other effect, and so on indefinitely. The causes which produce natural phenomena can all be traced to certain forces, one of the most import- ant of which is heat. 8. Natural Law. — If we observe any natural phe- nomenon, we will notice that the relation between cause and effect is constant and invariable. The same cause, acting in the same way, alwavs produces the same effect. Thus, unsupported bodies always fall to the earth ; a steel pen rubbed against a magnet alwa 3 "s becomes magnetic, and acquires the property of at- tracting iroiT filings ; the same pen, unless specialh^ protected, alwaj's becomes covered with rust when exposed for some time to damp air. MA TTER. 13 When, by observation, we have discovered the cause of any natural phenomenon, and have ascertained the order in which cause and effect follow each other, this order, expressed in language, forms what is called a natural law. Natural Philosophy has for its object the study of natural laws. This definition, it will be seen, embraces the study of all natural phe- nomena; but natural philosophy is generally restricted to the study of the laws concerning the physical changes that take place. 9. Method of Study. Experiment, — There is only one way by which we can discover natural laws, and that is by observation. If we wish to know what effect will follow a certain cause, we must make the trial, and observe what happens. If, after repeated trials, we obtain the same effects, we may conclude that Ave have discovered the law. 10. Force and Energy. — All changes in nature are caused by the action of certain forces. The term energy is employed to designate the amount of work Avhich can be accomplished by the action of any force. By the muscular force of the arm, a pound weight may be raised a certain distance, say one foot;' if the same force continue to act, the Aveight may be raised two feet. Here, in each case, the cause of the motion is the same, viz., the muscular force of the arm ; but the energy, or the amount of Avork done, is tAvice as great in the latter case as in the former. 11. Indestructibility of Matter and Energy. — It is believed that there exists in the universe a certain definite quantity of matter and of energj'’. Neither of these can by any known means be increased or diminished. Matter and energy may, during changes, disappear, but only to reappear in other forms. Thus, 2 14 NATURAL PHILOSOPHY. when a piece of paper or wood is burned, it disap- pears, and the heat set free is apparently lost ; but the paper or wood has, by the process of burning, been changed into invisible gases, and the heat set free has acted on the matter around it, and produced changes therein. Matter may change from one form to another, and energy may be converted into some other form of energy, but neither can be destroyed. Syllabus. Matter is anything which occupies space and prevents other things from occupying the same space. No two pieces of matter can be in the same place at the same time, for all matter fills the space it occupies, and prevents other matter from occupying the same space. Air and water are kinds of matter, since they occupy space and exclude other bodies from the space they occupy. We know of the existence of matter by means of our senses ; we can see, feel, smell, or taste it. Different kinds of matter are called substances; substances are either elementary or compound. Elementary substances are called elements. Matter is constantly undergoing change. A physical change is one which may happen without the substance losing those peculiarities or properties which generally enable us to recognize it. A chemical change is one which obliterates or removes such properties. Physics or Natural Philosophy studies the causes and efiects of physical changes ; chemistry studies the causes and effects of chemical changes. A phenomenon is a natural event, or anything which occurs in the usual course of nature. Nothing happens without a cause. The causes which produce natural phenomena may be traced to certain forces, one of the most important of which is heat. A natural law is that which expresses the order in which cause and effect follow each other. Natural laws are discovered by means of observation. QUESTIONS FOR REVIEW. 15 By the term energy, we mean the amount of work which can he accomplished by the action of any force, flatter and energy are both indestructible. During changes, matter and energy may both disappear, but only to reappear in some other forms. Questions for Review. How can you tell whether the things you see are composed of matter or not ? Prove that both air and water are forms of matter. Is a shadow a form of matter ? Why ? How do we become acquainted with different kinds of matter? What are substances? Name some different substances. What are elementary substances ? What are compound substances ? Give some examples of each. What is a body ? What do you understand by the properties of a body ? To what two different kinds of change is matter subject? What is the differ- ence between these changes ? Give an example of each of these kinds of change. What is Physics or Natural Philosophy ? What is chemistry ? What is the difference between the scientific and the common use of the word pheuomeuon? Name some different natural phenomena. What do you understand by the cause of a natural phenomenon ? What is an effect ? What relation always exists between cause and effect? Name any important cause which produces many natural phenomena. Define natural law. Give an example of any natural law. What has the study of natural philosophy to do with natural law ? In what way only can any law of nature be discovered ? Define energy. Can the amount of either matter or energy which now exists in the universe be increased or diminished? What is in- destructibility ? CHAPTER II. GENERAL, PROPERTIES OF MATTER. 12. Names of General Properties. — Bj the prop- erties of a body we understand those peculiarities or qualities which, enable ns to recognize it. Properties are either general or specific. General properties are those which are common to all matter. Specific or particular properties are those which are possessed only by certain kinds of matter. The most important of the general properties of matter are magnitude or extension, impenetr ability, divisibility, porosity, compressibility , expjansibility, mo- bility, and inertia. 13. Magnitude or Extension. — All matter occupies space or fills room, that is, it has size. This property is generally knoAvn by the name of magnitude or ex- tension. Matter extends or fills space in three difterent directions, namely, in length, in breadth, in thickness ; or, in other words, all matter possesses volume. 14. Units of Measurements. — In this country, the dimensions of a, body are measured in inches, feet, yards, or miles. In France and Europe, generallj', dimensions are measured in metres or in decimals or multiples of a metre. The following tables give the values of these units, viz. ; 16 GENERAL PROPERTIES OF MATTER. 17 English Measure. Measures of Length. Measures of Surface. Measures of Volume. 12 in. make one ft. 144 sq. in. =1 sq. ft. 1728 cub. in.= 1 cub. ft. 3 ft. “ “ yd. 9 sq. ft. = 1 sq. yd. 27 cub. ft. = 1 cub. yd. 1760 yds., or 5280 ft., make one mile. French Measure. Measures of Length. 1 metre equals 39.37 Eng. in., or 3.280 ft. 1 decametre, or 10 metres “ 393.7 1 hectometre, or 100 metres “ 3937. (( C( 1 kilometre, or 1000 “ “ 39370. “ “ or 1093.6 yds. 1 decimetre, or y’j- “ “ 3.937 1 centimetre, or “ “ .3937 1 millimetre, or '* " .03937 “ “ or in. nearly. Measures of Area or Surface. 1 square metre equals . . . 1550.06 sq. in., or 10.764 sq. ft. 1 square decimetre equals . 15.5006 “ “ 1 square centimetre “ . .155006 “ “ 1 square millimetre “ . .00155006 “ “ Measures of Volume. 1 cubic metre equals . . . 61027.1 cub. in., or 35.3166 cub.ft. Icubicdecimetre, or litre, equals 61.0271 “ “ .2202 gals. 1 cubic centimetre equals .0610271 “ “ The measures of surface are obtained by squaring the measures of length. Thus, 12 ins. X 12 = 144 square inches, or one square foot ; 3 ft. X3 = 9 square feet, or one square yard; 39.37 ins. X 39.37 = 1550.06 square inches, or one square metre. The measures of volume are obtained by cubing the measures of length; thus, 12 ins. X 12 X 12 = 1728 cubic inches, or one cubic foot. The following table will be found convenient in changing English into French units : 1 inch 1 foot 1 yard 1 mile 25.40 millimetres. .3048 metre. .9144 metre. 1.609 kilometres. The value of one square inch can be obtained by squaring 25.40 millimetres = 645.16 sq. millimetres. The value of one cubic inch can be obtained by cubing 25.40 =16387.06 cubic millimetres. 2* B 18 NATURAL PEILOSOPHY. Figure 1 represents the actual lengths of the French decimetre, and Figure 2 of four English inches. Since a decimetre equals 3.937 ^ inches, it is but a trifle shorter than four English inches. From to i? is one decimetre; from A to a is one centimetre, of which there are ten in one decimetre, or in the whole length of A B. From .A to 6 is one millimetre, of which there are ten in every centimetre. From C to D is four English inches; from C S _ to a is one inch ; from C to i is one-tenth of an inch. There are about twenty-five millimetres in one inch. 15. Impenetrability is that prop- erty wliicli prevents any ttvo particles of matter from occupying the same space at the same time. As we have already seen, magni- tude or extension and impenetrability are necessary to the existence of mat- ter. They are^ therefore, sometimes called the essential properties of matter. Experiment. — If an empty glass goblet be held, mouth downwards, in a vessel of water, it will be found that the water will not rise and fill the goblet, since it is already full of air. This proves that air and water cannot be in the same space at the same time. If, however, a vessel open at both ends, as, for example, a glass lamp ^ chimney, be dipped into water, it will be found that the water will rise as high inside the chimney as it is on the outside, for, as the water rises in the chimney, it pushes the air out at the top. Caution . — Unless the goblet be held with its mouth quite level with the water, some of the air will escape, and mar the experiment. Although, in many cases, two different bodies seem to occupy the same place, yet, in reality, this is not the case. Like the body moving through air or water, they merely move parts of the other body out of the space they occupy. Thus, a nail driven into a board can only enter it by crowding some of the particles of the board more clc.«ely GENERAL PROPERTIES OF 31 ALTER. 19 together. If, too, in the experiment just described, the goblet be pushed far down into the water, the water will rise some little dis- tance inside it, because the pressure has packed the particles of air more closely together. 16. Divisibility is tliat property of matter wlrich enables us to cut it up or divide it into a number of smaller pieces. So far as simple experiment shows, there would appear to be no practical limit to the di- visibility of matter. AYe can continue to divide it until the particles are too small to be directly seen, and even then, by using a microscope, we can carry the di- vision still further. The following examples will show the wonderful extent to which it is possible to carry the division of matter. Gold can be beaten into leaves so thin that it would take about three hundred thousand of such leaves, placed one upon the other, to make a pile one inch thick, and yet each of these leaves can be cut into very small pieces. A grain of musk will continue for yeans to give off its odorous particles to the air around it, without decreasing very perceptibly in weight. A very small quantity of certain coloring matters will give a de- cided tint to a large quantity of water. Now, since the water so colored may be divided into a very great number of parts, in each of which the color is distinctly visible, the quantity of coloring matter in each must be exceedingly minute. The microscope has revealed to us the existence of animalcules so small that millions can easily swim about in the space of a cubic inch without touching one another. 17. Atoms and Molecules. — Notwithstanding the above, and other wonderful instances of the extreme divisibility of matter, we have reasons for believing that matter is not infinitely or indefinitely divisible, but that if we could continue to divide a piece of mat- ter far enough, we would reach a limit beyond which it would be impossible further to divide it. These final particles of matter are called atoms. The word atom means that which cannot be cut. 20 NATURAL PHILOSOPHY. Since we know hj actual trial that we can divnde matter into very small particles, the atoms must be exceedingly small; but of their real nature, no definite knowledge has as yet been obtained. The atoms sel- dom, if ever, exist separately, but combine with one another, and form groups of two or more atoms, called molecules. The force which causes the atoms to com- bine is the chemical force. Molecules, though larger than atoms, are still exceedingly small — smaller, in- deed, than the smallest particles into which we have been able to divide bodies. The molecule of a compound substance is the small- est possible quantity of that substance that can exist. Water is a compound substance, composed of two atoms of hyi'lrogeu combined with one atom of oxygen. Any compound containing less than two atoms of hydrogen combined with one atom of ox}'- gen, would not be water. The molecule of water, or the smallest quantity of water that can exist, will therefore contain two atoms of h_\xlrogen and one of oxygen. The molecule of an elementary body consists of a group of atoms of that body. The very small particles of matter obtnined artificially, by con- tinued division or by grinding, are neither atoms nor molecules. The word particle is sometimes used in the sense of atom or molecule ; such use is only correct when we mean the smallest possible particle. 18. Porosity. — Neither the atoms nor the molecules touch one another; though they are nearer together in solids than in liquids or gases, yet it is believed they are not in actual contact, even in the most solid substances known. The spaces tvliich separate them are called pores. The size of the pores varies greatly. In some kinds of matter, the pores can readily be seen, as, for example, in most woods, or in sponges. GENERAL PROPERTIES OF MATTER. 21 In otliers we cannot see tlie pores, even Avitli tlie aid of a microscope. A¥e can, however, in many cases, show that they exist, by forcing liquids through them. Thus, if water be placed in a strong vessel of gold, and a powerful pressure be exerted on the water, it can be forced to pass through the vessel without rup- turing it. The water must therefore have passed through the pores. 19. Compressibility. — All matter, when subjected to sufficient pressure, can be made to occupy a smaller space ; or, in other words, all matter possesses the property of compressihility . Substances vary in their degree of compressibility. Of the three conditions in which matter is found, gases are the most compressi- ble, and solids the least. The compressibility of most solids and liquids is very small, considerable force being required to produce a perceptible change in their volume. The compressibility of- gases is very much greater than that of solids or liquids. All matter, when cooled, decreases in bulk. This decrease is called contraction., and is to be distinguished from compression. As a rule, gases contract more than liquids, and liquids more than solids, on the loss of equal quan- tities of heat. 20. Expansibility. — • As matter contracts or de- creases in bulk by a loss of heat, so also it expands or increases in bulk by an increase of heat. Gases ex- pand more than liquids, and liquids more than solids, by the same change of temperature. To the above general statement there exist exceptions, a few substances expanding on a loss, and cortracting on an increase, of heat. 22 NATURAL PHILOSOPHY. If, as is believed, the size of neither the atoms nor the molecules is changed during expansion or contraction, then the increase or decrease in the bulk so produced can only be due to the increase in the size of the pores or spaces between the atoms and molecules. As, therefore, all matter expands or contracts when subjected to changes of tem- perature, all matter must contain pores. Experiment. — If an empty glass bottle be held, mouth downwards, in a plate of w'ater, so that the mouth is just under the water, and the hand be made to cover as much of the outside of the bottle as possible, the heat of the hand will cause the air inside the bottle to expand, so that the bottle will no longer be able to hold it all, and it will be seen to bubble out from the mouth of the bottle. Caution . — The bottle should be compar- atively large, so as to hold a fair amount of air, and made of glass as thin as possible. Pig, 3.-Expansion of Air. quickly communicated to the air within. If the bottle be dipped too far down in the water, it will take a longer time for the bubbles of air to escape, on account of the pressure of the water on the outside. 21. Mobility is tliat property of matter -vs-iiicli en- ables it to be moved or to change its place. Since the earth is ahvays rotating on its axis and revolving around the sun, it is clear that nothing on the earth is ever actually at rest. We generally say, however, that a body is at rest when it is not chang- ing its position as regards neighboring bodies. Besides the more apparent motions that occur around us. like the flowing of a river, the flight of a bird, or the fall of a stone, there are other motions too minute to be seen. The molecules are never at rest, but are in rapid motions towards and from each other. These motions cause various phenomena of heat and light. 22. Inertia. — A body never begins to move, stops moving, or changes the direction in Avhich it has been moving, unless force of some kind acts upon it. In GENERAL PROPERTIES OF MATTER. 23 other words, matter can do nothing of itself towards changing its condition, either of rest or motion. If we attempt to move a comparatively large body from a state of rest, as, for example, a wheel revolving freely on an axis, we will find it necessary to exert onr strength for some time before we can get the wheel to move rapidly ; that is, we find that a body at rest apparently offers a resistance to changing its state of rest. On the other hand, when the wheel has been set in motion, we will also find it necessary to exert our strength, but this time in the opposite direction, before we can bring the body to rest again ; that is, we find that a body in motion apparently offers a re- sistance to chano'ino- its state or condition of motion. O O By the inertia of matter we understand its tendency to continue in whatever condition it may be, whether of rest or of motion, until some force acts upon it. Therefore it follows, from the property of inertia, 1st. That a body at rest tvill continue at rest forever., unless some force acts upon it ; and, 2d. That a body in motion will continue moving in a straight line forever, unless some force acts upon it. It is very easy to believe that a body once at rest will continue at rest forever, until acted on by some force, since such is our common experience. But it is difficult, at first, to believe that the same thing is true of a body in motion. If, for example, we throw a stone straight up in the air, we know that it will not continue moving upwards forever. In reality, it moves more and more slowly every moment, and at last entirely stops, and begins to fall to the earth. But in this case the body does not stop its own mo- tion. It is stopped because the earth is constantly pulling it down towards it, and because the air is 24 NATURAL PHILOSOPHY. resisting its motion. Could we go out into tlie empty space, beyond the influence of any other force, and throw the stone in any direction, it w’ould continue moving in a straight line in that direction forever, since, as it is inert, it has no more force or power to stop its motion than it has to begin to move. fl’he earth moves through the apparently empty space around the sun, and, since there is nothing to stop its motion, must continue moving forever. 23. Examples of Inertia. — When a train of cars begins to move, it takes some time before it reaches its full speed, on account of the inertia of the train. When, hoAvever, the train has attained this speed, con- siderable force must be exerted to stop it. Before a moving body can be brought to rest, it must lose an amount of energy equal to that wdiich caused its motion. Cannon-balls owe their great de- structive power to the fact that they have been set in motion by the explosion of gunpowder, and hence will overcome considerable resistance before stopping. If w'e jump from a rapidly moving coach, w'e are very apt to fall over, because, on reaching the ground, the motion of our feet is stopped while the rest of our body continues to move forward. A running jump will carry us much farther than a standing jump, be- cause, if we first run rapidly in the direction in which we wish to jump, the motion thus given to the body wdll help to carry it in that direction. Experiment. — If a book be stood upright on its end on a sheet of paper, it will be found that, if the paper be slowly pulled, the book can be moved without falling, since, in this case, the motion is gradually imparted to it. If, now, while the book is moving, we suddenly stop pulling the paper, the book will fall forwards, since its top keeps on moving after the part resting on the paper has stopped. GENERAL PROPERTIES OF MATTER. 25 If, on starting, we pnll the paper quickly, the hook will fall back- wards, because the part resting on the paper is moved forwards be- fore the top has commenced to move. Caution. — The experiment will be more impressive and apt to suc- ceed, if a long, heavy book be used. 24 :. Living Bodies Possess Inertia. — Living bodies, as well as those without life, possess inertia. We are conscious of having to exert oitr strength before we can move about from place to place. Thus, if we move an arm, we do it by means of the muscular strength or force of the arm. When we have once set our bodies in rapid motion, as, for example, in running down hill, we find it necessary to exert con- siderable strength in order to stop suddenly. 25. Resistances to Motion. — A body moving- through air or water can only advance by pushing the air or water out of its way. Since both air and water possess inertia, they cannot move them- selves out of the way, and therefore require that force act upon them. This force is taken from the moving body, and diminishes its motion. Eesistances of this kind are called fluid resistances., and are im- pediments to motion. When bodies are slid or rolled over one another, they meet another resistance or impediment to motion. Even the smoothest surfaces we can obtain are full of irregularities ; and when one body is slid or rolled over another, the projections of the one fitting into the de- pressions in the otlier, cause a resistance to moticn. Besides the irregularities of the surfaces, whenever two bodies are brought near each other, they attract or tend to hold on to each other. The re.sistances to motion produced in this Avay are called /Vfc/ioTj,. 26 NATURAL PHTLOSOPIIY. Friction, therefore, is caused 1st. By tlie irregularities of the surfaces in contact; and, 2d. By the attraction which the bodies have for each other. .ao>»»4o<> Syllabus. By the properties of a body, we mean those peculiarities or qual- ities which enable us to recognize it. Properties are either general or specific. Magnitude or extension, impenetrability, divisibility, porosity, com- pressibility, expansibility, mobility, and inertia are the most import- ant of the general properties of matter. Magnitude and impenetrability are sometimes called the essential properties of matter, because they are necessary to its existence. The unit of measurement used in this coantrj’’ is the inch and its multiples; that commonly used in France, and thronghout Europe, is the metre and its multiples, or subdivisions. One decimetre is very nearly equal to four English inches. The atoms and molecules are impenetrable. When a nail is driven into wood, or a stone is dropped into water, the molecules are pushed aside, not penetrated. Although, so far as experiment is concerned, our abilit}' to divide matter appears to be without limit, yet we believe that, if the division were carried far enough, tve would at length reach minute particles that could no longer be divided. These particles are called atoms. The atoms are believed to be unalterable, and their size unaffected by heat or cold. The atoms seldom, if ever, exist separately, but com- bine with one another, and form groups of atoms called molecules. Neither the atoms nor the molecules touch one another, even in the densest kinds of matter. The spaces between the atoms or the mole- cules are called pores. We know that even solid matter contains pores, 1st, Because many solids are permeable to liquids ; 2d. Because all matter is compressi- ble ; 3d. Because all matter contracts by cold. When matter expands or contracts by heat or cold, or when it is compressed, it is not the size of the atoms or molecules which is changed, but the size of the spaces or pores between the atoms or molecules. QUESTIONS FOR REVIEW. 27 The expansion or contraction of gases is greater than that of liquids, and that of liquids greater than that of solids. Since the earth is constantly moving, nothing on its surface is at actual rest. We generally consider, however, that a body is at rest when it is not changing its position as regards neighboring bodies. A body never begins to move, stops moving, or changes the direc- tion of its motion, unless force acts upon it. By inertia we understand the tendency matter has of continuing in whatever state it may be, whether of rest or motion, until some force acts upon it ; therefore, a body at rest will continue at rest for- ever, or a body in motion will continue in motion forever, unless force acts upon it. Living bodies possess inertia. Force is necessary to move our bodies or to stop their motion. A body moving through air or water loses all the energy it gives to the air or the water it pushes out of its way. Friction is the resistance which one body experiences in being slid or rolled over another, and is caused by the irregularities of the sur- faces, and the attractions the two bodies have for each other. Questions for Review. What is meant by the properties of a body ? What is the difference between general and specific properties ? Which two general proper- ties are sometimes called the essential properties ? Name the general properties of matter. Define magnitude or extension. What unit of measurement is gen- erally used in this country ? In Europe ? Define impenetrability. Is it the atoms and molecules or the pores that are impenetrable? Is matter divisible without limit? Can we prove this by experiment? Name any instances of the extreme di- visibility of matter. Define atom and molecule. What are pores? How do we know that pores exist ? Which is the more compressible, solids or liquids ? Liquids or gases ? When matter is compressed, is it the atoms and the molecules or the pores which are made smaller ? Which is the more expansible, solids or liquids ? Liquids or gases ? Describe in full any experiment by means of which the expansibility of air may be shown. 28 NATURAL PHILOSOPHY. Why can there be no such' thing as absolute rest on the earth? When do we generally regard a body as being at rest? What is necessary in order that a body may begin to move, stop moving, or change the direction of its motion ? What do you understand by the inertia of matter? Is inertia pos- sessed by bodies in motion as well as by those at rest ? Show by anv example that this is the case. What example can you give of a body continuing its motion forever? How does the amount of energy which a moving body must lose before it comes to rest, compare with the amount required to produce that motion ? Why can a person jump farther by a running than by a standing jump ? Describe any experiment by which the inertia of matter may be shown. Do living bodies possess inertia? What do we find necessary to do before we can begin to move from place to place? Is any force re- quired to stop our bodies when they have once been put in motion? What do you understand by friction ? Give an example of friction acting as a resistance to motion. CHAPTER III. THE THREE CONDITIONS OF MATTER. 26. Solids, Liquids, and Gases. — Matter is found in three different conditions, viz., the solid, the liquid, or the gaseous. Many kinds of matter can be made to assume any of these conditions. The molecules of a substance, it is believed, do not, change in size when the substance is passing from a solid to a liquid form, or from a liquid to a gaseous form ; it is only the distances which separate the mole- cules, and the intensity of the forces that act on them, that change. 27. The Molecular Forces. — The molecules do not touch one another, but are separated by spaces or pores, that are large when compared Avith the size of the molecules. The molecules are kept at certain dis- tances from one another by reason of forces which act on them, and Avliich are called the molecular forces. There are two molecular forces. One tends to draw the molecules together, and is called the force of mo- lecular attraction; the other tends to keep them apart, and is called the force of molecular repulsion. The molecular forces, therefore, act in opposite directions. Molecular repulsion is caused by the action of heat. The cause of molecular attraction is unknoAvn. The three conditions of mattei\ the solid, the liriuid, 3 * 29 30 NA TURAL PHILOSO PII Y. and the gaseous^ result from the different degrees with which these attractive and repulsive fforces are exerted on the molecules. 28. Solids. — In soli'ds, the force of molecular attrac- tion is greater than that of repulsion ; the molecules, therefore, are held together, and resist any force tend- ing to separate them. Solids can he fashioned into any form or shape, .since the molecules are all held together, and resist force tending to separate them. The force of molecular attraction varies in different solids. A piece of paper, or a match stem, may easily be pulled into smaller pieces, since the molecules are readily separated ; but, if we take a piece of sheet-iron no thicker than the pajier, and try to pull it to pieces, we would find that it would require very much greater force, because the molecules of the sheet-iron cling together with far greater force than do those of the paper. A steel wire, so thin as to be almost invisible at the distance of several yards, is strong enough to hold a man’s weight. In soft wax or butter the molecules are held together so feebly that but little force is required to alter their form. Indeed, these substances are very much like liquids. 29. Fluids. — Substances whose molecules move freely over one another are called fluids. There are two kinds of fluid substances, viz., liquids and gases. 30. Liquids. — In liquids, the forces of molecular attraction and repulsion exerted, are nearly equal to each other. The molecules, therefore, are not all held together, but are very nearly independent of one an- other, and, when acted on by any force, can move or slide easily over one another. From the great freedom of motion of their mole- cules, liquids possess no definite shape, but at once assume that of the vessel in which they are kept. A quantity of water poured into a bottle will at once THE THREE CONDITIONS OF MATTER. 31 take the shape of the inside of the bottle, and will keep that shape as long as it continues in the bottle; but let it be poured into a cup, plate, or tumbler, and it will at once take the shape of the inside of the cup. plate, or tumbler. 31. Mobile and Viscid Liquids. — The force of molecular attraction varies in different liquids. In some the molecules are attracted to one another with much greater force than in others, and in them, there- fore, the molecules do not move or flow over one another so readily. In molasses or tar, the molecules do not move over one another as readily as in alcohol or ether, because the molecules of molasses or tar are held together with greater force than are the molecules of the alcohol or ether. Liquids, like molasses or tar, in which the molecules do not flow or move over one another readily, are called viscid or viscous liquids. Those like alcohol or ether, in which the molecules flow or move over one another readily, are called mobile liquids. AYe have seen that some solids, like soft butter or wax, are scarcely to be distinguished from liquids. There are, on the other hand, many liquids, like very thick tar, which it is difficult to distinguish from solids. The solid condition, indeed, often passes almost imperceptibly into the liquid condition. Some solids, as, for example, the metals, exhibit, when subjected to sufficient pressure, many of the phenomena of flow. In the process of wire drawing, a stout rod of cold iron or copper may he drawn without fracture into a wire many hundred times the length of the original rod. A disc of metal, put under a powerful coining-press, is caused by the pressure to flow into all the cavities of the dies, thus assuming accurately their precise impressions. Heavy blocks of ice 32 NATURAL PHILOSOPHY. or stone, when subjected to long-continued pressure, may be consider- ably bent or flexed without fracture. 33. Gases. — In gases, the foree of repnlsion is stronger than that of attraction. Gases, as we have seen, are one form of fluids, and possess the ability to flow to a much greater extent than liquids. Since in gases the force of repulsion is greater than that of attraction, the molecules should be constantly getting farther and farther apart, or the bulk of the gas should be constantly increasing; but the mole- cules of all bodies on our earth are prevented by the force of gravity from getting beyond a certain dis- tance from one another. In gases, therefore, the force of gravity, together with that of molecular attraction, is equal to the force of repulsion. Since the force of repulsion is thus ever held in check, it is evident that a gas constantly tends to expand. AVe see the influence of this action in our atmos- phere. Towards its upper limit the air is much lighter than that nearer the earth’s surface, since the lower layers of air have to sustain the weight of those above them, and the particles are thereby packed more closely together. The increase in the force of gravity near the earth’s surface also causes a slight increased density of the lower layers. When a balloon rises very bigb above tbe earth’s surface, tbe gas it contains expands, and occupies a mucb greater bulk than it did near tbe surface. If balloons, at tbe time of tbeir ascent, are too full of gas, they often burst from tbis cause on reacbing moderately great beigbts, unless some arrangement is provided for tbe escape of the excess of gas. 33. Influence of the Pressure of the Air on Liquids. — AY ere it not for the pressure of the air. THE THREE CONDITIONS OF MATTER. 33 many bodies wbich now exist as liquids would be gaseous. W ater, and many other liquids, wlien placed iu an unlimited space from wbich all the air has been removed, will turn into the gaseous state. Gaseous bodies that are formed from liquids in this way, or by the direct action of heat, are called vapors. In many liquids the forces of molecular attraction and repulsion are not in equilibrium.^ until the pressure of the atmosphere adds its force to that-^f attraction. 34. Influence of Heat on the Condition of Matter. — By the action of heat, most solids become changed into liquids, and most liquids into vapors or gases. The action of heat is to increase the distance between the molecules, and thereby to lessen the force of at- traction. When solids are changed into liquids by the ac- tion of heat, we say that they have been melted or fused. Some substances are very easily melted or fused. Ice, for example, melts at quite a low temperature. Butter softens when brought into a warm room. In these substances but a slight increase of tempera- ture is necessary to overcome the force of attraction. Other substances are only melted or fused by the action of intense heat. Cast-iron re- quires the heat of a blast-furnace in order to melt it. Substances that are very difficult to melt or fuse are called refractory substances. When substances which have been melted or fused lose the heat which caused them to fuse, they again become solid. This process is called solidification. Thus, water solidifies or freezes when it loses sufficient heat. Cast-iron is obtained in any desired form by pouring it, when melted, into moulds, in which it solidifies on cooling. All liquids are changed into solids if they lose sufficient heat. C 34 NATURAL PniLOSOPHT. By the action of heat, liquids become changed into gases or vapors. This process is called vaporization. On the loss of the heat which caused the vaporiza- tion, the vapor again becomes a liquid. This process is called liquefaction or condensation. As a rule, most vapors are liquefied by mere loss of heat. Ordi- nary gases require both loss of heat and compression in order to bring their molecules sufiiciently near one another to become liquids. Until quite recently, a number of gases had never been liquefied by cold or pressure. These were called the incoercihle gases. Bj' subjecting them, however, to intense cold and pressure, they too have been changed into liquids. There are, therefore, no gaseous substances that can now be considered as incoercible. oX»io Syllabus. Matter exists in three conditions, viz., as solids, liquids, and gases. In the same substance these conditions differ from one another, not on account of the size of the molecules, but on the distances which sep- arate them. A group of two or more atoms is called a molecule. The molecule of a compound substance is the smallest quantity of that substance that can exist. The molecules are kept in their relative positions by the forces of molecular attraction and repulsion. The cause of molecular attrac- tion is unknown. Molecular repulsion is caused by heat. Solids are bodies in which the force of molecular attraction is greater than that of repulsion. Solids possess definite shape, because the molecules are all held together, and resist any force tending to separate them. The force of molecular attraction varies in diflferent solids ; some are easily pulled in pieces, while others can only be broken by the appli- cation of considerable force. Liquids are bodies in which the forces of molecular attraction and repulsion are equal to each other. The molecules are not all held together, and can move or slide readily over one another. Bodies whose molecules move or slide easily over one another are called fluids. There are two kinds of fluids, viz., liquids and gases. QUESTIONS FOR REVIEW. 35 Liquids have no definite shape, but take at once the shape of the vessel into which they are poured. Liquids that flow readily are called mobile liquids. Those that do not flow readily are called viscid or viscous liquids. The solid condition passes almost imperceptibly into the liquid con- dition. Solid bodies, when subjected to great pressure, exhibit many of the phenomena of flow. Gases are bodies in which the force of attraction is greater than that of repulsion. The pressure of the air or the force of gravity, however, makes the force of attraction equal to that of repulsion. Gases are constantly tending to expand ; their molecules possess great freedom of motion. Heat causes solids to become liquids, and liquids to become gases. When a solid has been changed into a liquid by the action of heat, we say that it has been melted or fused. Substances differ in the amount of heat necessary to fuse them. Those which are very difficult to fuse are called refractory substances. When a substance which has been melted is allowed to cool, it again hardens or becomes solid : this is called solidification. By vaporization we mean the change of a liquid into a vapor by the action of heat. When a vapor is sufficiently cooled, it again becomes a liquid : the process is called liquefaction. By the combined action of cold and pressure all known gaseous bodies have been changed into liquids. Questions for Review, In what three conditions may matter exist? Define atom, molecule. What do we understand by the molecule of a compound substance ? By the molecule of an elementary sub- stance ? Are the small particles obtained by grinding either atoms or molecules ? What are the molecular forces? Explain in full the action they exert upon the molecules. What are solids? Why should solid bodies possess definite shape? Give any example which shows that the particles of different kinds of solid substances are held together with different force. What are fluids ? What two kinds of fluid substances are there ? What are liquids ? Why should liquids possess no definite shape ? Have the molecules of liquids no attraction for one another ? 36 NATURAL PHILOSOPHY. What is the difference between mobile and viscid liquids ? Give an example of each. What is the cause of this difference? Show that the solid state often passes insensibly into the liquid state. What instances can you give to show that solids sometimes exhibit the phenomena of flow ? What are gases? By what is the force of molecular repulsion aided in gases ? Why should a gas be constantly endeavoring to expand ? Why is that portion of our atmosphere which is near the earth's surface denser than that which is far above it ? What influence does heat exert on the condition of matter? When is a substance said to be melted or fused ? What happens when a sub- stance which has been melted or fused is allowed to cool ? What do you understand by vaporization ? Under what circum- stances does vaporization occur ? What is meant by liquefaction or condensation ? In what two ways may liquefaction or condensation be caused ? Can all gaseous sub- stances be liquefied by the combined action of cold and pressure ? CHAPTER IV. FORCE AND MOTION. 35. Force is anything which makes a body begin to move, which stops its motion, or Avhich changes the direction in which it has been moving. Force is necessary to produce or modify motion, on account of the inertia of matter. All natural phenomena are caused hy force acting upon matter. 36. Varieties of Force. — Force manifests itself in a variety of ways, and thus arise different varieties of force. We have already briefly mentioned the force of gravity, which causes bodies to fall to the ground ; the force of magnetic attraction, which causes magnets to attract iron filings; the chemical force which causes chemical changes, like the rusting of a pen ; the muscular and vital force which cause the movements of living bodies; the /orccs of fluid resistance and friction which modify the motion of bodies ; the force of molecular attraction, and the force of molecular repulsion or heat which keeps the molecules of matter at certain distances from one another. Besides these forces there are a number of others, as, for example, magnetic repulsion, electric attraction and repulsion, light, and elas- ticity. 37. The Representation of Force. — In order to ascertain the effect produced by any force, we must know 4 37 38 NATURAL PHILOSOPHY. 1st. The point of application, or tlie point of tlie body at wbich tbe force acts. 2d. The direction in which the force acts ; and, 3d. The intensity with which the force acts. It is convenient to represent forces by straight arrows ; the intensity of the force is represented by the length of the arrow; the direction of the force is that in which the arrow would fly; and its point of application is considered as being situated at the end of the arrow which is placed against the string of the bow. Thus, in the figure, we represent the weight of the body A, or the force with which it is pulled down by the attraction of the earth, by the arrow y B. The amount of this force is represented by the length of the arrow ; the force is represented as acting at the point y. We generally express the amount or intensity of a force in pounds avoirdupois. When forces are represented by arrows, as in the figure, a certain portion of the length, as, for example, one inch, is taken to represent one pound. Thus, if in the figure the arrow y B were two inches long, the figure might represent a force of two pounds acting at the point y, in the direction of y B. B Pig. 4. The Represen- tation of Force, 38. The Direction in which the Force Acts. — If we take hold of a body free to move and pull it, the body moves towards us ; if we push the body, it moves from us. The direction in which the force acts, therefore, de- termines the direction in which the body moves. FORCE AND MOTION. 39 39. The Point of Application of the Force. — If we place the end of a ruler against a book lying on a table, and push the book, we will notice that the kind of mo- tion it acquires depends upon the part touched by the ruler. If, for example, the ruler be placed exactly against the middle of one of the ends of the book, a push will move it straight forward in the direction we are pushing; but if the ruler be nearer one end than the other, although the book will still advance when pushed, it will at the same time move partly around. Tlie point at which the force is applied determines, therefore, the nature of the motion. 40. The Intensity of the Force. — If we push the book with more force in one instance than in another, we will find that its motion will be more rapid when the greater force is acting upon it. The intensity of a force, therefore, determines the rapidity of the motion. 41. Mass and Velocity. — If one book is larger than another, we will find that more force is required to give' it in the same time the same rapidity of motion as that given to the smaller book. By the mass of a body we mean the quantity of matter it contains. In the same substance the mass is proportional to the number of molecules. By velocity we mean the distance through which a moving body passes in a given time. The distance is generally measured in feet and the time in seconds, thus, by a velocity of six feet per second, we mean a velocity that will carry the body through six feet in one second of time. 40 NATURAL PHILOSOPHY. In tlie following table, taken mainly from Ganot, will be found tbe velocities in feet per second of a number of different objects. Table of Velocity. Snail Moderately slow river .... Moderately rapid river .... Man walking ...... Quick military step Moderate wind High wind Hurricane ....... Eailway train Steam-ship ....... Eagle Sound in air at 32° Fah Martini-Henry rifle-hullet .... Point on equator by rotation of earth Centre of earth by revolution around the sun i^ee/ per second, b TOSTJ 3 . 6 . 3 to 4 434 10 40 120 to 160 36 to 75 22 100 . 1090 . 1330 . 1520 . 101000 42. Momentum. — We have seen that tbe amount of energy required to produce motion in a body depends : 1st. On the mass of the body. Tbe greater tbe mass, tbe greater tbe amount of energy expended in produ- cing a given motion, and 2d. On the velocity. In tbe same body, tbe greater tbe velocity tbe greater tbe amount of energy re- quired to produce it. In order, tberefore, to compare tbe total quantity of motion possessed by one body witb tbat possessed by another, or in order to obtain tbe total quantity of motion in any body, we must multiply the mass of the body by its velocity. The momentum of a moving body., or the quantity of motion it possesses., is equal to tbe mass of tbe body multiplied by its velocity. FORCE AND MOTION. 41 If a given force acting on a mass of ten pounds moves it at tlie rate of ten feet per second, the momentum, or the amount of energy of mo- tion, would be ten times ten, or one hundred ; if the same force acts on a mass of one pound, it must, in order to give it the same momentum, impart to it a velocity of one hundred feet per second, since 100 X 1 = 100. This result would not follow, unless there was no other resist- ance to the motion of the body than that arising from its inertia. In a body moving through air or water, the increased fluid resistance would cause the same force to produce a smaller velocity. 43. Examples of Momentum. — A drop of rain may scarcely bend the blade of grass on which it falls ; a moderately large hailstone, falling with about the same velocity, may cut the leaves and branches from trees ; while a rifle-bullet, from its more rapid motion, carries death in its path. A floating chip is harmless when it strikes the sides of a small boat, but a floating log may crush the boat if caught against a wharf, or other fixed obstacle. Even a powerful ship, caught between two large ice- bergs moving in opposite directions, is as certainly crushed as an egg-shell when trodden on. If, while in motion, we strike our bodies against any fixed obstacle, as a tree, the injury received will depend on the speed with which we were moving. If we were walking at a moderate speed, the injury would probably be but slight ; if running, we may break a bone ; if thrown from a carriage while in rapid motion, we may lose life. 44. Momentum the Measure of Energy of Mo- tion. — Since a body begins to move solely from energy expended on it, it follows that the momentum at any given time represents the amount of energy of motion that the body possesses at that time. The momentum.! therefore! ^ measure of the amount of energy of motion lohich a body possesses. 4* 42 NATURAL PHILOSOPHY. From the inertia of matter, it follows that motion once having been imparted to a body by force acting on it, the body must continue to move until it has lost all of its energy ; if it meets no resistance, or is acted on by no other force, it must continue to move in the same direction, and with the same velocity forever. 45. Force not Affected by the State of Rest or Motion. — A force acting on a body will produce the same effect, whether the body be at rest or in motion. Common experience proves this. A body dropped to the floor of a rapidl^r-moving car strikes the floor directly under the place from which it fell. The car-floor does not move from under the body while it is falling, since the body is moving forward as rapidly as the car. By its rotation, the earth is rapidly moving towards the east; and yet we find no more difficulty in running towards the west than towards the east. A mounted acrobat at a circus, in jumping through a hoop, does not jump forward; if he did, he would be carried over the horse’s head. He simply jumps up into the air, and, his body having the same velocity as the horse, he falls again on its back, just as the body dropped to the floor of the car, strikes the floor directly under the place from which it fell. Therefore., when two or more forces act on the same body at the same time, . each produces the same effect as if it acted alone. 46. Varieties of Motion. — When a body moves through equal distances in successive seconds, its mo- tion is said to be uniform. When a body moves through unequal distances in successive seconds, its motion is said to be varied. FORCE AND MOTION. 43 If the velocity of the body is changed at a uniform rate, we say that its motion is uniformly varied. When the velocity regularly increases, the motion is said to be uniformly accelerated. When the velocity regularly decreases, the motion is said to be uniformly retarded. As gravity is constantly acting on a falling body, it must have a uniformly accelerated motion, since gravity is constantly adding to the motion the body has already acquired. A body thrown vertically upwards must have a uniformly retarded motion, since gravity is constantly decreasing its motion. Bodies whose motion is either uniform or varied, may move in straight lines or in curves, that is, their motion may be rectilinear or curvilinear. Rectilinear motion is that in which the body moves in strai2:ht linos. Curvilinear motion is that in which the body moves in curved lines. In rectilinear motion, the body continues to move in the same direction. In curvilinear motion, the body is constantly changing its direction. When a body moves around a fixed point, as, for example, a wheel on an axis, the motion is called rotary. 4:7. Components and Resultants. — A body cannot move in more than one direction at the same time. No matter, therefore, how many forces may act at the same time on a body, their combined action can only produce motion in one direction; and this motion could have been p)roduced by a single force of suffi- cient intensity acting in the proper direction. Any single force which will produce the same eft'ect 44 NATURAL PHILOSOPHY. on a body, as a number of separate forces, is called their resultant. The separate forces are called the components. 48. Direction of the Forces, — When several forces act at the same time on a body, they may act 1st. In the same straight line. 2d. At some angle with one another. 3d. Parallel to one another. 49. Forces Acting in the same Straight Line. — When two or more forces act in the same straight line, they may act either in the same or in opposite direc- tions. When they act in the same direction, the re- sultant is equal to their sum. When two forces act in opposite directions, the resultant is equal to their difference; and if the two forces are of equal inten- sity, they will be in equilibrium.^ and the body may be at rest. 50. Forces Acting at some Angle with One Another. — When a body is- acted on by two forces in directions that form an angle Avith each other, the body Avill not move in the direction of either force, but in some direction between them, that is, the re- sultant will lie somewhere between the two compo- nents. Thus suppose, for example, that a man is rowing a boat across a riverwith a force that would in a certain time car- ry the boat from Pig. 5— The Parallelogram of Forces. ff. tO .5 in the FORCE AND MOTION. 45 direction of the line A B, but that in the same time, by the velocity of the stream, he would be carried from A to B, then, by the combined action of these two forces, he would be carried to C along the line A 0. The direction and intensity of this force are ascer- tained as follows : Draw the line B C parallel to A D and D C parallel to A B. Then, if the point (7, where these two lines meet, be connected by a straight line with the point yl, this straight line will give the direction in which the body will move when under the action of both forces. Since A B and A D rep- resent the direction and intensity of the two forces, the line A C will represent the direction and inten- sity of the force produced by their combined effect. If, therefore, we know the lengths of A B and A D, we can obtain the length of A C either by direct measurement or by calculation. In this case (7 is the resultant, and A D and A B its components. The figure A B C D, being a four-sided figure bounded by parallel lines, is called a parallelogram, and this construction is generally called the j>aral- leloyram of forces. The parallelogram of forces affords another example of the fact already stated, that when two or more forces act on the same body, each produces the same effect as if it acted alone. Thus, by the force of the man rowing, the boat would be carried from Aio B-, subse- quently, by the force of the stream acting for an equal time, it would be carried from B to C. We see that each force has produced the same amount of effect as if it acted alone, for, while the man has rowed the boat through a distance equal to D C, the stream has carried it through a distance equal to B C. When two forces act in any direction, forming an angle with eacb other, the resultant is found in the same manner. 46 NATURAL PHILOSOPHY. Fig. 6. — Parallelogram of Forces. Thus, suppose two forces, A C and A B, act together on the point A, one tending to produce motion along the line A C, and the other along the line A B ; then, as before, completing the par- allelogram A B C D, its diagonal, A i>, represents the di- rection and inten- sity of the resultant. Wheu three or more forces act at the same point, their resultant may be obtained by first finding, as before, the resultant of any two of the forces ; and then with this resultant, and any other of the forces, a second resultant is found, and so on until a final re- sultant is obtained which expresses the action of all the forces. Thus, suppose three forces, A B, A C, and A B, act as represented on the point A. Completing the paral- -> lelogram A B H C.^ its di- agonal A H '\?> the resultant X 0 of the two forces A B and A C. Now taking this re- sultant, and the third force Fig. 7.-Composition of Three Forces. completing the parallelogram A H 0 D, its diagonal, A 0, represents the resultant of the three forces. 51. Parallel Forces. — 'When two parallel forces act in the same direction, the resultant is always equal to their sum ; but the point of application of the resultant does not coincide with the points of appli- cation of either of the two forces. When the forces are of equal intensity, it is situated midway between them ; but when they are FORCE AND MOTION. 47 Fig. 8,- - Parallel Forces Acting in the Same Direction. unequal in intensity, its distances from the points of application of the two forces are inversely proportional to the intensities of these two forces. Thus, suppose the bar A B, of uniform thickness and density, be supported at its centre C ; it will then be in equilibrium, since, being acted on by two equal par- ^ allel forces, viz., the weights aI of the parts CB and C A, the resultant of these forces will be found midway be- tween their points of appli- cation, or at (7; and since the bar is supported at this point, it will be in equilibrium. Suppose, however, we conceive the bar to be divided into two unequal lengths, A D and D B ; then, since the weight of .4 i) is greater than that of D B, we have two parallel but unequal forces, which may be considered as acting at the points E and F, the middle points oi A D and D B. The point of application of the resultant of these forces will not, therefore, be found midway between E and F, but will be at C, which is as much nearer E than F, as the force acting at E is greater than that acting at F, that is, the distance of C from E and F is inversely as the intensity of the forces acting at E and F. Should additional weights be hung to the bar at the points E and F, the equilibrium will not be disturbed, provided the weight at E is as much greater than the weight at as the distance CF is greater than the distance C E. When two parallel forces act in opposite directions, the point of application is never situated between them, but is in the extension of a line joining the points of application of the two forces, on the side of the greater force, at a distance from the points of application of the two forces inversely as their intensities. Thus, let two parallel forces, B C and A D, act at the points B and A, as shown. The resultant E F takes the same direction as that of the greater force B 0; its intensity is equal to the difference of the intensity of i? C and A D, and its point of application E is so situated that the distances, E B and E A, are inversely as the forces B C and A D. JJ If two parallel forces of equal inten- Fjg. g.— Parallel Forces Acting in sity act in opposite directions, they can- Opposite Directions. 48 NATURAL PHILOSOPHY. not be replaced or held in equilibrium by a single force. These forces form what is called a couple, and tend to rotate the body on which they are acting. 52. Centrifugal Force. — The inertia of a moving body will cause it to continue moving in a straight line forever. To stop its motion or to change its direction, some other force must act on it. Since a body in circular motion is constantly changing its direction, it is clear that motion of this kind can never be produced by the action of a single force, because one force would be required to set the body in motion and another to constantly act to change the direction of the motion so produced. A stone tied to a string and whirled around, will continue moving in a circle as long as the string is held in the hand; but if Ave let go the string, the stone will no longer move in a circle, but will fly oft' in a straight line, in the same direction as that in which the stone was moving at the time the hand let go the string. The string is constantly keeping the stone at a fixed distance from the hand, and preventing it from moving oft' in a straight line. Since, however, the stone neither moves towards the hand nor away from it, these two forces must be equal to each other; they are not, however, directly opposite, and therefore do not neutralize each other, and the body moves in their resultant. The force which causes the body to tend to move oft' in a straight line in the direction in which it was moving at any time, is called the Centrifugal Force. Centrifugal force, or the centre-flying force, is merely the result of inertia. The force Avhich prevents the body from flying from the centre is sometimes called the Centripetal Force. FORCE AND MOTION. 49 The motion of the earth around the sun aifords a good instance of the so-called centrifugal and cen- tripetal forces. The earth, in consequence of the motion originally given to it, is constantly tending to move away from the sun. The sun, however, is constantly attracting it ; and these two causes make the earth move in an almost circular path around the sun. 53. Examples of Centrifugal Force. — Drops of mud thrown by centrifugal force from the wheels of a carriage in rapid motion move off from it in straight lines. Grindstones and the fly-wheels of engines when put into very rapid rotation are sometimes burst by the action of centrifugal force. The tendency of dif- ferent portions of the wheel to continue moving in the direction in which they were moving at any given moment, at last becomes stronger than the cohesion of the particles, and the stone or wheel flies into a number of pieces, which move with consid- erable velocity in different directions. The shape of the earth is not that of a perfect sphere. The equa- torial diameter, or the distance through the centre at the equator, is somewhat greater than the polar diameter, or the distance through at the poles. This bulging of the earth at the equator was caused by the action of centrifugal force. Syllabus. Force is anything which makes a body begin to move, which stops its motion, or which changes the direction in which it has been moving. All natural phenomena are caused by force acting on matter. The principal varieties of natural force are the force of gravity, the forces of magnetic and electrical attraction and repulsion, the forces of molecular attraction and repulsion, the chemical force, the 5 D 60 NATURAL PHILOSOPHY. muscular force, the forces of fluid resistance and friction, and the forces of light and elasticity. To ascertain the effect produced by a force we must know, 1st. The point of application of the force, or the point at which it acts. 2d. The direction in which it acts; and, 3d. The intensity or energy with which it acts. Forces are generally represented by arrows: the force acts in the direction in which the arrow flies ; the intensity of the force is repre- sented by the length of the arrow ; the point of application is at the end of the arrow which is placed against the string of the bow. The direction in which the force acts determines the direction in which the body moves ; the point at which the force acts determines the nature of the motion, and the intensity of the force determines the velocity of the motion. The mass of a body is the quantity of matter it contains. The velocity of a body is the distance through which it moves in a unit of time. The momentum of a body is the quantity of motion it possesses, and is equal to the mass of the body multiplied by its velocity. When we know the momentum of a body, we can tell the amount of energy that has caused its motion. A force acting on a body produces the same effect, whether the body is at rest or in motion. Two or more forces acting on a body produce the same effect as if each force acted alone. Motion may be either uniform or varied. Varied motion may be either accelerated or retarded. Wlien two or more forces act on a body at the same time, they may act, 1st. In the same straight line ; 2d. At some angle with one an- other ; 3d. Parallel to one another. The resultant of a number of forces is any single force which will produce the same effect as all the separate forces. The separate forces are called components. We can determine the direction and intensity of the resultant of several forces which act in directions at an angle with one another, by means of the principle of the parallelogram of forces. Two or more forces are in equilibrium when they neutralize each other’s effects, or produce rest. The Centrifugal force is the force with which a body moving in a circular path is constantly endeavoring to move in a straight line. Centripetal force is the force which constantly changes the direction of a moving body, and so causes it to move in a curved path. QUESTIONS FOR REVIEW. 51 Questions for Review. What is force ? By what are all natural phenomena caused ? Name the principal varieties of natural force. What three things are necessary in order to ascertain the effect pro- duced by any force ? How are forces generally represented ? What effect is produced in the motion of a body by the direction in which a force acts ? What effect is produced by the point at which the force acts ? What effect is produced by the intensity of the force ? Define mass, velocity. Give the velocity in feet per second of any four moving objects. What do you understand by the momentum of a body ? On what two things does the momentum of a body depend ? Why is the mo- mentum the real measure of the amount of energy of motion a body possesses ? Give any examples of the different effects produced by bodies whose momenta are different. Is the effect produced by a force acting on a body the same when the body is at rest as when it is in motion ? Why will a body dropped in a rapidly-moving car fall straight down to the floor ? Is any more energy required to move a body towards the west than towards the east ? When two or more forces act on a body at the same time, will each produce a similar effect as if it acted alone ? Illustrate by examples. What is the difference between uniform and varied motion? What two kinds of varied motion are there? Define rectilinear motion, curvilinear motion, rotary motion. When are forces said to be in equilibrium ? What do you understand by the resultant of several forces ? What is meant by the components of a force ? When several forces act on a body at the same time, in what dif- ferent directions may they act ? When several forces act on a body in the same straight line, how is their resultant found ? Describe the principle of the parallelogram of forces. How do we determine the resultant of several forces acting on the same body in directions parallel to one another ? What do you understand by centrifugal force ? By centripetal force ? Could a single force cause motion in a circle ? CHAPTER V. THE MECHANICAL POWERS. 54. Machines. — A machine is any arrangement of parts by means of which force is so transmitted from one point to another that its intensity or direction, or both, are modified. The force Avhich is used in mov- ing a machine is called the power. The resistance to be overcome, or the work to be done, is called the wo7'Jc, weight .1 or load. A pair of scissors affords a good example of a simple machine. Here the power, which is the strength of the fingers, is applied at the handles, in a direction which causes the handles to come together. The work accomplished by the scissors is the cutting of some material placed between the two blades. 55. The Principle of Velocities. — When there is no other resistance to the motion of a body than that arising from its inertia, a given amount of force acting for a certain time on a given mass will impart to it a given velocity ; but the same force, acting for the same time on another mass twice as great, will impart to it but half this velocity. Fig. 10.“ A Simple MacMne. 52 THE MECHANICAL POWERS. 53 If a straiglit rod, A i?, supported on a fixed point, F.^ nearer A tlian B, be moved so as to as- sume tire position shown by the line A' B', then, since the end B moves over Fig. ll.— The Principle of Velocities, the curved path B B' in the same time that the end A moves over the curved path A J.', the velocity of B must be as much greater as the velocity of A as B B' is greater than A A'. But B B' is as much greater than A A' as the line F B w greater than the line FA. If then FB be twice as great as F A, the velocity of the end B will be twice that of the end A. If now a weight of one pound be hung at the end 5, then, disregarding the weight of the bar A B, the pound at B will exert a force capable of lifting as much more than one pound at A^ as the velocity of B is greater than the velocity of A ; and, since in this case the velocity of B is twice that of A, one pound at B will lift two pounds at A. A, however, will move with a velocity but half as great as that with which B moves; for, since B is acting on a mass twice as great as its own, it can in the same time impart to A but half the velocity. The rod, A B, moving about the point, F, is in fact a simple machine. Power applied at either end will overcome a resistance at the other end, and the power and weight will be in equilibrium, or, in other words, the power will be able to balance the weight when the power multiplied by the distance through which it moves is equal to the weight multiplied by the distance through which it moves. 5 * 54 NATURAL PHILOSOPHY. The bar will then be in equilibrium because the momentum at either end of the bar will be equal to that at the other end. Thus, at ^ a mass of one is moving with a velocity of two, and the momentum is therefore equal to 1 x 2 = 2, while at a mass of two is moving with a velocity of 1, and the momentum is therefore equal to 2 x 1=2. This principle will enable us to determine the re- lation existing between the power or force used in driving any machine and the work done by the machine ; /or, disregarding friction and fluid resist- ances, we have only to notice the distance through which the power must he moved in order to cause a given movement of the weight. Suppose, for example, in any machine that, in order to move the weight through one foot, the power moves through ten feet, then a power of one will move a weight of ten. Disregarding the weight of the rod A B, and supposing, as before, the distance A F to be one-half of F B, then we can see, from what has been said about parallel forces, that if one pound be hung at B and two pounds at A, they will balance each other ; since the two par- allel forces of different intensities, acting in the same direction, are in equilibrium when their resultant is at F, wdiich is as much farther from B than from A as the force at B is less tlran that at A. 56. No Energy Gained by Machines. — It must not be supposed, from the example just given, that a ma- chine creates energy. No more work can be done bv any machine than that which has been expended in moving it. In fact, from the resistances which all machines offer to motion, we never get as much effec- tive work out of a machine as that Avhich has been put into it. All that any machine can do is to change the amount of resistance that can he overcome, by chang- ing the time in tuhich it is done. THE MECHANICAL POWERS. 55 Suppose, in Fig. 11, F B were ten times the length of F A.! then one pound at B could move ten pounds at A. Here it might seem as if energy sufficient to raise nine pounds had been created ; but this is not so, since the single pound moves through ten times the distance as that through which the ten pounds are raised, or .exerts its force through ten times the space. To make this clearer, suppose that, by exerting his entire strength, a man could just lift one hundred pounds. Then, by the u.se of the simple machine above described, he could raise 10 X 100 = 1000 lbs. ; but to raise these thousand pounds through one foot, he would be required to continue to exert a force of one hundred pounds through ten feet ; and this would clearly be the same as if he divided the thousand pounds into ten separate parcels of one hundred pounds each, and for ten successive times exerted his strength of one hundred pounds through one single foot. Though machines gain nothing in energy, yet their gain in con- venience is often considerable. In the case of the simple machine just described, there is a saving in the time that would otherwise be lost in passing from parcel to parcel ; besides which, one man is enabled to raise weights without dividing them, which it would otherwise be im- possible for him to raise. In general, whatever be the nature of the work for which the machine is designed, its perfection depends on the convenience it se- cures in the performance of that work. 57. Mechanical Powers. Simple Machines. — Ho matter how complicated any piece of machinery, it is made up of various combinations of a number of simple machines. These simple machines are some- times called the mechanical powers. The mechanical povjers are the lever.^ the wheel and axle.^ the pulley the inclined plane, the wedye, and the screw. The six mechanical powers just named are in re- ality modifications of the lever and of the inclined plane. In all that will be said about simple machines, it is 56 NATURAL PHILOSOFIIY. F w Fig. 12. — Lever of the 1st Class. comes w supposed that no force is lost during its transmission from one part of the machine to another. 58. The Lever. — 1. The lever consists of an inflex- ^ ihle rod or bar moving about a fixed point, called the fulcrum. A force or power applied at one part of the lever over- a resistance or lifts a weight at another part.- -P The difierent forms of levers may be arranged in three class- es, according to the relative positions of the fulcrum, the power, and the weight. In the first class, the fulcrum is between the power P and the weight ; in the second class, the weight is between -- . '■'■j the fulcrum and the power; w and in the third class, the Fig. 14. — Lever of the 3d Class. . . , , , « . power IS between the luicrum and the weight. Examples of levers of the first class are found in a pair of scissors ; in a crowbar, when used to raise blocks of stone ; in the common balance for weighing, and in a pair of pincers. Examples of levers of the second class are found in nut-crackers, where the nut to be cracked, is placed between the fulcrum, E, Fig. 13. — Lever of the 2d Class. and the part, P, where the power is applied. A Fig. 15. -Levers of the 2d Class. opened bv a hand applied to the knob is another example : the weight of the door is between the hinges, which act as the fulcrum, and the knob where the power is ap- plied. A Avheelbarrow is another example. THE MECHANICAL POWERS. 57 Examples of levers of the third class are found in tlie sugar-tongs, tlie fire- tongs, and in the com- mon foot-treadle. The Arms of the Le- ver. — 2. The shortest distance from the ful- crum to the direction in which the power acts, is called the arm of the power; the shortest distance from the fulcrum to the direction in which the Aveight acts, is called the arm of the weight. These two distances are called the arms of the lever. When the f power acts, as / \ in Fig. 17, in a direction, B P. ^ \ ^ at right angles to the length of O the lever, then ' FB the arm of the power ; but if it acts in any oblique direction, as B P', then F B', the shortest distance from the fulcrum to the direction, B' P', of the power, is the arm of the poAver. V p Fig. 17.— Tlie Arms of a Lever. P The Effects of the Lever. — 3. The effect produced by the poAver in any lever depends on the relative lengths of the arm of the poAver and the arm of the Aveight ; for, as the distances through which the poAver and the Aveight move are directly as the arms of the poAver and of the Aveight, the power, multiplied by the arm of the poAver, is equal to the Aveight multi- plied by the arm of the weight. 58 NATURAL PHILOSOPUY. In Figs. 12, 13, and 14, a bar of the same length is shown, em- ployed as a lever of the first, second, and third classes respectively. In Fig. 12, if F P, the arm of the power, is three times as great as F W, the arm of the weight, then a force of one pound acting at P will balance a weight of three pounds at IF. In Fig. 13, by placing the power and fulcrum at the extremities of the lever, the distance, F IF, the arm of the weight, may be made one- fourth that of F P, the arm of the power, so that a force of one pound at P will balance four pounds at IF. The same bar, therefore, when used as a lever of the second class, can bo employed to overcome a greater resistance than when used as a lever of the first class. In Fig. 14, if F P he one-fourth as great as F IF, then a force of one pound acting at P will balance but one-fourth of a pound at IF.- it will move it, however, with a velocity four times greater than its own. In levers of the third class, the velocity of the weight is always greater than the velocity of the power. 59. The Wheel and Axle. — In the tvlieel and axle, the power applied at the circumference of a wheel is employed to raise a weight attached to a rope wound around an axle. The figure shows a form of the wheel and axle called Pig. 18— The Windlass. windlass. The power is applied to either a wheel or winch at Tl', and raises a weight, if, attached by a rope to the axle, -4. The wheel and axle is simply a modification of the lever : the fulcrum is at the axis ; the weight is ap- plied at one side of the axle, while the power is a]3- plied at some point on the wheel. Since, by one complete turn of the wheel, the weight is only raised the length of the rope wound once around the axle, a force of ojte pound applied at the wheel will raise a iceight of as many more pounds hung to the axle as the circumference of the wheel is greater THE MECHANICAL POWERS. 59 than the circumference of the axle. TVater-Avlieels and steering-apparatus for vessels employ the principle of the wheel and axle. 60. The Pulley, — The pulley consists of a wheel turning on its axis, and having an edge over which a flexible band or rope passes. Pulleys may be fixed or movable. In the fixed pulley, no advantage is gained except the change in the direction of the motion, since, if the rope is pulled down one foot by the power, it will raise the weight through an equal dis- tance. In the movable pulley, the block or frame. A, to which the weight is at- tached, is movable. If the power, P, moves downwards through two feet, the weight, IF, would be raised through but one foot, since the rope is pulled from both C and D. In such a pul- ley, therefore, a force of one pound at P would raise a weight of two pounds at IF. The fixed pulley is a continuous lever of the first class with equal arms. The lever includes both the wheel and axle and the 'pulley. 61. The Inclined Plane. — Instead of raising a weight directly through a given height, it may be raised gradually through the same height by moving it up an inclined plane. In this way heavy casks or barrels are moved out of deep cellars. The efficiency of the inclined plane depends on the direction in which the power is applied. We will con- sider but two cases, viz. : 1st, when the power is applied in the direction of the length of the plane ; 2d, ichen it is applied in the direction of the base of the plane. 60 NATURAL PEILOSOPHY. An example of the first case is seen in the figure, where, to raise the barrel through a height equal to the height of the plane, we must roll the barrel over the whole length of the plane. If the lenfjth of an inclined plane he four times as yreat as its heifjht, a force of one pjov.nd will he able to 'move a weight of four pounds up the plane. In the second case, in order to raise the weight through the distance of the height, the power moves through a distance equal to the base of the plane ; if, then, the base of an inclined plane he three times as great as its height, a force of one pjound would he able to ptoll three pounds up the plane. 62. The Wedge. — The wedge is a modified form of inclined plane, in which, instead of moving the weight up the jfiane, the plane is moved under the weight. The Avedge is used when great force is to be exerted in a small space, such, for ex- ample, in splitting Avood or stone, or in Pig. 21. The Wedge. pj-0gsiijg opg or juices from seeds. The edges of such cutting tools as scissors kniAms, chisels, hatchets, and razors are forms of Avedges. 63. The Screw. — The scrcAV is another modifica- tion of the inclined plane, and has the same relation to a simple plane that a spiral staircase has to a straight one. If a piece of paper be cut in the form of a right-angled triangle and Avrapped around a pencil, as shown in the cut, the edge of the paper, Avhich cor- THE MECHANICAL POWERS. 61 responds to the length of the plane, will form a spiral line around the pencil in a direction which will be the same as that of the thread of a screw. As the power which turns the screw moves it through one complete turn, the screw, and anything against which it is pushing, as, for exam- pie, the movable plate of a copying-press, 22,— The Prin- Fig. 23, advances through the distance between any two consecutive threads. If the power acting- on a screw move through a circumference of 24 inches, and the distance between any two consecutive threads be yo of an inch, a power of one jDound applied at the head of the screw w'ould move a weight of 240 pounds at the other end, since 23.— A Copying-Press, the power would move through a distance 240 times as great as that through which the weight moves. The wedye and screw are modifications of the inclined plane. 64. Perpetual Motion. — Many foolish men, igno- rant of the fact that machines do not create energy, but only transmit the energy imparted to them, have art- fully endeavored so to design a machine that, when it has once been set into motion, will not only con- tinue to move forever afterwards, but would even give motion to other things, Avithout losing any of its own motion. If a body could be set in motion where it would be exposed to no friction or fluid resistances, it would then continue to move forever. Such conditions, hoAA'ever, are impossible on the earth ; and even if they did exist, 62 NATURAL PHILOSOPHY. a macTiine so set in motion would be useless, since it would possess only a certain quantity of energy, namely, that applied to start it ; and if it should be used to give motion to other machines, or to do any work, it would cease moving as soon as it had expended an amount of energy equal to that originally imparted to it. Syllabus. A machine is any arrangement of parts by means of which power is so transmitted from one point to another that its intensity or direction, or both, are modified. The force causing the motion of a machine is called the power ; the resistance to be overcome is called the work or load. If a certain force acting on a body having a certain mass produce a given velocity, the same force acting for the same time on a body with twice the mass will produce but one-half as great a velocity; or double the force acting on a given mass, will produce twice as great a velocity as half the force acting on the same mass. If a straight rod be moved while unequally balanced, the ends will pass through unequal spaces in the same time, and therefore possess un- equal velocities. If weights be hung at the extremities of such a rod. they must be unequal, in order to produce equilibrium or to balance the rod. If a given force acts at the longer end of such a rod, it will exert at the shorter end a force as much greater as its own, as the velocity with which it moves is greater than the velocity with which the other end moves. If unequal weights be balanced at the extremities of a bar supported at a point nearer one end of the bar than the other, either weight multiplied by its distance from the point of support will be equal to the other weight multiplied by its distance from the point of support ; or, if the bar be moved about the point of support, either weight mul- tiplied by the distance through which it moves will be equal to the other weight multiplied by the distance through which it moves. No machine creates energy ; it merely changes the amount of work that can be done by changing the time in which it is done. All machines, however complicated, are made up of various combi- nations of a few simple machines, called the mechanical powers. SYLLABUS. 63 The mechanical powers are six, viz., the lever, the wheel and axle, the pulley, the inclined plane, the wedge, and the screw. These are all modifications of the lever or of the inclined plane. The wheel and axle, and the pulley are modifications of the lever ; the wedge and the screw are modifications of the inclined plane. A lever is a straight rod or bar moving freely about a point called the fulcrum. There are three classes of levers ; in the first, the fulcrum is between the power and the weight ; in the second, the weight is between the power and the fulcrum ; in the third, the power is between the weight and the fulcrum. Scissors, pincers, the common balance, and the crow-bar when used to lift weights, are examples of levers of the first class. Nut-crackers, a wheelbarrow, and a door opened by a hand at the handle, are examples of levers of the second class. Sugar-tongs, fire-tongs, and the common foot-treadle, are examples of levers of the third class. In all levers, the arm of the power is the shortest distance between the fulcrum and the direction of the power ; the arm of the weight is the shortest distance between the fulcrum and the direction of the weight. The power and weight will always balance each other when the power multiplied by the arm of the power is equal to the weight multiplied by the arm of the weight. In the wheel and axle, a force of one pound applied at the circum- ference of the wheel will raise as many more pounds of weight hung to the axle as the circumference of the wheel is greater than the cir- cumference of the axle. Pulleys are fixed or movable : in fixed pulleys, nothing is gained except a change of direction ; in the movable pulley, the power can raise a weight as much greater than its own as the distance through which it moves is greater than the distance through which it raises the weight. In the inclined plane, if the power is applied parallel to the length of the plane, it will exert a force as much greater than its own as the length of the plane is greater than the height of the plane ; but if the force is applied parallel to the base of the plane, it will exert a force as much greater than its own as the base of the plane is greater than the height. The wedge is used wherever great force is to be exerted in a small space. Any force acting to turn a screw will cause the screw to move with a force as much greater than its own, as the circumference of the circle through which the screw moves is greater than the distance between any two contiguous threads. 64 NATURAL PHILOSOPHY. Perpetual motion is impracticable : 1st, Because all moving bodies on the earth encounter resistances ; 'and, 2d, Because, even if a body could move without friction and fluid resistance, it would cease moving as soon as it had expended in giving motion to other bodies an amount of energy equal to that originally imparted to itself. Questions for Review. Define machine, power, weight or work. A straight bar is supported so as to move freely about a point nearer one end of the bar than the other; when will weights Lung at the ends of the bar be in equilibrium ? 1 f no energy is gained by a machine, why is it that, when properly using a lever, we can, by exerting a force of only one hundred pounds, move a weight of one thousand pounds ? Name some of the advantages gained by the use of machines? What do you understand by a simple machine or mechanical power ? Name the mechanical powers. Under what two heads may all the mechanical powers be arranged ? Name the powers which are included under each of these heads. What is a lever ? What are the arms of a lever ? Into what three classes may all levers be arranged ? What are the relative positions of the fulcrum, power, and weight in each of these classes ? Name some levers of the first class ? of the second class ? of the third class ? In which class is. each of the following, viz., a foot-treadle, scissors, sugar-tongs, a door opened by a hand applied at the knob, a wheelbarrow, a balance, and a pair of pincers ? Describe the wheel and axle. Why will a certain weight hung at the circumference of the wheel lift a greater weight hung at the cir- cumference of the axle ? What two kinds of pulleys are there ? What is the nature of the advantage gained by the fixed pulley? By the movable pulle}'? If two inclined planes have the same height, but one is twice as long as the other, how much less force would be required to roll the same barrel up the longer plane than up the shorter one ? Why ? For what is the wedge generally used ? Give some examples of wedges. Show that the screw is a modified form of inclined plane. How can the power of a screw be estimated ? If the power acting on a screw move through a circumference of four inches, and the distance between any two contiguous threads be j’jj of an inch, with how much force would the screw advance when a force of one pound was applied at the head? CHAPTER VI. GRAVITATION. 65. Effect of Gravity. — We have seen that unsup- ported bodies fall to the earth because they are attracted towards it by the force of gravity. The real nature of gravity is not thoroughly under- stood, but, from repeated observations and experiments, it is believed that every particle of matter in the uni- verse exerts an attractive force for every other particle of matter, and that this attractive force would eventu- ally bring all matter to one place were it not for op- posing causes. By the force of gravity^ we mean the attractive force that one mass of matter exerts upon another. The weight of a body is caused by the earth’s attraction for the body. 66. English and French Systems of Weight. — The unit of weight both in this country and in England is the pound. Unfortunately, the pound is of two distinct kinds, viz., the pound avoirdupois and the pound troy. The grains are alike in both of these pounds, but the other subdivisions are different. The avoirdupois pound contains 7000 grains, and the troy pound contains 5760 grains ; in the former there are 16 ounces, and in the latter 12 ounces. In France, the unit of weight is the gramme and its multiples and subdivisions. The gramme is the weight of a cubic centimetre of water at the temperature of its greatest weight, viz., at 39°.2 Fahr., and is ec[ual to about 15.432 English grains. The following table gives the names and values of these multiples and subdivisions. 6* E 65 66 NATURAL PHILOSOPHY. Grammes. Grains. 1 Kilogramme = 1000 15432.34 1 Hectogramme = 100 1543.23 1 Decagramme = 10 154.32 1 Gramme = 1 = 15.43 1 Decigramme = 0.1 1.543 1 Centigramme = 0.01 .154 1 Milligramme = 0.001 .0154 . Law of Universal Gravitation.- —The law universal gravitation was discovered bj Sir Isaac Newton, an English philosopher, after a long series of observations and calculations. It may be stated as follows, viz. : Every particle of matter in the universe attracts every other particle of matter with a force that is directly pro- portional to the mass, and inversely proportional to the square of the distance. One thing is directly proportional to another wlien it increases or decreases in the same ratio that the other increases or decreases. Thus, when we say that the force of attraction is directly proportional to the mass, we mean that, if the mass of a hodj’ were doubled, the attraction it would exert for other bodies would also he doubled ; or if its mass were trebled, its attractive force would be trebled. One thing is inversely ptroporiioival to another when it increases or decreases in the same ratio that the other decreases or increc^es. Thus, if one is made twice as great, the other becomes but half as great. When we say, for e-^ample, that every particle of matter attracts every other particle with a force that is inversely propor- tional to the square of the distance, we mean that if at a certain distance the attraction between two bodies was represented by one, then, if this distance were made twice as great, the attraction would be but one-fourth, viz., inversely as the square of the distance. 68. Attraction Proportional to the Mass. — The greater the mass the greater the attraction. An un- supported body falls to the earth because the earth attracts it ; the body also attracts the earth, and al- though the pull which the earth has for the body is GRAVITATION. 67 no greater in amount than that which the body has for the earth, yet the quantity of matter in the earth is so much greater than that in the body, that the motion imparted to the earth is as much less than that imparted to the body as the mass of the earth is greater than the mass of the body ; hence, although the earth rises to meet the falling body, its motion is imperceptible. 69. Attraction Inversely Proportional to the Square of the Distance. — The farther a body is carried above the earth’s surface the less the attrac- tion, and, since the weight of a body is due to the earth’s attraction for it, the less the weight. A body on the earth’s surface is approximately four thousand miles from the centre of the earth. If a pound weight be carried four thousand miles above the earth’s sur- face, it would weigh but one quarter of a pound, since, as its distance from the earth’s centre is doubled, the earth’s attraction for it is diminished to one-fourth. Since the earth is bulged out at the equator, a body when at the equator is farther from the earth’s centre than when at the poles. The same body, therefore, would weigh more at the poles than at the equator ; 191 pounds at the equator would weigh 195 pounds at the poles. If, however, we take a body below the surface of the earth, it would weigh less than at the surface, since that part of the earth that is above it, would pull in the opposite direction to that part of the earth below it, and would therefore decrease its weight. At the centre of the earth, a body would have no weight, since the attraction would be the same in all directions, that is, the body would be pulled as much in one direction as in another. The combined effect of mass and distance on the amount of attrac- tion, is seen in the tides of the ocean, which are caused by the attraction of the sun and moon. The mass of the sun is very much greater than that of the moon, but since the moon is so much nearer G8 NATURAL PH ILOSO PH Y. the earth than the sun, the influence which the moon exerts in causing the tides is greater than that exerted by the sun. In order to understand the efiects of gravity, tve must, as with any other force, ascertain, 1st. TJie direc- iion in which it acts ; 2d. Its point of application, and dd. Its intensity. 70. The Direction of Gravity. — Gravity acts in a vertical direction. A vertical line is one tvhich is perpendicular, or at right angles to a horizontal line. A horizontal line is one at right angles to a vertical line, or is one which extends in the same direction as a limited water surface. The direction of gravity can be ascertained by the use of the plumbdine, Avhich con- sists of a weight, W, attached to the end of a string. If the other end of the string be held in the hand when the weight has come to rest, the string will be stretched ill a vertical direction, and the plumb-line will point directly to- wards the eentre of the earth. As it is the string which pre- vents the weight from falling to the earth, and the string comes to rest in a vertical position, the direction of the vertical must be that in which gravity acts on the body. w fig. 24. — The Plumh-Line. 71. The Point of Application. Centre of Grav- ity. — As gravity acts alike on all the particles of matter in a body, there must be as many separate points of application as there are particles. The GRA VITATION. 69 direction in wlricli gravity acts on eacli of tliese parti- cles is vertically downwards. The separate pulls, which may be regarded as so many separate components, may all be replaced by a single resultant, which, since they all act in the same direction, is equal to their sum. This resultant is equal to the weight of the body, and its point of appli- cation is called the centre of gravity^ because it is a point at which the whole weight of the body may be regarded as collected. In the figure, the vertical dotted lines, a, J, c, d, e,/, etc., represent the separate pulls which gravity exerts on the particles of a body. G A represents their resultant, and G, the point of application of this result- ant, is the centre of gravity of the body. Fig. 25.— The Centre of Gravity. 72. Method of Determining the Centre of Grav- ity. — The centre of gravity of any body may be de- termined as follows : Suspend the body by a string attached at any part, and allow the body to come to rest. Notice the direction in which the string, if ex- tended downwards, would pass through the body. The centre of gravity will be situated somewhere in this line. Attach the string to some other part of the body, and again suspend it, and observe as before the direction of the line extended downwards from the string when the body is at rest. The centre of grav- ity of the body will be situated somewhere in this line. As the centre of gravity is a point, a point situated in two lines must be at their intersection. The point, therefore, where these two lines intersect each other will be the centre of gravity of the body. 70 NATURAL PHILOSOPHY. Experiment. — In a rectangular plate of tin, bore small holes at a, &, and c, as shown in Fig. 26. Tie a string to the plate at h, and sus- pend the plate by the string, and when it lias come to rest, draw the line & d in the direction of the string extended. Then attach the string at some other point, as c, Fig. 27, and, proceeding as before, draw the line c e. The centre of gravity of the plate will be found at g. where the line h d is cut by the line c e. Find in the same way the centre of gravity of any other body. Fig. 27.— Method of Finding the Centre of Gravity. 73. A Body Supported at its Centre of Gravity will be at Rest, — Since, in Fig. 26, tlie plate of tin comes to rest in the position sliotvn, tbe shaded por- tion, h c d /, must be of the same weight as the un- shaded portion, aide-, for were one of these por- tions heavier than the other, it would fall, and the line h d would take some other direction. So also in Fig. 27, the shaded portion, c e or, is of the same weight as the unshaded portion, c e f. The same is true of any other position of the body : the part of the body on one side of the line of direction is always equal in Aveight to tbe part on the opposite side. J. body at rest siqyported at its centre of gravity tcill there- fore remain at rest, since the weight is then equally dis- tributed about the point of support. GRA VITA TION. 71 74. Equilibrium of Bodies Supported on an Axis. — A body supported ou an axis, around wbicli it is free to turn, will be in equilibrium only, wlien the point of support and the centre of gravity are in the same ver- tical line. The point of support may be in three different posi- tions, viz., 1st. Above the centre of gravity ; Below the centre of gravity ; and, 3d. At the centre of gravity. These three positions correspond to three different kinds of equilibrium, viz., stable, unstable, and neu- tral equilibrium. 75. Stable Equilibrium. — In stable equilibrium, the point of support is above the centre of gravity. Any motion of the body around the point of support raises the centre of gravity, and the body when left to itself again assumes a position of stable equilibrium. If a disc be cut from a flat piece of card-board, and a hole be made at S by a large needle, so as to allow the card to move freely around the needle, and the card held as shown in Fig. 28, it will come to rest in a po- sition of stable equilibrium, since rinm. the point of support, /S', is above the centre of grav- ity, 6^, and in the same vertical line with it. 76. Unstable Equilibrium. — In unstable equilib- rium, the point of support is below the centre of gravity. Any motion of the body causes the centre of gravity to fall : the body does not afterwards tend to assume its old position of equilibrium, but assumes one of stable equilibrium. If the pasteboard disc be held as shown in Fig. 29, it will be in a position of unstable equilibrium. Fiff. 28.— StaMe Equilit- 72 NATURAL PH ILOSOPHY. since tlie point of support, is below the centre of gravity, and in the same ver- tical line Avith it. If the disc be slightly moved, the centre of grav- ity falls, and the disc assumes the Fig. 29.— UnstaUe Equilib- . . , . , i j. .c rinin, position shown in the last hgure. 77. Neutral Equilibrium. — In neutral equilibrium, the point of support coincides with the centre of grav- ity, and the body will remain at rest in whatever posi- tion it may be moved to about the axis. If the pasteboard disc be held as in Fig. 30, it will be in a posi- Fig. 30.— Neutral Eqnilib- tion of neutral equilibrium, since the point of support is at the cen- tre of gravity, and no matter how the disc be turned, it will remain at rest. 78. Equilibrium of Bodies Resting on a Flat Sur- face. — When a body has more than one point of sup- port, as, for example, when some portion of the bodv is resting on a fiat surface, it is not necessarj* that the centre of gravity be above any one of these points of support in order that the body may be in equilibrium. It is sufficient if the vertical line, passing through the centre of gravity, passes within the base on which the body rests. The equilibrium may be stable, unstable, or neutral. When the centre of gravity is as loiv as it can get, the body is in stable equilibrium. The loAver the cen- tre of gravity, and the greater the base on Avhicli the body rests, the more stable the equilibrium. When the relative positions of the eentre of gravity and the GRA VITATION. 73 Fig. 31.— Stable Equilib- rinm. Fig. 32.— Stable Eqnilib- linnij but less Stable than at A. point of support remain the same in any position of a body, it is in neutral equilibrium. A book placed as at A, Fig. 31, is in stable equilibrium, since it is resting on a large base, and its cen- tre of gravity is low. When standing as at i?. Fig. 32, it is still in stable equilibrium; but since the base on which it rests is smaller than when placed as at 4, and its centre of gravity is higher, the equilibrium is less stable at B than at A. When placed as at (7, Fig. 33, the equilibrium, though still stable, is less stable than at since the base on which it rests is still smaller, and the centre of gravity higher. If the book could be rested on the corner of one of its covers, it would be in a position of unstable equilibrium. If it could be rested on the edge of one of its covers, it would still be in unstable equilibrium ; though the equilibrium would be less un- stable than in the preceding case, since, in the latter case, it would have a num- ber of points of support, while in the former it would have but one. less Stable than 8/t B A sphere resting on a level table is in neutral equilibrium, because no movement of the body can change the relative positions of the centre of gravity and the point of support. A wagon loaded with brieks is in more stable equi- librium than one loaded with hay, because the centre of gravity is nearer the ground. 74 NATURAL PHILOSOPHY. In a boat loaded with people, the equilibrium is more stable if the passengers remain seated. When they stand up, the centre of gravity is raised, and the boat if small may upset. Kxperiment.- d Fig. 34.— Experiment in Stable Equilib- rium. A stout pin, a, Fig. 34, is stuck upright in a cork placed in the mouth of a narrow bottle, c. The blunt end of a stout needle, e, is stuck in a cork, d, on the sides of which a number of penknives,/ and g, are placed, as shown in the figure. If the point of the needle be now carefully placed on the head of the pin, it will be found that it will rest there in a position of moderately stable equilib- rium, and the cork and knives can even be moved around without falling. Caution . — As the head of a pin is more or less rounded, it may be rubbed flat with a file. 79. The Laws of Falling Bodies. — The laws of falling bodies were discovered by Galileo, an Italian philosopher. They may be expressed briefly as fol- lows, viz. ; 1. First Law. — The velocity of a falling body is not affected hy its mass. Gravity acts on each of the atoms of a body ; if the atoms were all separated from one another, each would fall to the earth with the same velocity, because the same force acts on each. Their being united in one mass makes no difference, since gravity acts on each as though it were alone, therefore the number of atoms in any body, or its mass, has no effect on its velocity. Should we yoke two equally fast horses abreast of each other, so as to give them perfect freedom of motion, the two together would not be able to run any faster than either separately. It is the same with the motion of atoms towards the earth. 2. Second Law. — The velocity of a falling body is not affected by the shape or the nature of the body. GRA VITATION. 75 So far as our experience goes, tliis law would appear to be incorrect, since a piece of gold in the shape of a ball, will fall more rapidly through the air than when beaten out into gold-leaf; again, a small piece of cork falls less rapidly through the air than a piece of iron of the same size. It is the resistance of the air which causes these apparent exceptions to the law. In a vacuum, or empty space, all bodies, whatever their size or material, fall with the same velocity ; a feather and a leaden bullet let fall from the same height, at the same time, would reach the bottom of the empty vessel, Fig. 35, at the same instant. Even in the air, two iron weights, one weighing, say, an ounce, and the other a pound, if allowed to fall from the hand at the same time, will reach the floor in so nearly the same time that the eye is una- ble to detect any difference. 3. Third Law . — The velocity acquired hy a falling body at the end of any given time is proportional to the times .1 and is as the numbers 1, 2, 3, 4, etc. Thus, in a body falling freely from a state of rest, the velocity at the end of the first second is about 32 ft. per sec. ; the velocity at the end of the second sec- ond is 2 X 32 = 64 ft. ; the velocity at the end of the third second is 3 x 32 = 96 ft. 4. Fourth Law. — The distances fallen through in successive times increase as the odd numbers 1, 3, 6, 7, etc. A body falling freely from a state of rest passes Fig. 35.— Bodies Faliingthrongli anEmptySpace. 76 NATURAL PHILOSOPHY. tlirouD-li about sixteen feet durincr tlie first second of O O its descent. But tlie velocity of falling bodies is con- stantly increasing, since gravity is constantly giving to the body a fresh impulse, vrhich is added to the velocity already acquired. At the beginning of the first second of its fall, the velocity of the body is equal to nothing, since it starts from a state of rest. From the moment of falling, the velocity regularly increases. Its mean or average velocity during any second would therefore be equal to the mean between the velocity at the beginning and that at the end of that sec- ond. Thus, the mean velocity during the first second = — ^ — = 10; or the final velocity at the end of the first second is 32 /h At the beginning of the second second, the body has acquired a ve- locity that, if gravity ceased to act, would carry it during the second second through 32 ft. But during the second second, gravity would carry it over an additional distance of 16 ft., and therefore, during the second second, the body will fall through 32+ 16 = 48 ft. But 48 = 3X16. The body during the second second falls through three times the distance that it fell during the first second. At the beginning of the third second, which is of course the same as the end of the second second, the final velocity is according to the third law, 2 X 32 ft. = 64 ft. During this second, gravity, as be- fore, carries it an additional distance of 16 ft., and the distance fallen through equals 64 + 16 = 80 ft. But 80 = 5 X 16- The body, during the third second of its fall, passes through a distance five times as great as during the first second. 5. Fifth Law. — The total spaces pjassed throucjli are proportional to the squares of the times. During the first second, the body falls through 16 ft ; during the second second, it falls through 48 ft. ; at the end of the second sec- ond, it has fallen through a total distance of 48 + 16 = 64 ft. But 64 is four times as great as 16, that is, at end of the second second the body has fallen through a space of 2 X 2. or 2 squared, greater than what it fell during the first second, or, in other words, the whole space is proportional to the square of the time. During the tliird second, the body falls through 80 ft. ; at the end of the third second, the bod}' has fallen through a total distance of 16 + 48 + 80 = 144. But 144 = 9 X 16, and 9 = 3X3, or, as before, the total distance is proportional to the square of the time. GRA VITA TION. 'll 80. The Pendulum. — A pendulum consists of a body, 5, suspended by a string or rod, , a 5, from a fixed support, a, on wbicli ^ it is free to move. The body, i, is / called the hob of the pendulum. j When the pendulum is at rest, it / assumes the position of the vertical, / a h. If, now, the bob be raised, so / as to assume the position shown at a j c, it will, when allowed to fall, move / towards its old position, a h, along the / curved line c h. When, however, the *3- — i position a b is reached, the pendulum 30 ,— The Pendu- does not cease moving ; the momen- turn it has acquired in falling from c to ^ will carry it past this position to d. When it reaches c/, it again falls towards 6, and acquires momentum sufficient to carry it to c, and so on, the pendulum continuing to swing between c and d. Each complete swing from c to d^ or from d to c, is called a7i oscillation. The time it takes the pendulum to move through each complete swing is called the tmie or duration of an oscillation. The curved line 6 c, or i (/, which marks the distance the pendulum has been moved from the vertical a b, is called the amplitude of the oscillation. 81. The Laws of the Pendulum. — Fi7-st Law. In the same pendulum., if the amplitude of the oscillation is not very great., the time of oscillation for different a7nplitudes is nearly the same. Unless the pendulum is connected with a spring- er weight, the resistance which the air offers to its movement will cause it to swinu; through smaller and smaller arcs. When, however, these arcs are not 7 * a 78 NATURAL PHILOSOPUY. very large, the time required to move through the longer arc will be the same as that required to move through any shorter one. Second Law. — In ]}endulums of different lengths.^ the duration of an oscillation is proportional to the square roots of the lengths. That is, the longer the pendulum the slower its oscillation. If the lengths of two pendulums are as 1 and 9, the duration of their oscillations will be as v'T and 1 / 9 , or as 1 is to 3, that is, a pendulum nine times the length of another, will move three times more slowly, or the time of its oscillation will be three times as great. 82. The Intensity of Gravity. — The intensity or energy with which gravity acts ou any given mass of matter, can be ascertained by the weight of that mass. This, however, varies in different latitudes, being, as we have seen, greater at the poles than at the equator. 83. Intensity of Gravity and the Pendulum. — Since gravity is the cause of the motion of the pendu- lum, Ave can determine the variations in the intensity of gravity in different parts of the earth by noticing the time of oscillation of the pendulum. If Ave car- ried the same pendulum from the equator to the poles, Ave Avould find that its oscillations would become grad- ually more and more rapid, thus showing that the force of gravity Avas becoming greater and greater. Syllabus. The force of gravity is caused by the attractive force which par- ticles of matter exert on one another. Every particle of matter in existence attracts every other particle with a force that is directly pro- SYLLABUS. 79 portional to its mass, and inversely proportional to the square of the distance between them. The mass of the earth is so great that our experience of the force of gravity is confined almost entirely to a force tending to draw bodies down towards the earth’s centre. If one body has twice the mass of another, its attractive force will be twice as great. If two bodies are twice as far apart at one time as at another, their attractive force at the greater distance will be four times less than at the smaller distance. The force of gravity acts in a vertical direction, and tends to pull bodies towards the centre of the earth. This direction can be shown by means of a plumb-line. The point of application of gravity is situated at the centre of grav- ity, or the point in a body, at which the resultant of all the pulls which the earth exerts on the particles of the body, acts. The weight of a body may be considered as concentrated at the centre of gravity. A body supported at its centre of gravity wiU he in equilibrium, because its weight is evenly distributed around this point. There are three kinds of equilibrium, viz., stable, unstable, and neu- tral. A body supported on an axis, around which it is free to move, will not be in equilibrium unless the point of support and the cen- tre of gravity are in the same vertical line. When the point of sup- port is above the centre of gravity, the equilibrium is stable ; if at the centre of gravity, the equilibrium is neutral ; if below the centre of gravity, the equilibrium is unstable. In bodies resting on a level surface, the equilibrium is most stable, if the centre of gravity is in the lowest position possible ; if the centre of gravity is not as low as it can be, the equilibrium may be stable or unstable; if the relative positions of the centre of gravity and the points of support are not altered by any movement of the body, the equilibrium is neutral. The broader the base of a body, and the lower the centre of gravity, the more stable the equilibrium. The velocity of a falling body is independent of its mass, and is not affected by the nature or shape of the body. The velocity acquired by a falling body at the end of any given time, is proportional to the time. The distances fallen through in successive times increase as the odd numbers 1, 3, 5, 7, etc. The total spaces fallen through are proportional to the squares of the times. In the same pendulum, if the amplitude of the oscillation is not 80 NATURAL PHILOSOPHY. very great, the time of oscillation for different amplitudes is nearly the same. In pendulums of different lengths, the duration of an oscil- lation is profiortional to the square root of the length. The intensity of gravity at the surface of the earth is greater at the poles than at the equator, because at the poles a body is nearer the earth’s centre than when at the equator. The intensity of gravity can lie determined by the use of the pen- dulum. The same pendulum will oscillate more rapidly at the poles than at the equator, because the force of gravity is greater at the poles than at the equator. J-CJ',^00 Questions for Review. By what is the weight of a body caused? State the law of uni- versal gravitation. What do we mean when we say that one thing is inversely proportional to another ? If the mass of a body be doubled, how much will its attractive force be increased ? If the distance between two bodies be doubled, how much will their attractive force be decreased ? In what direction does the force of gravity act ? How may this direction be ascertained ? Define centre of gravity. Why may the centre of gravity be re- garded as the point of application of the force of gravity ? How may the centre of gravity he ascertained experimentally ? Wliy should a body supported at its centre of gravity be in equi- librium ? What three kinds of equilibrium are there? "UTien a body is sup- ported at a point around which it is free to move, what must be the relative position of this point and the centre of gravity for the body to be in equilibrium ? What kind of equilibrium will exist if the point of support be above the centre of gravity? At the centre of gravity? Below the centre of gravity? When will a body resting on a flat horizontal surface be in stable equilibrium ? When will it be in unstable equilibrium ? When wOl it be in neutral equilibrium? Upon what does the degree of stability of a body resting on a flat horizontal surface depend ? State the laws for falling bodies. Wliy should the velocity of a falling body be independent of its mass? How can we prove that bodies of different kinds and shapes fall with the same velocity ? QUESTIONS FOR REVIEW. 81 To what is the velocity acquired hy a falling hody at the end of any given time proportional ? Through how much greater distance will a hody fall during the second second of its descent than during the first second ? To what are the total spaces fallen through proportional ? Define oscillation of a pendulum ; time or duration of an oscillation ; and amplitude of an oscillation. In the same pendulum, when the amplitude of the oscillation is not very great, does the pendulum take any longer time to move through a long arc than it does to move through a short one ? How does the length of the pendulum afiect the duration of its oscil- lation ? How can we determine, hy means of the pendulum, the variations of the intensity of gravity at different parts of the earth ? . F CHAPTER VII. COHESION AND ADHESION, AND PROPER- TIES PECULIAR TO SOLIDS. 84. Force of Molecular Attraction. — Tlie force of molecular attraction may hold together molecules of the same or of different kinds of matter. Cohesion is the name given to the force of molecu- lar attraction when it holds together molecules of the same kind of matter. Adhesion is the name given to it when it holds together the molecules of different kinds of matter. 85. Cohesion. — The force of cohesive attraction binds together the molecules of the same kind of matter. This force varies greatly in different sub- stances. In some, such as iron and steel, the cohesion is very great ; in others, such as soft butter or puttv, the cohesion is feeble. It is the cohesion of a solid that causes it to retain its shape. The force of cohesive attraction appears to act only at very small distances. If we once overcome the cohesion between the particles of a piece of iron or china, we cannot, by merely pressing the broken edges together, cause the force of cohesion to again bind them. It would appear as if we could not bring the molecules sufficiently near one another. Two fresh 82 K I COHESION AND ADHESION. 83 surfaces of lead, however, may be caused to cohere with considerable force, by being merely pressed to- gether. Two clean plates of polished glass, such as are used for mirrors, will often cohere so strongly, when laid one on the other, as to make it impossible to separate them without fracture. Experiment. — Cast a cylinder of lead about one inch in length and a quarter of an inch in diameter. This can be done by boring a hole of the proper size in a piece of hard, dry wood, and pouring melted lead into the hole. Cut the cylinder in half by resting a sharp knife against its side and striking the knife a few blows with a hammer. Attach strings or wires to the ends of the pieces, as shown in the figure. If now the freshly cut surfaces be firmly pressed to- gether, they will cohere with sufficient force to sustain a heavy weight placed below, on a pan made by tying strings to the four corners of a square piece of stout pasteboard. Caution. — Be sure that the mould is quite dry before pouring in the lead. Do not touch the cut surfaces of the lead, as the hands, even when clean, will tarnish or grease the lead, and the experiment fail. The surfaces Fig. 37.— The Cohesion should be smooth. If the cylinder is not cut Lead, smooth by the knife, it may be filed smooth, and then made bright and clean by a knife. Each time the cylinders are used, it will be necessary to reclean the surfaces. 86. Cohesion of Liquids. — ^Tbe molecules of liq- uids move over one another so easily that we might suppose that they possessed no cohesion. They do, however, exert a cohesive attraction for one another ; but this, of course, is much less than in solids. Had liquids no cohesion, the drops of dew on leaves would be pulled by gravity into thin flat layers, in- stead of having their almost spherical shape. The force of cohesion varies in different liquids. 84 NATURA L PHIL OSO PH F. 87. Adhesion, — ^When the force of molecular at- traction holds together the molecules of different kinds of matter, we name the force adhesive attrac- tion, to distinguish it from cohesive attraction. Thus, the hand when dipped in water is wet; here we say the water adheres to the hand, because the attraction is exerted between different kinds of molecules, namelv, between those of the hand and those of the water. Chalk-marks adhere to a blackboard, but the molecules of chalk cohere to one another. 88. Chemical Attraction or Affinity. — The force of molecular attraction is to be carefully distinguished from that of atomic attraction or chemical affinity. The former, as we have seen, may attract and bind together the molecules of the same or of different kinds of matter. The latter may attract and bind together the atoms of the same or of different kinds of matter. It is the chemical attraction between the atoms that pro- duces the molecules of elementary and of compound substances. By the operation of molecular attractions, physical changes are produced in matter; by the operation of the atomic attractions, chemical changes are produced. Thus, a piece of iron exposed to moist air becomes cov- ered with rust. This rust is a compound body, and is formed by the atoms of the iron exerting an attraction for the atoms of oxygen in the air. The body formed by the combination, or union, of these different atoms, though containing both iron and oxygen, yet possesses the properties of neither ; its constituents have under- gone a chemical change. The above distinction between molecnlar and atomic attractions is not in all cases so clear. In the solution of certain solids by liquids, there appears to be a combination of the molecules of the solid and COEESION AND ADHESION. 85 liquid, which can scarcely he regarded as either purely physical or chemical. A similar combination occurs in salts which possess water of crystallization. These combinations might be regarded as partly chemical combinations, but it will be more convenient to consider all attractions not evidently chemical, as varieties of adhesion. 89. Varieties of Adhesion. — The force of adhesive attraction manifests itself in a variety of ways. We will consider the adhesion which occurs, 1st. Between solids. 2d. Between liquids. 3d. Between solids and liquids. 4th. Between solids and gases. 5th. Between liquids and gases. 90. Adhesion between Solids. — The resistance which friction offers to motion is caused not only hy the irregularities of the surfaces, but also by the adhe- sion which they exert for one another. When these surfaces are both of the same kind, the friction is in- creased by their cohesion. Cements afford examples of adhesion between dif- ferent kinds of solids. Thus, mortar placed between stones or bricks adheres to them, and so binds them together. Glue adheres to the pieces of wood or cloth between which it is placed. Paste or gum causes paper to adhere to walls. Dried paint adheres to wood-work, and ink-marks to paper. 91. Adhesion between Liquids. — i. Mixture. Oil and water will not mix, because there is but little ad- hesion between their molecules. The same is true of mercury and water. Milk and water, however, mix readily, because the molecules of the one adhere to those of the other ; the same is true of many other liquids. 8 86 NATURAL PHILOSOPIIT. 2. Solution. The solution of one liquid by another may also be regarded as a species of adhesion. Thus, castor-oil is dissolved by alcohol. Many cases of solution, however, are caused by the influence of a partly chemical attraction. Thus, pure concentrated alcohol and water combine ; but a weak solution of alcohol will mix with water in any proportion. 3. Diffusion. If a vessel filled witli liquid fie care- fully lowered fieneatfi tfie surface of some ligfiter liquid with wfiicfi it is capafile of mixing, tliough no agita- tion lias occurred, yet after a time, which differs for dif- ferent liquids, the two will be found to be thoroughly mingled, as much of the heavier liquid being now found in the upper portions of the vessel as in the lower. Phenomena of this kind are known by the general name of diffusion, and are caused by the at- tractions which exist between the unlike molecules. 92. Adhesion between Solids and Liquids. — i. Wetting. If we plunge the hand into water, it be- comes wet, because the water adheres to it ; but if plunged into mercury, it is not wet, because there is but little adhesion between the hand and the mercury. Water-proof stuffs contain substances that are not readily wet by water. When rain falls on such fabrics, it either runs off of itself, or can be easily shaken off". 2 . Solution. The solution of a solid hry a liquid is another example of adhesion between a solid and a liquid. When a lump of sugar is thrown into a glass of water, the sugar gradually disappears and becomes mixed throughout the liquid, which has now a sweet- ish taste. We call this change solution; by solution a solid becomes changed into a liquid. The solvent powers of water are much greater than those of any other common liquid. As a rule, the solvent power of any substance COHESION AND ADHESION 87 increases with the temperature ; thus, hot water will dissolve more sugar than cold water. This is not. however, always the case ; cold water will dissolve more lime than hot water. Some cases of solution are caused by the influence of a species of chemical attraction, such, for example, as the solution of concentrated lye by water. Fig. 38 The Liquid Wets the Tube, 93. Capillarity. — If a tube, J., of large diameter be dipped into a liquid wbicb wets it, tlie liquid will stand as high on the outside of the tube as on the inside; but if the tube be of small diameter, as at the liquid will rise so that it will be higher inside the tube than outside it, as is shown in Fig. 38. If the tube is still smaller, as at O', the liquid will rise higher than before, aud the difference between the inside and outside levels will be greater. When the liquid does not wet the walls of the tube, if the tube be of large diam- eter, as shown at Fig. 39, the level inside the tube is the same as that outside it; but if the diameter be small, as at F", the liquid will be depressed within the tube, so that the level of the liq- uid inside the tube will be below that on the outside. In a tube of still smaller diameter, as at (7, the difference of level is greater. These phenomena are known by the name of ca^jillar- ity. A capillarij tube is one whose diameter is small, or hair-like. Fig. 39,- -The Liquid does not Wet the Tube. 88 NATURAL PHILOSOPHY. By capillarity is meant the elevation or the depression of liquids in tubes of small diameter. The principal phenomena of capillarity may be briefly stated as follows, viz. ; 1st. When a capillary tube is dipped into a liquid, the liquid will rise in the tube if it wets the tube, but will be depressed, if it does not wet the tube. 2d. The amount of the elevation or depression in- creases as the diameter of the tube becomes smaller. 3d. The amount of elevation or depression varies with the kind of liquid and the material of the tube. 94. The Cause of Capillary Phenomena. — Capil- lary phenomena are caused by the difference between the cohesion of the liquid molecules for one another and their adhesion to the walls of the capillary tube. A liquid rises in a capillary tube ■which it wets, because the adhesion between the liquid and the walls of the tube draws the liquid towards the walls of the tube. A liquid is depressed in a capillary tube which it does not wet, because the cohesion of the liquid draws the liquid away from the walls of the tube. If the liquid wets the tube, then the adhesion between it and the tube, or the force which pulls the liq- uid towards the walls, is greater than the force of cohesion which tends to keep the particles of the liquid together. If the liquid does not wet the tube, then the force of cohesion is greater than that of adhesion. 95. Familiar Examples of Capillarity. — The phe- nomena of capillarity occur in loose, porous substanees whenever the spaces, or sensible pores, between the different parts of the substance are of capillary dimen- sions. Thus, the oil rises in the wick of a lamp through the capillarity of the spaees between the strands. A lump of sugar placed Avith only its lower end in milk or Avater is soon Avet throughout ; here the elevation COHESION AND ADHESION. 89 of the liquid is clue to capillarity. A towel hung so that only its lower part clips into water is soon wet for a considerable distance above the level of the water, from the same cause. 96. Osmose is the unequal mixing of two different liquids through the pores of a membranous substance which separates them. If two liquids that are capable of mixing, be placed in compart- ments of the same vessel, and separated from each other by only a thin wall of bladder or other membrane, through the pores of which the liquids can slowly pass, they will not remain separate, but will mix with each other. This mixing, however, is different from that produced by mere diffusion, since more liquid passes in one direction than in the other. Suppose, for example, sugar and water were placed in one compartment, and pure water in the other ; then it would be found that more pure water would pass into the compartment contain- ing the sugar and water, than the sugar and water would pass into the other compartment, so that, after standing for several hours, the level of the liquid would he higher in the compartment containing the sugar and water than in the other. The cause of osmose is not thoroughly understood. It is probable, however, that these phenomena are due mainly to the adhesion which exists both between the molecules of the two liquids and between them and the walls of the membrane. 97. Adhesion between Solids and Gases. — Tbe adbesion existing between solids and gases is seen in tbe absorption of gases by solids. Charcoal, for exam- ple, has a wonderful power of absorbing various gases and condensing them within its pores. Freshly -burned charcoal can absorb nearly a hundred times its bulk of some gases. When thrown on decaying animal or vegetable substances, it removes most of their bad odors by absorbing the disagreeable gases as fast as they are given off. The smell of tobacco clings for some time to the clothes of one who has been smok- 8 * 90 NATURAL PHILOSOPHY. f ing. This is due to the adhesion between the smoke and the clothes. 98. Adhesion between Liquids and Gases. — Most liquids have the power of absorbing various gases. W ater possesses this property in a remarkable degree. All water which has been exposed for some time to the air will be found to contain a considerable amount of air in solution. If a tumbler of clear Avater stand for some time, minute bubbles of gas Avill be seen on the inside of the glass. These bubbles come from the air which was contained in solution in the Avater. 99. Properties Peculiar to Solids. — The solid con- dition of matter is characterized by certain properties peculiar to it. The most important properties pecu- liar to solids, are malleability., ductility, hardness, brit- tleness, tenacity, solid elasticity, and crystalline form. 100. Malleability is the property certain solids possess of being wrought into different shapes under the hammer or roller. Most of the metals are malle- able to a considerable extent, and on this property much of their value depends. Gold is one of the most malleable metals knoA\m. It can be beaten into leaves so thin, that it takes about 300,000 such leaA’cs to make a pile one inch in thickness. The malleabil- ity of metals under the hammer is someAvhat different than under the roller. Gold, lead, silver, tin, and cop- per are very malleable. 101. Ductility is the property certain solids pos- sess of being draAA'n out into Avire. This property, Avhich is possessed most generally by the metals, is nearly the smne as malleability, since in both, the molecules of the body, AA’hen subjected to pressure or strain, are caused to floAV or moAm OA'er one another COHESION AND ADHESION. 91 without fracture. The different metals, however, are not malleable and ductile to the same extent. Platinum, silver, iron, and copper are very ductile. 102. Hardness is that property in virtue of which certain substances resist being scratched or worn by others. We can tell which of two substances is the harder, by rubbing them together ; that Avhich is the harder will scratch the other. The terms hard and soft are relative, since a body which is hard, when compared with one substance, may be soft when com- pared with another ; thus, glass will scratch marble, therefore glass is hard when compared with marble, but the diamond will scratch glass, so that glass is soft when compared with the diamond. Hardening, Annealing, Tempering. — Certain metals possess the re- markable property of having their hardness changed by heat. If they are heated to about redness, and then suddenly cooled by being plunged into cold water, they become hard and brittle. Steel pos- sesses this property in a remarkable degree. The process is called hardening. When steel is highly heated and allowed to cool slowly, it becomes soft, ductile, and malleable. This process is sometimes called softening or annealing. If a bar of hardened steel is struck a sharp blow with a hammer, it is apt to be fractured ; but if softened, it can be wrought into any desired shape, such, for example, as a knife-blade, an axe, or a hatchet, and these articles can afterwards be hardened by high heating and subsequent rapid cooling. As a rule, the hardness so obtained is so great that the articles would be too brittle for use; in this case, a portion of the hardness may be removed by heating to a lower tem- perature, and then allowing them to cool. This process is called draw- ing the temper. 103. Brittleness. — A substaace is brittle when it is readily broken into pieces. This property is almost the opposite to that of malleability. Brittle substances are generally hard. 92 NATURAL PHILOSOPHY. 104. Tenacity. — By the tenacity of a substance is meant its power to resist being pulled apart by a force acting in the direction of its length. The tenacity of a body is due to the cohesion of its molecules, and, as cohesive attraction varies greatly in different sub- stances, the tenacity must always vary. As the force tending to pull the molecules apart is applied in the direction of the length of the body, the tenacity must increase with the area of transverse sec- tion, that is, a section made at right angles to the length ; for, if in the two bars, A and 5, Fig. 40, of the same material, A has a smaller area of transverse section, abed, than efgh of B, then the total tenacity of A must be less than that of B, since there are fewer mole- cules in any cross section, as abed, tending to hold the bar together than there are in any cross d h Fig. 40. — Influence of Sectional Area on Tenacity. section efgh. If, then, the area efgh be twice as great as abed, the bar, B, would require twice as great a force to pull it apart as the bar A, and so also with any other proportion. The tenacity of a bar or beam is independent of its length. The increase in the length does not affect the number of molecules in any area of cross section, and, of course, the bar would break at its weakest part. Since, however, the weight of a beam, supported at one end, tends to break the beam, we can see that the brealcing weight is less with an increased length. In the following table is given the tenacity of a few important substances in pounds per square inch of area of cross section. I It COHESION AND ADHESION. 93 Steel . 120,000 to 160,000 Iron . . 50,000 to 120,000 Silver .... 41,000 Copper .... 37,000 Gold Zinc 65,000 2,800 Oak wood . . . 26,000 Pine wood . . . 15,000 105. Limits of the Size of Structures. — The size of any structure cannot exceed certain limits, which vary with the strength of material employed and with its weight per unit of volume : for, since all ma- terials have to sustain their own weight, if this should exceed their tenacity, the structure would fall. We have seen that the tenacity of a body increases as the area of its cross section. If, then, the dimensions of a beam be doubled, that is, if it be made twice as long, twice as deep, and twice as broad, the area of its cross section will be four times as great, and it will be able to sustain four times as great a weight acting in the direction of its length. But at the same time, by doubling its dimensions, we have increased its volume eight times ; hence its weight has also been increased eight times. If we increase the dimensions of the beam three times, its tenacity is 3 X 3 = 9 times greater, and its weight, 3X3 X3 = 27 times greater ; if increased four times, its tenacity is 4 X 4 = 16 times, and its weight 4 X 4 X 4 = 64 times. It is apparent, then, that any bar would soon become large enough to fall apart by its own weight. 106. Elasticity. — By the elasticity of a body we mean the property it possesses of regaining its orig- inal shape after being compressed, stretched, bent, or twisted. All kinds of matter, whether solid, liquid, or gas- eous, will, when compressed, tend to some extent to regain their original bulk, or, in other words, possess the property of elasticity. Elasticity developed in a hody hy compression is therefore a general property of matter. Solids are the only form of matter which can be stretched, bent, or twisted, and by any of these changes elasticity is developed. Elasticity developed in a hody hy stretching, bending, or twisting, is therefore a property peculiar to the solid condition of matter. 94 NATURAL PHILOSOPHY. Elasticity developed hy stretching.! or tension, is seen in the return to its former length of a piece of caout- chouc, or India-rubber, or a wire after it has been stretched by a weight. Elasticity developed hy bend- ing, or flexure, is seen in the recoil of a bow which has been bent and then released. Elasticity developed hy twisting, or torsion, is seen in the untwisting of a string or wire which has been twisted. 107. The Measure of Elasticity. — The degree of elasticity of a body is measured by the force with wdiich it tends to regain its original shape, when that shape has been changed by compression, stretching, bending, or twisting. A body which in resuming its original form gives out a force equal to that by which its form has been changed, is said to be perfectly elastic. Liquids and gases, as a rule, when compressed, resume their original volume on the removal of the pressure, and are therefore elastic. Within certain limits of compression, etc., nearly every solid body will resume its original shape on the relief of compression, bend- ing, stretching, or twisting, and is therefore elastic ; but with many solids, the limits of elasticity, or the limit beyond which aiiy further compression, bending, stretching, or twisting would produce a permanent change of form, are so small that they may be consid- ered as inelastic. 108. Crystalline Form. — We recognize an animal or a plant by a certain form peculiar to it. In the same way many lifeless substances occur in forms, called crystals, peculiar to them. Although in most solids these crystals are too small to be seen, yet they nearly always exist. For example, ice is composed of crystals of beautiful shapes. COHESION AND ADHESION. 95 Crystals are more or less regular in shape, and although of many different forms, yet are all modifica- tions of a few simple forms. Experiment. — Place a quarter of a pound of common alum in as much hot water as will completely dissolve it. Strain the solution through a piece of muslin, and pour the clear liquid into a cup or howl, iu which has been placed a piece of rough stone wrapped with colored yarn. Set the liquid aside in a quiet place over night, and in the morning beautiful shining crystals will be found covering the stone. By slipping a thin knife under the stone it may be separated from the bottom of the cup or bowl. Caution . — Do not move or shake the cup after once putting it away. If it is wished to obtain large crystals, use more water to dissolve the alum, and let it stand longer to crystallize. The force of crystallization which arranges the molecules of solids in the form of regular crystals, is only a variety of the force of cohesive attraction. Syllabus. The force of molecular attraction when acting on the molecules of the same kind of matter, is called cohesive attraction, and when acting on the molecules of different kinds of matter, adhesive attraction. The force of cohesive attraction is manifested principally by solids ; it is not absent, however, in liquids. The force of molecular attraction binds together molecules of either the same or of different kinds of matter. The force of atomic attrac- tion, or chemical affinity, binds together the atoms of either the same or of different kinds of matter. The force of adhesion may exist between all three forms of matter, viz., solids, liquids, and gases. The friction of two surfaces is caused partly by their adhesion. Such substances as glue, paste, paint, and ink owe their efficiency to their power of adhering to the surfaces on which they are put. Water and vinegar, or water and milk, mix when put together, be- cause they adhere to one another ; oil and water, or water and mer- cury, do not mix, because they do not adhere. Diffusion is that property in virtue of which two different gases, or two different liquids, will mix with each other, even when the lighter liquid or the lighter gas is placed above the other. 96 NATURAL PIIILOSOPnY. It is by the action of diffusion that the different gases which com- pose our atmosphere are kept from settling in layers, according to their difference of density. A solid is dissolved by a liquid, when the adhesion between the two is sufficient to overcome the cohesion between the molecules of the solid. By capillarity we mean the elevation or the depression of liquids in tubes of small internal diameter. When the liquid wets the tube, it is elevated in it ; when it does not wet it, the liquid is depressed. When the liquid wets the tube, the adhesion between the lube and the liquid is greater than the cohesion between the molecules of the liquid, and the liquid is therefore drawn up the tube. When, how- ever, the cohesion of the liquid is greater than its adhesion to the tube, the liquid is drawn down the tube. Osmose is the unequal mixing of two liquids through the pores of a membrane or wall separating them. Many solids possess the power of absorbing gases and condensing them ; this power is due to the adhesion between the solid and the gas. Most liquids possess the same property. The most important of the properties peculiar to solids are mallea- bility, ductility, hardness, brittleness, tenacity, solid elasticity, and crystalline form. Many solids, when subjected to great pressure, exhibit to some degree the phenomena of flow ; they are malleable or ductile, that is, can be beaten out in thin sheets, and pulled out into thin wires without frac- ture. These properties are called malleability and ductility. Solids differ in their hardness, or their ability to resist being worn or scratched. Certain metals when highly heated and then suddenly cooled become hard and brittle, but w'hen heated and then slowly cooled become soft and ductile. Brittle substances are easily broken into small pieces when struck. The tenacity of a bar or rod is its power to resist being pulled apart by a force applied in the direction of its length. The tenacity is greater in some substances than in others. In the same substance the tenacity is proportional to the area of cross section. By the elasticity of a body we mean its tendency to resume its orig- inal bulk on the removal of any force which has acted to change its shape. Elasticity may be developed in all bodies by compression, and in solids by stretching, bending, or twisting. Most solids have a peculiar shape, or crystalline form, which is char- acteristic of them. This shape is caused by the force of molecular attraction acting in certain directions on the molecules. QUESTIONS FOR REVIEW. 97 Questions for Review. Define cohesion, adhesion. What is the difiference between them? Describe an experiment by which the cohesion of two pieces of lead may be shown. What facts can you mention to prove that liquids possess cohesion ? What is the difference between molecular attraction and chemical affinity ? Name five varieties of adhesion. Explain some of the phe- nomena produced by the adhesion of solids. What is meant by diffusion? What two kinds of diffusion are there ? Of what use is diffusion in our atmosphere ? By what is the solution of a body caused? What do you understand by capillarity? Why should a liquid be elevated in a capillary tube when the liquid wets the tube ? Why should it be depressed when the liquid does not wet the tube ? Name some phenomena produced by capillarity. Wliat is osmose ? When does it occur ? Define absorption. To what do solids owe their power of absorption ? Do liquids possess the power of absorbing gases ? Define malleability, ductility. How do these properties prove that solids possess to some extent the power of flowing ? What do you understand by the hardness of a substance ? Prove that hard and soft are relative terms. Describe the process of harden- ing ; of annealing ; of drawing the temper Define brittleness. What do you understand by the tenacity of a substance ? What effect is produced on the tenacity of a beam by increasing the area of its cross section ? By increasing its length ? What natural limit exists to the size of structures ? Explain your answer in full. What is meant by elasticity? How may it be developed in all kinds of matter ? How may it be developed in solids ? How is the elasticity of a body measured ? What is meant by the limits of elasticity ? Define crystalline form. Describe an experiment by which crystals of alum can be formed. 9 G Part IL Fluids. CHAPTER I. HYDROSTATICS. 109. Hydrodynamics and its Divisions. — Hydro- dynamics is that branch of Natural Philosophj" which treats of the conditions of rest and motion in fluid bodies. It includes Hydrostatics, Avhich treats of liq- uids at rest ; Hydraulics, which treats of liquids in motion, and Pneumatics, which treats of gases either at rest or in motion. 110. Compressibility of Liquids. — Liquids are but slightly compressible. Thus, Avater, when subjected to a pressure of fifteen pounds to the square inch, is com- pressed but the of its volume. Many other liq- uids are even less compressible. We may therefore regard liquids as practically incompressible. Liquids, hoAveA’er, are, as a rule, more compressible than solids. When solids are compressed, they are not generally confined at the sides, and the particles spread. They appear, therefore, to he more compressible than liquids. If -we should confine solids in strong A'es- sels, so as to prevent them from spreading laterally, as we must do with liquids, Ave Avould find solids to he less compressible than liquids. 98 HYDROSTATICS. 99 111 . Transmission of Pressure. — Law. Liquids transmit in all directions.^ and vdtliout sensible loss of intensity.! the pressure exerted on any part of their 7nass. Liquids transmit pressure equally in all directions as a necessary result of tlie very great freedom with Avhich their molecules move in any direction over one another. The pressure due to the weight of a solid is exerted only in one direction, viz., vertically downwards. A solid, therefore, may be in equilibrium when supported only at its base. The pressure due to the weight of a liquid is exerted in all directions. A liquid, therefore, to be in equilibrium, must be supported at the sides as well as at the base. 112. Liquid Pressure as a Mechanical Power. — Since pressure is transmitted through liquids as well in one direction as in another, and as nothing is lost during the transmission, it follows that the total press- ure sustamed by any surface is proportional to its area. This fact furnishes us with an additional mechanical power. If, for example, two vessels, A and B, Fig. 41, filled with water, he furnished with pistons, C and 1), that is, with parts arranged so as to move ^ freely up or down the vessels without allowing the water to ^ pass them, and these vessels he connected by a tube, L Tig. 41, - Liquid Pressure as a . y , . Meohauical Power, we have a simple machine by which we can modify the effects of a force to any desired extent. Suppose the areas of the two pistons, C and ii, he respectively 1 and 100 square inches. If, then, the piston, C', he pushed down with a force of 1 lb., the piston, D, will be raised with a force of 100 100 NATURAL PHILOSOPHY. lbs. ; for, since a pressure of 1 lb. is exerted at C on the area of 1 sq. in., it must exert a pressure of 1 lb. on every square incb of surface in the two ve.ssels. But the piston, /), is the only other part of the vessel that can move, and, as it contains 100 square inches, it must be pushed upwards with a force of 100 lbs. As with any other machine in which the intensity of the power is increased, the distance throngli which the power moves is as much greater than that through which the weight moves as the power is less than the weight ; if, for example, the piston. C. he moved by the power through 1 foot, the piston, D, will only raise the weight through the of a foot. 113. The Hydrostatic Press. — The macbine just described forms wbat is known as tbe hydrostatic press. A lever, P, Fig. 42, is attached to the piston. A, and by its movement oil or water is pumped iuto the larger vessel in which the pis- ton, .A, moves. The jires- sure thus exerted causes the piston, A, to rise. Sub- stances to be compressed, such, for example, as hay or cotton, are placed be- tween a platform, (?, at- tached to the piston, B, and a strong frame, Z), attached to the press. This press is used to compress various substances, to extract oil from seed, to raise heavy weights, and for various other purposes. Fig. 42.— The Hydrostatic Press. 114. Pressure Caused by the Weight of a Liquid. — Each molecule of liquid has to bear the weight of all the molecules directly above it. The greater the depth of the liquid, the greater the number of these HYDROSTA TICS. 101 molecules. The pressure.^ therefore., exerted hy any liq- uid increases with the depth of the liquid. But diftereut liquids vary iu tlieir densities, that is, a cubic inch of some liquids weighs more than a cubic inch of others. The pressure, therefore, exerted hy any liquid increases loith its density. A vessel filled with mercury or molasses, would have a greater pressure on its bottom and sides than if it were filled wifh water, because mercury or molasses is denser than water. As liquids exert pressure equally well in all direc- tions, the downward, upward, and lateral pressure at any point must be equal to one another. 115. The Downward Pressure, or the Pressure on the Base. — If a vessel with vertical Avails be filled with liquid, the pressure oai its base Avill be equal to the weight of the liquid in the vessel ; but if the walls be inclined, the pressure on the base may be either greater or less than the weight of the Avater in the vessel. In the vessel. A, a b a' v a" b" Fig. 43, whose walls are vertical, the pres- sure on the base is equal to the Aveight of Avater in the vessel, c d c' d' c" d" In B, the pressure on ^3.— The Pressure on the Base. the base is greater than the weight of the Avater in B, and in C the pressure on the base is less than the Aveight of the water in C. It might be supposed that the pressure on the base of any vessel Avould ahvays be equal to the Aveight of the liquid in the vessel ; but in the vessel, B, the par- ticles at the surface not only transmit their pressure 9 * 102 NATURAL PHILOSOPHY. to that part of the base directly under them, but also to all parts of the base c' cZ'; the total pressure on the base is, therefore, the same as if there were the same depth of liquid over all parts of the base as there is over the middle parts ; that is, the total pressure is equal to the weight of the column of water, a' h' c' d' . In the vessel, C, only the particles of the liquid imme- diately above the base exert a pressure on the base ; the inclined walls receive the pressure of the remain- ing particles. If the three vessels, A, B, and have equal bases, and are fdled with water to the same height, the press- ures on the bases of all will be the same. 116. Rule for Calculating the Pressure on the Base. — The pressure on the base of a vessel contain- ing liquid, is equal to the weight of a column of liquid whose base is that of the vessel, and whose height is equal to the vertical distance from the middle of the base to the surface of the liquid. The weight of a cubic foot of water is about equal to 62.3 lbs. avoirdupois ; the weight of a cubic inch is .036 lbs. If, therefore, a vessel, whose base has an area of 2 sq. feet, be filled with water to the depth of 3 feet, the base will sustain a pressure of 2 x 3 x 62.3 lbs. = 373.8 lbs. 117. Pressure on the Side of a Vessel. — The press- ure on any side of a vessel containing a liquid is equal to the weight of the volume of liquid obtained by mul- tiplying the area of that side by the vertical distance from the middle of the side to the surface of the liquid. If the rectangular side of a vessel is two feet long, and the vessel be filled two feet deep with water, then the HYDROSTATICS. 103 area of the side covered by water — 2 x 2 = 4 sq. feet ; the distance from the centre of gravity, which will be at the middle of the side, is one foot. Then the pressure on the side = 4 sq. ft. x 1 ft. = 4 cubic feet. But the weight of 4 cubic feet of water = 4 x 62.3 = 249.2 lbs., which equals the pressure on the side. 118. Upward Pressure. — The upward pressure at any part of a liquid mass is equal to the downward pressure at that part. Experiment. — Take a glass cliimney, B, Fig. 44, with straight sides, and cut a disc, c d, of mica, such as is used for tlie doors of stoves, large enough to cover the base of the chimney. Attach a string to the middle of the disc. Now, placing the disc so as to cover the smoother end of the chimney, and holding the disc in its place by the string, push the chimney vertically downwards in a vessel, A. filled with water. When some little depth has heeu reached, the string need no longer be held, as the upward pressure caused by the water in A endeavoring to flow into B will hold the disc in its place. To find the amount of this upward pressure, pour water carefully into the chimney until the mica disc falls off. This will he found to take place when the level of the water inside the chimney is the same as Fig- 44,— Upward that on the outside. At this moment, the downward pressure caused by the water in B trying to flow out, is equal to the upward pressure caused by that in A trying to flow in. Since the sides of the chimney are vertical, the downward pressure is equal to the weight of the water in B ; hence the upward pressure on c rf is equal to the weight of a column of water whose base is c d, and whose height is the depth of the liquid above cd. Caution. — The end of the chimney must be quite even and flat. If mica cannot be obtained, a thin, smooth sheet of metal may be used. If much difficulty be experienced by water leaking in, the end of the chimney may be slightly greased with tallow. 119. Surface of Liquids at Rest.— When, a liquid is at rest, its upper surface is everywhere level or hori- zontal ; for, were it higher at one point than at another, 104 NATURAL PHILOSOPHY. the greater pressure so produced would press it up at the lower points until the whole surface became level. It is only the surface of a comparatively small ex- tent of water that is horizontal. Large masses like the ocean are curved. Lor any mass of water to he in equilibrium, all parts of its surface must be at equal distances from the earth’s centre. 120. Equilibrium of Liquids in Communicating Vessels. — A liquid in communicating vessels is in equilibrium when it stands at the same height or level in all the vessels; for, were it higher in one vessel than in another, the greater pressure so produced would cause it to mount in the other vessels until it stood at the same height. The water rises out of the water- pipes in the streets and fills the pipes in the houses, because the level of the water in the basins, or reser- voirs in which it is stored, is equal to or greater than the height of the houses. In artesian wells, the water is forced up from great depths by the pressure of water contained between two curved impervious strata. EYDROSTA TICS. 105 When two liquids of different densities are placed in communicating vessels, they Avill be in equilibrium Avhen the level of tlie denser liquid is lower than the level of the other liquid. The heights of the liquid columns will be inversely as their densities. 121. Bodies Immersed in Liquids. Buoyancy. — Bodies immersed in a liquid weiyli less than they do in air. They lose as much of their weight as the weight of the liquid they displace. This important principle was discovered by Archimedes. Suppose, for example, the cube, ahcd, Fig. 46, be com- pletely immersed in water ; then the pressures on the opposite sides being equal, neutralize each other. The downward pressure is equal to the weight of a column whose base is the top of the cube, and whose height is e of ; the upward pressure is equal to the weight of a liquid column whose base is the bottom of the cube, and whose height is e c. The upward pressure, therefore, exceeds • Fig. 46 — Cause of the downward pressure by a weight Buoyancy, of water equal to the volume of the cube. This excess of upward pressure being exerted in a direction opposite to that of the force of gravity, causes the body to lose as much of its Aveight as the weight of the water displaced ; this effect is called buoyancy. 122. Experimental Proof of the Principle of Archimedes. — The correctness of the principle an- nounced by Archimedes may be demonstrated by means of the apparatus shown in Fig. 47. A closed cylinder, A, of such a size as to exactly fill the hollow cylinder. 106 NATURAL PHILOSOPHY. A, is suspended below it, and tbe two attached to the I pan of a balance, and exactly counterpoised in air by weights at C. This being done, the ' cylinder, B, is completely inimersed in water, as shown in the figure. The buoyancy of the water on B causes it to lose weight, as is shown b\' the other pan of the balance falling. If now the cylinder, J., be exactly filled with water, equilibrium will be restored. The weight lost by Fig. 47. — Principle of i?, therefore, is equal to the weight Archimedes. ^ Volume of Water which will just fill A\ but this volume is tbe same as that dis- placed by i?, therefore the weight lost by B is equal to the weight of the water it displaces. 123. Floating Bodies,^ — A body placed in a liquid, f will float if it displaces a bulk of liquid equal to its : weight, because then the buoyancy of the liquid holds the bod}'- up with the same force that gravity pulls it down. Buoyancy acts at a point called the centre of buoy- ancy., Avhich is situated at the centre of gravity of the displaced liquid. 124:. Equilibrium of Floating Bodies. — A floating body will be in equilibrium when the centres of gravity and buoyancy, G and 0, are in the same ver- tical line. Thus,inFig. 48, the boat at A is in equilibrium, since the Fig. 48.-Eqnilibrinm of Floating Bodies. buoyancy are equal and directly opposed to each other ; | i HYDROSTATICS. 107 Fig. 49. — Stable Equilib- rium. but if tbe boat is moved into the position represented at B, it is no longer in equilibrium, since gravity and buoyancy are no longer directly opposed to each other, but tend to turn the boat around, until they are both in the same vertical line. A floating body will be in stable equilibrium when the centre of buoyancy is above the centre of gravity, or when the centre of gravity is as low as it can get, as, for example, in Fig. 49, where the centre of gravity is as low as it can be. A floating body is in unstable equilibrium when the centre of buoyancy is below the centre of gravity, as is seen in Fig. 50. A floating body will be in neutral equilibrium when the relative positions of the centres of gravity and buoyancy are not affected by any movement of the body. A sphere floating in a liquid, as shown in Fig. 51, would be in neutral equilibrium. In the above examples, it will be no- ticed that the centre of buoyancy is the point of sup- port of the floating body, and bears the same relation to the centre of gravity as does the point of support of bodies which can move freely on an axis, as will be seen by comparing Figs. 49, 50, and 51 with Figs. 28, 29, and 30. The equilibrium of a ship or boat is more stable as its centre of gravity is lower. When a ship is not heavily laden, ballast is put in the lower part of the vessel in order to lower the centre of gravity. Fig. 50. — Unstable Eqnilibrinm. Fig. 51.— Neutral Equilibrium. 108 NATURAL PIIILOSOniY. 125. Specific Gravity. — By the specific /jravity of a hody^ we mean the weight of that body as compared with the weight of an equal bulk of some other body taken as the standard of comparison. In determining the specific gravity of solids and liquids, we compare their weight with the weight of an equal bulk of water ; and in determining that of gases and vapors, with the weight of an equal bulk of air. Rule. — To determine the specific gravity of a solid or a liquid^ divide the weight of the body in air by the v:eight of an equal volume of water. The same rule may be expressed as follows, viz. : or Sp Gr ^ Weight of body in air. W' Weight of equal hulk of water. Fig. 52.— A general Formula for Specific Gravity. To determine the specific gravity of any gas, we di- vide the weight of the gas by the weight of an equal volume of air, or other gas, taken as a standard. In order, therefore, to determine the specific gravity of any body, it is only necessary to ascertain the weight of the body and the weight of an equal bulk of water or gas. Suppose, for example, we wisli to determine the specific gravity of a piece of iron. We first find the weight of the iron in air, Avhich Ave A\ill suppose to be 778 grains. We then find the AAmight of an equal bulk of Avater, which Avill be, say 100 grains. The specific gravity of the iron, therefore, is 7.78, or, in other words, the iron is 7.78 times heavier than Avatcr. HYDRO ST A TICS. 109 126. Methods of Obtaining Specific Gravity. — By the Balance. For Solids. Since a body im- mersed in water loses exactly as mucli weight as the weight of the water it displaces, it is easy to obtain the specific gravit}^ by this method. For, attach the body to a string tied to one of the pans of a balance ; exactly balance the body by adding weights to the other pan; these weights will give us the value of IF, that is, the weight of the solid in air. Then, while the solid is still attached to the balance, immerse it in water, and find how much weight it loses. This weight will give us IF', or the weight of a bulk of water equal to that of the solid. Then IF divided by IF' will give the specific gravity of the solid. Sup- pose, for example, a solid weighs in air 200 grains, and loses in water 150 grains, then its specific gravity = Hg = 1.33. 127. For Solids Lighter than Water. — If the solid is lighter than Avater, attach it to another solid, as, for example, a piece of copper, heavy enough to sink it in water. Find how much weight the two lose when im- mersed in water. Find how much weight the copper itself loses when immersed in water ; then the differ- ence between this weight and the weight that both lose will give the weight the light body loses in water. Divide the Aveight of the light body in air by the Aveight it loses in Avater, and the quotient Avill be the specific graAuty. Suppose the lighter solid weighs 6 grains, and that Avhen both are immersed in AA'ater they lose 10 grains, and that the heavier solid loses Avhen immersed 1 grain. Then the lighter solid must lose 9 grains, and its specific gravity must equal I or .66. 10 no NATURAL PHILOSOPHY. 128. For Liquids. — A closed bulb, A, Fig. 53, partly filled with mercury or other heavy substance, is attached to a string, and suspended from one of the pans of a balance. First find the weight the bulb loses when immersed in the liquid whose specific gravity is desired. This will be the weight of a quantity of liquid equal to the volume of the bulb, D ; then find the weight the bulb, A, loses when immersed in water ; this will be the weight of a volume of water equal to that of the bulb. Then divide the weight the bulb loses in the j) liquid, whose specific gravity is desired, b}" the Fig. 63. weight it loses in water, and the quotient will Specific- , ^ Gravity be the specific gravity. Thus, suppose the Bulb. J)^ loses 184 grains when immersed in sulphuric acid, and 100 grains when immersed in water ; then = 1.84, the specific gravity of the sulphuric acid. Ui 129. By the Specific-Gravity Bottle. — The spe- cific gravity of a liquid may be very conveniently found by means of a bottle. The bottle is first weighed when empty. It is then filled with the liquid, sa}^ milk, whose specific gravity is desired, and again weighed : this weight, less the weight of the bottle, will give the weight of a quantity of milk that will exactly fill the bottle ; the bottle is then emptied of milk and filled with water, and again weighed. This weight, less the weight of the bottle, will give the Aveight of a quan- tity of water that will exactly fill the bottle ; then the weight of the milk that Avill exactly fill the bottle, divided by the weight of Avater that Avill exactly fill the bottle, Avill give the specific graAdty of the milk. Thus, suppose the bottle, Avhen empty, Aveighs 300 I I I HYDROSTATICS. Ill grains, and when filled Avith milk, 1326 grains, and when filled with water, 1300 grains ; then 1326 — 300 = 1026 grains, the weight of the milk, and 1300 — 300 = 1000 grains, the weight of the water, and = 1.026, the specific gravity of the milk. 130. By the Hydrometer. — The specific gravity of a liquid may be very easily found by the use of floating instruments called hydrometers. One form of this instrument is seen in Fig. 54. It is made of glass, and is hollow except at its lower end, which contains mercury, so as to make it float upright when placed in any liquid. It is evident, that when the instrument is placed in any liquid, it will sink until it displaces a bulk of liquid equal in weight to its own weight. But the denser the liq- uid, the less deep it will sink, since the less Avill be the bulk of liquid required to equal in weight the weight of the instru- ment. We determine the specific gravity of Pig. 54. — Hy- any liquid in Avhich the hydrometer is drometer. placed by comparing the distance to which the instru- ment sinks Avhen placed in water with the distance to Avhich it sinks Avhen placed in the liquid Avhose specific gravity is desired. A scale, already calcu- lated, marked on the tube generally gives the specific gravity. Instruments of this kind, when used to determine the specific gravity of milk, are called lactometers ; and of alcohol, alcoholometers. In the folloAving table, the specific gravity of a fcAV common substances Avill be found : 112 NATURAL PHILOSOPHY. Solids. Iron 7.78 Zinc 7.19 Lead 11.35 Copper — 8.90 Silver 10.47 Gold = 19.30 Platinum = 22.06 Granite 2.75 Ice = .87 Cork = .24 Liquids. Mercury = 13.50 Sulphuric acid = 1.84 Milk = 1.026 Ocean water = 1.026 Alcohol = .792 Ether = .715 I Since the weight of a cubic foot of water = 62.3 Ihs., if we know the specific gravity of any substance, we can easily calculate the weight of a cubic foot of that substance ; thus, the sp. gr. of gold = 19.30 ; then a cubic foot of gold = 19.30 x 62.3 = 1202.39 lbs.; so, also, if we know the weight of a body and its sp. gr., we can calculate its volume. Thus, what is the vmlume of 100 lbs. of gold ? Since one cubic foot of gold has a weight of 1202.39 lbs., 100 lbs. must occupy the 1 2 01^3 y ^ cubic foot. Syllabus. Hydrodynamics treats of the conditions of rest and motion in fluids. It includes Hj’^drostatics, Hydraulics, and Pneumatics. H 5 ’drostatics treats of liquids at rest; Hydraulics, of liquids in motion ; and Pneu- matics, of gases either at rest or in motion. Liquids are almost incompressible. They transmit in all directions, and without sensible loss of intensity, the pressure exerted on any portion of their mass. The total pressure sustained by any surface immersed in a liquid is proportional to its area; this gives us another mechanical power, as is seen in the hydrostatic press. The pressure caused by the weight of a liquid increases with the depth and density of the liquid. Since liquids exert pressure equally in all directions, the downward, upward, and lateral pressures at any point must all he equal to one another. SYLLABUS. 113 The pressure on the base of a vessel filled with liquid is sometimes greater and sometimes less than tlie weight of the liquid in the vessel. To obtain the pressure on the base, calculate the weight of a column of liquid whose base is the base of the vessel, and whose height is the ver- tical distance from the middle of the base to the surface of the liquid. To determine the pressure on the side of a vessel, calculate the weight of a volume of the liquid equal to the area of the side multiplied by the vertical distance from the middle of the side to the surface. The upward pressure at any point is equal to the downward press- ure at that point. The upper surface of a comparatively small extent of water is every- where level or horizontal. The surface of a large body of water like the ocean is curved. When a liquid is placed in communicating vessels, it will be in equi- librium only when its surface is at the same level in all the vessels. Bodies immersed in liquids lose as much of their weight as the weight of the liquid they displace, because the excess of upward press- ure which buoys them up is equal to the weight of the liquid displaced. A body floats when the weight of the liquid it displaces is equal to its own weight, since the force which pulls it down is then just equal to the force which holds it up. A floating body is in equilibrium when the centre of buoyancy and the centre of gravity are in the same vertical line. The centre of buoyancy is the centre of gravity of the displaced liquid. If the centre of gravity of a floating body in equilibrium is as low as it can get, or if the centre of buoyancy is above the centre of grav- ity, the equilibrium will be stable. If the centre of buoyancy is be- low the centre of gravity, the equilibrium will be unstable. If no motion of the body can alter the relative positions of the centres of buoyancy and gravity, the equilibrium will be neutral. The specific gravity of a body is the weiglit of the body in air divided by the weight of an equal bulk of some other substance. To find the specific gravity of a solid or liquid, divide the weight of the body in air by the weight of an equal bulk of water. To find the specific gravity of a gas, divide the weight of the gas by the weight of an equal bulk of air. The specific gravity of a body may be found by the balance, by the specific-gravity bottle, or by the hydrometer. The weight of any volume of a substance is equal to its specific gravity multiplied by the weight of an equal volume of water. ' 10* H 114 NATURAL PHILOSOPHY. Questions for Review. Define Hydrodynamics ; Hydrostatics ; Hydraulics ; Pneumatics. How does the compressibility of liquids compare wdth that of solids? State the law for the transmission of pres.=ure in liquids. Describe the construction of the hydrostatic press. What relation exists between the pressure caused by the weight of a liquid and its depth ? Between the pressure caused by the weight of a liquid and its density ? Why should the upward, downward, and lateral pressures at any point of a liquid he equal to one another ? Give an example of a vessel in which the pressure on the base of the liquid arising from the weight is greater than the whole weight of the liquid. Give an example in which this pressure is less than the whole weight of the liquid. State the rules for calculating the pres.sure on the base of a vessel filled with liquid ; on the vertical wall of a vessel ; the upward pressure at any part of the liquid. Describe an experiment by which the amount of the upward press- ure of a liquid may be determined. What will be the shape of the surface of a comparative!}' small ex- tent of a liquid at rest? tWiy ? When will a liquid placed in communicating vessels be in equi- librium ? What do you understand by the buoyancy of a liquid ? How much weight will a body lose when immersed in a liquid? When will a body float in a liquid ? When will a floating body be in equilibrium ? What is meant by the specific gravity of a body ? With what do we generally compare the weight of solids and liquids? Of gases? State the general rule for obtaining the specific gravity of a body. State the general formula. Describe the method of obtaining the specific gravity of a solid by means of a balance, 1st. When the body is heavier than water ; 2d. When the body is lighter than water. Describe the method of obtaining the specific gravity of a liquid by means of a balance. By means of a specific-gravity bottle. By means of a hydrometer. 1 CHAPTER IT HYDRAULICS. 131. Hydraulics treats of liquids in motion. It studies tlie flow and elevation of liquids, and the ma- chines for moving liquids, or to be moved by them. 132. Pressures on a Vessel Containing Liquid. — The walls of a vessel filled with liquid and exposed to the air are subjected to two pressures, viz. ; 1st. The pressure of the liquid from within out- wards. 2d. The pressure of the air from without inwards. If a hole be pierced in the side of a vessel containing liquid, the liquid will escape only when the pressure from within outwards is greater than the atmospheric pressure. If the vessel be open to the air at the top, the pressure of the air tends to force the liquid out with the same force that it tends to keep it in. In this case, therefore, the liquid tends to run out with a force equal to the pressure caused by the depth of the liquid above the opening. Water flowing from a narrow-necked bottle does not escape in a steady stream, but at more or less regular intervals partially stops flowing, when a few bubbles of air enter the neck of the bottle with a gurgling sound, and the full flow again begins. The partial stoppages are due to the pressure of the atmosphere, which forces bubbles of 115 116 NATURAL PHILOSOPHY. air into the bottle against the pressure of the escaping liquid. The air thus forced into the bottle occupies the space left by the liquid which has escaped. After a certain quantity of liquid has escaped, the piress- ure of the air against the mouth of the bottle is greater than the pressure forcing the liquid out. Some air then enters, and more liquid escapes. Were a hole made in the bottom of the bottle, the liquid would escape in a steady stream. 133. Velocity of Escape. — If holes he bored in the side of an open barrel filled with water, it will be found, as might be supposed, that the tvater will flo\y most rapidly out of the hole tvhich is nearest the bot- tom. The water flows out of the openings solely by reason of the pressure of the liquid at the opening ; and as the amount of pressure exerted by a liquid increases with the depth, the greater the depth of the hole below the surface the greater the velocity Avith which the liquid escapes. If the holes in the side of the barrel be all of the same size, it can be proved that the liquid escapes the most rapidly from the lowest hole ; for, if a vessel be held for one minute before each of the holes, so as to catch all the liquid which escapes from that hole in one minute, it will be found that the hole nearest the bottom of the barrel will discharge more than any of the others ; the velocity of escape of the liquid from this hole must, therefore, be greater than from any other. An opening in tlie side of a vessel tbrougli wbich. liquid escapes is called an orifice. The \mrtical dis- tance from the middle of the orifice to the surface of the liquid is called the head. The amount of liquid Avhich iloAVS out of an orifice in a given time is called the flow. 134. Rule for Calculating the Velocity of Escape. — W e have seen that the Amlocity of escape of a liq- | uid increases Avith the depth of the orifice below the j surface, that is, Avith the head. Torricelli, an Italian philosopher, discoAmred that the A'elocit}' Avith Avhich HYDRAULICS. 117 the liquid escapes is exactly the same as the velocity it would acquire in falling in an empty space through the distance of the head. This fact is expressed ap- proximately by the following simple formula, viz. : V = 8 VH, when V = the velocity of escape per second, and H = the head. The rule may be expressed as follows, viz. : The ve- locity in feet per second with which a liquid escapes from an orifice.^ is equal to eight times Ulc square root of the head. Suppose, for example, that an orifice was four feet below the surface of a liquid, then the velocity of escape would be8Xl^'i = 8X2 = 16 feet per second. 135. Method of Ascertaining the Flow. — To as- certain the flow\ or the amount of liquid escaping from an orifice in a given time., multiply the velocity of escape hy the area of the orifice. Since the velocity obtained from the preceding formula tvill be in feet, if the area of the orifice be in square inches the velocity must first be reduced to inches ; the product will then give the volume of the flow in cubic inches per second. The product of the area of the orifice and the velocity must give the volume of the discharge, as the following simple reasoning will show. Suppose the area of the orifice to be one square inch, and the flow twelve inches per second ; then in one second a mass of water one inch thick and twelve inches long would flow out from the orifice, the vol- ume of which would of course be 12 cubic inches. Since the velocity of escape from any vessel varies AV'ith the head, if this decreases as the liquid runs out, the quantity discharged from any given orifice must be greater during the first second than during the second, and greater during the second second than during the third, and so on. AVe cannot, therefore, ascertain the quantity discharged in, say 60 seconds, by multiplying 118 NATURAL PHILOSOPHY. the quantity discharged during the first second by 60, unless the head is kept constant by allowing w'ater to run into the vessel as fast as it runs out. The flow, as calculated by the preceding rule, will be found in practice to be greater than the amount which actually escapes. This is because the issuing stream contracts shortly after leaving the orifice. The amount which actually escapes is about equal to two- thirds of the calculated amount. 136. The Flow of Liquids through Horizontal Pipes. — When a liquid flows through long pipes, a. produced by the escape of the liquid is sometimes j called the reaction of the escaping jet. i Fig. 59. — Reaction Vase. 14:4:. The Turbine Water-Wheel water-Avheel, advantage is taken of the reaction of the escaping jet. . — In the turbine i 1 1 Fig. 60.— The Turbine Water-Wheel. The figures represent one form of turbine in per- spective and in horizontal section. This form of wheel SYLLABUS. 123 is submerged in water in such a place that the water can readily escape from the wheel after it has given motion to it. The top of the wheel is covered, to pro- tect it from the direct pressure of the water. The movable part of the wheel is seen at a a a a, which is attached to the shaft, A. The water enters below through openings between the fixed curved guides, (j per limit of the atmosphere. At the level of the sea, the pressure of the atmos- phere will sustain a column of mercury 30 inches in height above the level of the mercury in the cup. If the area of the open end of the tube be one square inch, then this column of mercury will weigh about fifteen pounds. Therefore.^ the pressure of the air at the level of the sea is about equal to 15 lbs. to the square inch. Fig. 62. — The Barom- 151. The Barometer. — Torricelli’s tube forms an instrument called a barometer, by means of which we can tell the variations that occur in the pressure of the atmosphere. As the pressure increases, the mercury rises in the barometer, and as it decreases, it falls. As 130 NATURAL PHILOSOPHY. a rule, the rise of the mercury in the barometer at any place is followed by clear weather, and its fall, by foul i | weather. The barometer is used, especially at sea, to J observe approaching changes in the weather. Its use, I however, for this purpose requires considerable expe- i | rience. J Since it is only the air above the mercury in the cup H that keeps the mercury in the tube, it follows that if || we carry a barometer up a high mountain, the height i of the mercury in the tube will decrease as we ascend. ■ I The barometer, therefore, can be used to measure the \ height of mountains or other elevations. 152. Accuracy of a Barometer. — The space above l- the mercury in the top of the tube should be a com- | plete vacuum.^ that is, should contain no air or other I matter except the unavoidable vapor of mercur}'. If j air w'ere present in this space, it would prevent the ^ mercury from rising as high as it should. To test the j completeness of the vacuum, the tube may be gently 1 inclined, when, if the vacuum is complete, a sharp me- j tallic click will be heard as the mercury fills the tube. The mercury also should be pure, since otherwise its : density would be changed, which would of course aftect i the height of the column. Barometers can be made with other liquids than mercury ; the height of the column will then depend on the specific gravity of the ' liquid. Thus, if water were used, the height would be about 34 feet, for since mercury is 13.5 heavier than water, the height would be 13.5 u X 30, or 405 inches, or about 34 feet, were it not for the fact that the .| vapor of water formed in the tube would somewhat decrease the height. t 153. Pressures Expressed in Atmospheres. — We \ find it very convenient to express the pressure exerted by a column of liquid or gas in what are called atmos- pheres of pressure. Thus, if the pressure is equal to PNE UMA TICS. 131 15 lbs. to tlie square inch, we call it a pressure of one atmosphere ; if it is equal to a pressure of 60 lbs. to the square inch, it is a pressure of four atmospheres. 154. The Air-Pump. — In order to make manifest i the pressure which the atmosphere exerts on any object, it is necessary to remove the pressure of the ' air from one side of the object. This is most con- veniently done by means of the air-pump. Fig. 63 represents one form of this instrument. An air-tight piston, P, moves in the cylinder, C. Open- ings are provided at a and c, in the bottom and top of the cylinder, and at 5, : in the piston. These openings are alternately I shut and opened by con- trivances called valves, which act like doors. In the air-pump these I valves all open up- I wards. The cylinder is connected by a tube, e, with a flat plate, J/, on I which is placed a glass I vessel, P, called a receiver. By successive movements I of the piston, the air in the receiver, P, is gradually removed, when the receiver is said to be exhausted. The way in which the air is removed is as follows : i When the piston is raised from the lower part of the cylinder, a vacuum is left below it, into which some ; of the air from the receiver, P, at once passes, lifting i by its tension the valve a. When the piston is pushed i down, the air in C is compressed, the valve a shut, and I the valve h opened, and the air below the piston is i ! 132 NATURAL PHILOSOPHY. now transferred above it. As tlie piston is again 1 raised, some more air passes from R to the cylinder, and the air above the piston is forced out of the cyl- inder through the valve c. 155. Illustrations of Atmospheric Pressure. — The receiver, A, can easily be lifted from the plate of the air-pump, provided the air is not removed from the inside. As soon, however, as the receiver is exhausted, the pressure of the air on the outside fixes it so firmly to the plate, that, if the receiver is moderately large, it will be almost impossible for a person to remove it from the plate until air is again allowed to enter. Two hollow hemispheres of brass Avith smooth plane edges, if simply pressed together so as to form a hol- low sphere, and then connected with the air-pump so that the air may be removed from the inside, are j held together so firmly by the pressure of the air on the outside that it is very difficult to pull them apart. 1 When a receiver with an open top, over which a piece of bladder has been tightly stretched, is placed on ' the plate of an air-pump and exhausted, the pressure I of the air on the bladder bursts it with a loud report. 156. Simple Experiments in Atmospheric Press- i ure. — The following experiments can be shown without the use of an air-pump. ist Experiment. — Place the open end of a hollow key to the mouth, ' | and vigorously sucking out the air, quickly press it against the lip, ' 1 and it will be held there by the pressure of the air. i 2d Experiment. — Select a small wine-glass with a smooth edge. Jj Place some small pieces of paper loosely in the glass, and set fire to I them. As soon as they are nearly consumed, quickly press the glass I to the hand, and it will then be pressed against it with considerable j force. The heat expands the air and drives part of it out of the I ' glass, which is then held against the hand by the outside atmospheric i j pressure. J PNE UMA TICS. 133 Fig, 64, — An Experiment in Atmospheric Pressure. 3d Experiment. — Try the same with a glass preserving jar, only place the open end of the jar below the surface of water in a soup plate. As the jar cools, the water will rise and partly fill the jar. Caution . — It is not necessary in either of these experiments to have the glass very hot. Select thin glass, which is less apt to crack when suddenly heated or cooled. 4th Experiment — Fill a smooth-edged tum- bler with water, place a piece of stout paper over the top, and pressing the palm of the hand against the paper, slowly invert the tumbler. The hand may now be removed from the p.aper, and the water will not run out, since the pressure of the air keeps it in. 5th Experiment. — Select an empty tomato or other tin can, from the top of which the small, round piece of tin only has been removed, leaving an opening about two inches in diameter. Tie a piece of mosquito netting firmly around this end of the can, stretching it smoothly over the top with the fingers. Fill the can with water by pouring it through the meshes of the netting. Place a piece of smooth, stiff paper over the open end, and invert as in the previous experiment. The paper will then be held against the can by the pressure of the air. Now holding the can as shown in Fig. 65, cautiously slide the paper from the netting, a7id the water will still remain in the can, although the open end is only protected by the mosquito netting. 6th Experiment. — Prepare a can as described in the 5th exper- iment, but in addition punch a hole with a nail in the bottom of the can. Holding a finger firmly against this hole, fill, invert, and re- move the paper as before, and the water will not run out : now remove the finger momentarily from the hole, as shown in Fig. 66. The press- ure of the air is then exerted downwards on the water as well as upwards, and the water flows out by its own weight. Eeplace the finger, and the flow after a moment ceases, since the pressure of the at- mosphere is greater than the weight of the water. This curious experi- 12 Fig. 65.— An Experiment in Atmospheric Pressure. 134 NA T UR A L PHIL 0 SO PHY. merit proves, 1st. Atmospheric pres.sure ; 2d. Considerable adhesion of i the water to the netting, and, 3d. Considerable cohesion of the molecules of the water. To succeed with it, read carefully the following Caution. — The netting must be free from grease, and the open end i of the can smooth ; above all, the can must in all cases be held with its t open end as nearly horizontal as possible. If you do not at first succeed ^ with this or any other experiment, try again until you are successful. The ability to suc- cessfully experiment can only be acquired by practice. The common leather sucker depends for its I ■ operation on the pressure of the air. j 157. Buoyancy of Air. — A body j wlieu weiglied iu air loses as muck , weight as the weight of the air it dis- i places. For ordinary purposes this '■ lo.ss of Aveight may be disregarded; ^ it becomes more considerable in pro- portion as the bulk of the thing weighed exceeds that of the Aveights used to balance it. Suppose, for example, a pound of feathers be balanced in air by a pound of lead ; then, since the bulk of the feathers is greater than that of the lead, the buoyancy of the air must decrease their weight more than the weight of the lead, ajid therefore more than one pound of \ ' feathers must be taken to balance one pound of lead. | < 158. Balloons. — solid lighter than AA'ater, im- mersed beloAV the surface of Avater, Avill, unless held in place, rise through the Abater until it reaches the sur- face, Avhere it Avill float. Balloons rise through the air for the same reason, for, being filled AA’ith some light gas '' or heated air, their AA'eight, Avhich tends to pull them doAA'u, is less than the buoyant force of the displaced air which tends to push them up. "When these tAA'o forces are exactly equal, the balloon AA'ill neither rise nor fall. The ascensional or liftiii'j power of a balloon can therefore be found by subtracting the AA'eight of } ‘ Fig. 66. — An Experiment in Atmospheric Pressure. PNE UMA TICS. 135 the balloon, enclosed gas, and car from the weight of an equal bulk of air. 100 cubic inches of air at the sea-level weigh about 31 grains. 159. Effect of Pressure on the Volume of a Gas. — Gases, as we have seen, are the most compressible forms of matter. As the pressure on any bulk of gas is increased, its volume is diminished, and, conversely, as the pressure is decreased, the tension of the gas causes its volume to increase. The law according to which these changes occur was discovered by Mariotte and Boyle, and may be expressed as follows : At the same temperature.^ the volume occupied hy any hulk of air is inversely proportional to the pressure it supports. This law is very nearly true for all gases. Suppose, for example, a certain quantity of air occupies, at the ordi- nary pressure of the atmosphere, the volume of one quart ; then, if the pressure on this mass of air be increased to two atmospheres, its volume will be reduced to one-half a quart; if the pressure be increased to three atmospheres, its volume will be reduced to one-third of a quart ; if to ten or one hundred atmospheres, to -j-'j- or of a quart. Again, if the pressure of one atmosphere on the quart be reduced to one- half an atmosphere, the tension of the air will cause it to expand to two quarts ; if it be reduced to of an atmosphere, it will expand to one hundred quarts. The ability of a gas to expand on the relief of pressure appears to be almost unlimited. 160. Effect of Pressure on the Specific Gravity or Density. — Since the mass of a gas remains the same, however its volume may be changed by press- ure, the density or specific gravity must increase as the pressure increases, or, in other words, the density must he proportional to the pressure. Since the lower layers of the atmosphere sustain the weight of the upper layers, the density of the air near the sea-level is greater than that of the air over the top of a mountain. Assuming the height of the atmosphere to be from 50 to 200 miles above the sea-level, by far the greater mass of the air lies within a few miles of the general surface. 136 NATURAL PniLOSOPET. Fig. 67.— The Siphon. 161. Machines Depending for their Action on Atmospheric Pressure. — In the following machines, advantage is taken of the pressure of the atmosphere. 1. The Siphon. — The siphon consists of a tube bent as shown in Fig. 67. When the shorter arm is placed below a water- surface, and the tube exhausted by the mouth applied at 6, the pressure of the air on the water in m causes the water to rise through the height, m 71, and flow out of the open end of the tube, h. The greater the differ- ence of level, a h, between the water- surface, m, and the open end, h, the greater the velocity with which the water will escape. 2. The Suction Water-Pump . — -In the common suc- tion-pump, Fig. 68, the w'ater is forced up out of the well into the body of the pump by the pressure of the at- mosphere. In its simplest form, this pump is essentially the same as the common air-pump. The valves open upwards ; one or more valves, h h, are placed in the piston, and one, a, at the lower end of the cylinder, or pump- barrel. As the piston is raised, a vacuum is left below it in the pump- barrel, into which rushes the air from the pipe dipping down into the well, ir. As the air is thus sucked Fig. 68. — The Suction- out of the pipe, the pressure of the Pump. water in IF forces it up through the valve, a. As the piston descends, the valve a closes and the valves h h open, and the SYLLABUS. 137 water passes above tne piston. When the piston is again raised, the iVater escapes at the mouth of the pump at D. In the figure, the valves are represented in the position they w'ould occupy in the up-stroke of the piston. 3. The Force-Pump. — In the force-pump. Fig. 69, there is no valve in the piston, P. A pipe, T., enters the side of the cylinder near the bottom. A valve, a, which opens outwards, is placed where this tube enters the cylinder. The valve h is placed, as before, at the lower end of the barrel. On the downward stroke of the piston, the water, instead of passing above the piston, is forced through the valve a up through the pipe, T. In the figure the valves are represented in the posi- tions they would occupy during the up-stroke of the piston. The height, m n, through which the water is raised by the pressure of the air in the siphon, the suction-, or the lifting-pump, can never exceed 34 69.— The loroe- feet, since, as we have seen, a column of water Pnmp. of this height exerts a pressure equal to that of the atmosphere. In practice, pumps seldom raise water higher than 28 feet from the level of the well to that of the lower valve. Syllabus. Gases, like liquids, 1st. Transmit pressure as well in one direction as in another; 2d. Exert equal upward, downward, and lateral pressures at the same part of their mass, and 3d. Exert a buoyant force on any body immersed in them. The atmosphere consists of a mixture of four parts by volume of nitrogen and one part of oxygen. It also contains carbonic acid and the vapor of water. The height of the atmosphere has been variously es- timated at from 50 to 200 miles ; its upper surface, however, is undefined. 138 NATURAL PniLOSOPIIT. The atmosphere has weight, and therefore exerts a pressure on all things on the earth’s surface. The reason that this pressure is not generally felt, is because it is exerted equally in all directions. The fact that the atmosphere exerts a pressure on the earth, was dis- covered by Torricelli by means of an instrument called the barometer. The mercury is sustained in the barometer tube by means of the pressure which the air exerts on the surface of the mercurj' in the cup in which the barometer tube dips. At the level of the sea, the height of the barometric column is about 30 inches. This is equal to a pressure of 15 lbs. on each square inch of surface. The barometer is used, 1st. To indicate approaching changes in the weather, and 2d. To measure the height of mountains or other eleva- tions. The accuracy of the barometer depends on, 1st. The completeness of the vacuum in the upper part of the tube, and 2d. On the purity of the mercury. By a pressure of one atmosphere, we mean a pressure of 15 lbs. on each square inch of surface. When the pressure of the air is removed from any one side of a body, its pressure on tbe other side at once becomes manifest. Bodies weighed in air lose an amount of weight equal to tbe weight of the air they displace ; therefore, a pound of feathers balanced in air by a pound of lead is actually heavier than the lead. Balloons rise through the air because the weight of the air they dis- place is greater than their own weight. At the same temperature, the volume occupied by any bulk of air i.? inversely proportional to tbe pressure it sustains. Its density or spe- cific gravity is directly proportional to the pressure. The lower layers of the atmosphere are denser than the upper layers, because tbe lower layers have to sustain the wmight of all the air above them. A liquid is forced up through the short arm of a siphon, or from a well into the body of a pump, by the pressure of the air. In the suction-pump and in the force-pump, there is a valve placed in the lower part of the cylinder, directly over the pipe leading down into the well. In the suction-pump, a valve or valves are placed in the piston. In the force-pump the piston is solid, and a valve is placed in the side of the cylinder, near the bottom, directly over the point where a pipe enters the cylinder. The greatest distance that water can be raised from a well to the barrel of the pump is 3-1 feet. It is seldom raised more than 2S feet. QUESTIONS FOR REVIEW. 139 Questions for Review. Name any properties that are possessed in common by both gases and liquids. Of what gaseous substances does the atmosphere consist ? What is known of the upper limit of the atmosphere ? Why do we not feel the pressure which the air exerts upon us ? How did Torricelli prove that the atmosphere exerts a pressure on everything it touches ? What is the atmospheric pressure in pounds per square inch ? Describe the barometer. For what purposes is the barometer used? Upon what does the accuracy of a barometer depend? How high would the column of a water barometer be ? What is meant by a pressure- of one atmosphere ? Describe the operation of the air-pump. Describe in full any simple experiments by which the pressure of the air can be demonstrated. Describe an experiment with a tomato can and a piece of mosquito netting. What effect has the buoyancy of air on the weight of bodies? What causes balloons to rise through the air? How may the ascen- sional power of a balloon be calculated ? What effect has an increase of pressure on the volume of a gas ? What effect has a decrease of pressure? What effect has either an increase or a decrease of pressure on the density of a gas ? State the law of Mariotte and Boyle. Why are the lower layers of the atmosphere denser than the upper layers ? Describe the construction and operation of the siphon. How many valves are there in a suction-pump ? Where are they placed ? How do they open ? In what respects are the suction-pump for water and the air-pump alike? How many valves are there in a force-pump? Where are they placed ? How does this pump differ from a suction-pump ? Why cannot water be lifted by the pressure of the air higher than 34 feet from the surface of water in a well to the barrel of a pump ? 1 ! Part III. | Sound and Heat. I CHAPTER I. i THE CAUSE, TRANSMISSION, REFLECTION, ' AND REFRACTION OF SOUND. 162. Sound Defined. — Sound is caused by a vibra- * tory or wave- like motion of the air or other medium, the effect of which is transmitted by the ear to the brain. ’ W e use the word sound in two distinct senses, viz. : 1st. As the impression which is produced on the brain, and 2d. As the thing which causes the impression, viz., as the vibrations of the air or other medium. It is evident, however, that the sound we hear is the im- pression, while the thing that causes the impression is the vibrations of the air. 163. Nature of Wave-Motion. — Since sound is caused by a vibratory or wave-motion of the air, it is necessary, before studjdng the phenomena of sound, to obtain clear ideas of the nature of wave-motion. » When a wave-motion is started in a body of deep * TiO I SOUND. 141 water, it appears as if the whole mass of water at the surface were moving in the direction in which the wave is advancing. If, however, we observe any light body floating on the water, we wdll notice that it merely rises and falls, and does not advance with the wave. The water, therefore, cannot be moving bodily forward. Similar wave movements are seen in the shaking of carpets or cords. If a cord, A i?. Fig. 70, fixed at A, be smartly shaken bv the hand at B, a wave-motion, as shown by the con- Fig. 70.— Waves in a String. string in a direction from B to A. On reaching J., it will be reflected, and will move back towards the hand along the dotted, curved line. These motions give the particles of the string the appearance of moving alter- nately from B to A and from A to B. This, of course, is not the case, the particles merely rising and falling; being sometimes above the straight dotted line, as at B a E, and sometimes below it, as at ^ i D. In wave-motion., the particles move alternately hack- wards and forwards through comparatively short dis- tances, while the waves themselves may move in only one direction through considerable distances. Thus, let the dots from D to B, Fig. 71, represent a row of particles which, when at rest, will have the position as shown in the straight line B Fig. 71.— Motion of the Particles. B. If, now, a wave moves along the string from B to Lious curved line, will move along the 142 NATURAL PHILOSOPHY. 2), the particles do- not move from B io D \ they sim- ply move at one moment below the straight line D B, and at the next moment above it. 164. Definitions. — The alternate motion to and fro, from one side of the position of equilibrium to the other, is called a vibration.^ undulation.^ or wave. Thus, in Fig. 70, the portion B a E above the straight line A B., together with the portion E h D below this line, form one complete wave movement. The length of the wave is the distance, B 21, measured along the straight line, or is the shortest distance be- tween any two sets of particles that are moving at the same time in the same direction. The amplitude of the wave is the line 6 2 or c e, Fig. 71, Avhich marks the greatest distance that any particle has moved out of its position of equilibrium. The wave period., or the time of vibration, is the time required for any particle to make one complete movement from one side of its position of equilibrium to the other. In Avaves of the same length, the wave period, or time of vibration, is the same, AvhateAmr may be the amplitude of the Avave. If, therefore, a tightly stretched cord, such as a string in a harp or Auolin, be pushed by the hand, it will, before coming to rest, move to and fro betAveen its position of rest through a gradually smaller and smaller amplitude; but the time of its vibration aaTII be the same near the end of its motion as near the beginning. 165. The Cause of Sound. — If AA"e cause a bell to sound by striking it, we can, by lightly touching its sides, feel that, while it is sounding, its sides are shak- ing to and fro; and if it is sounding loudly, Ave can SOUND. 143 even, see its sides shaking. When these shakings cease the sound ceases, as we can prove by pressing the hand against the sides, and thus stopping the vi- brations, when the sound at once ceases. It can, in a like manner, be shown that all bodies which are pro- ducing sound are vibrating. Sound is caused hy the shakings or vibrations of the sounding or sonorous body. A tuning fork consists of a bar of steel, of the form shown at a 6, Fig. 72, and firmly supported on a hollow case, c, of dry wood. When the fork is sounded by rubbing the sides near the top by a rosined bow, the arms, a'b, move alternately towards and from each other, and, setting the air around them into Avaves, produce a musical sound. Experiment. — Partially fill a thin glass goblet with water, and rub the moistened finger lightly against the edge, so as to cause a clear musical note. The surface of the water will then he rufified with min- iature waves, produced hy the shakings or vibrations of the sides of the goblet. Caution .' — To obtain a strong, clear note, the motion of the finger must be regular. 166. Manner in which the Motion is Conveyed from the Vibrating Body to the Ear. — The sides of the vibrating body set the surrounding air into waves, which move out from the body in all directions. These Avaves reaching the ear, cause us to have the impres- sion of sound. 167. Nature of Sound-Waves, — When any sono- rous body, as, for example, a bell, is set into vibration, as its sides are moving outwards, they croAvd the par- ticles of air immediately in front of them into a smaller 144 NATURAL PHILOSOPHY. space, and thus cause a condensation of the air. But when its sides are moving in the opposite direction, the particles of air not being able at once to follow them, get further apart, and thus produce a rarefaction of the air. A complete vibration of the bell consists of a mo- tion of its walls outwards and inwards. A complete wave-motion of the air consists of an alternate con- densation and rarefaction. The sound-waves that are produced in the air by the vibrations of a sonorous body move outwards from it in all directions, and consist of alternate condensa- tions and rarefactions of the air. They surround the sonorous body in the shape of constantly increasing spherical shells. The motions are called waves of con- densation and rarefaction. The motion of the air particles in a sound-wave is al- ternate! v backwards and forwards in the same direction as that of the line in which the wave is advancing. The amplitude of a sound-wave depends on the degree of rarefaction and condensation. The greater the num- ber of particles crowded into the condensed space, and the smaller the number in the rarefied space, the greater the amplitude. Some idea of the nature of these waves may be obtained from Fig. 73, where the shaking of the bell is represented as causing waves in the air around it. In these waves, the particles of air are alternately crowded together and separated from one another, as shown by the dark and light shadings. The nature of the motion of the particles of air may be roughly represented by placing a number of glass marbles in a straight, wooden trough, so that they all touch one another. If, now, a marble at one end of the trough be rolled sharply against the marble next S 0 UND. 145 it, the balls will not move on together ; the first ball gives its motion to the second, and then stops ; the second gives its motion to the third, and then stops ; and so on through all the balls to the end, which, hav- ing no ball to give its motion to, moves on. So with the sound-waves, the vibrating body strikes the parti- cles of air near it ; these give their motion to the par- ticles of air beyond them, and then stop ; and these particles to others beyond them, and so on until at last the waves reach the ear and we hear the sound. 168. A Medium Necessary to Transmit Sound. — - Since sound is transmitted by waves, something must exist between the sounding body and the ear to be set into waves, in order that the sound may be carried from one point to another. If a bell be struck in a vacuum, no sound will be heard, since there would be no medium to be set into waves to carry the sound. In Fig. 74, a bell is suspended by a thread inside a glass globe, Ji. If the air be removed from the globe by means of the air-pump, no sound will be heard when the bell is struck. 13 K Fig. 74.— BeU in Globe. 146 NATURAL PHILOSOFHY. Sound-waves are transmitted througli the air be-« cause the air is elastic. Any elastic substance canm transmit sound. Therefore, gases, liquids, and solids 1 will all act as sound media, that is, will transmit! sound. I Experiment. — Hold a bell under water, and ring it. The sound T can be heard by those standing near the water. Here the water must } have transmitted the motions of the bell. I Caution. — Of course the bell will sound differently in water than I in air. 1 Experiment. — Remove the bottoms of two small tomato-cans; I moisten a piece of bladder or stiff paper and stretch it tightly over the | open end of each, and secure by tying. When quite dry, pierce a hole 1 in the middle of each end with a large darning-needle. Get a piece f of string 15 or 20 feet long, and run one end through each of these J holes from the outside of the can to the inside. Tie a piece of stick ( Fig. 75. — The String-Telephone. to each end of the string to prevent its slipping out. If, now, the string is stretched rather tight, a person, by placing the ear at the opening of one of the cans, can distinctly hear all another person whispers into the open end of the other can. This instrument is called the string-telephone. Caution . — The bladder must be quite tight. A faint whisper at one end should be distinctly heard at the other end. t Experiment. — Try the same experiment, using cig.ar-hoxes and wire « instead of the tomato-can, string, and bladder. Bore a hole in the j SOUND. 147 bottom of each box for the insertion of the wire. Talk into, or listen at the open end of the boxes. The wire may then be stretched for a considerable distance in a straight line, as from one house across a street to another. Caution . — Select for this purpose boxes with thin, elastic bottoms. 169. Velocity of Sound. — Time is required for sound to travel from one place to anotlier. We can see a distant man strike a blow with a hammer some time before we hear the sound. We see the lightning which causes the thunder before we hear the thunder. The velocity of sound in air varies with the tempera- ture. At the temperature of freezing water, sound travels 1090 feet in every second. Sound travels in warm air more rapidly than in cold air. For each degree of temperature above the freezing-point, or 32° Fah., the velocity increases about lyg feet. In the same medium, all ordinary sounds have the same velocity ; since we can enjoy the music of an orchestra as well when we are at a distance from it as when we are near, the different sounds produced must all move with equal velocity, for, if they did not, some sounds would reach us sooner than others, and thus cause discord. The velocity of sound in toater is about four and one- half times greater than the velocity in air. The velocity of sound in elastic solids is generally greater than in either air or water. Thus, the velocity in cast-iron is about ten and one- half times greater than the velocity in air. 170. Reflection of Sound. — When nothing inter- feres with their progress, sound-waves move in all directions in straight lines. But when they meet with a suitable obstacle, they are, like other elastic bodies. 148 NATURAL PHILOSOrEY. reflected or thrown off from it at an equal angle to that at which they struck it. This change in the di- rection of the sound-waves is known as the reflection of sound. Tlie smooth and hard surfaces of elastic bodies are the best reflectors of sound-waves. Cloth.s, curtains, or other draperies scarcely reflect the w'aves at all. A smooth water surface forms an excellent reflector of sound. 171. Echoes. — When an impression on the brain has been made by the sound-waves through the ear, the sensation continues for a short time. If, therefore, two sounds follow each other too rapidly, the ear will be unable to distinguish them as separate sounds, and the effect of one continuous sound will be produced. Sounds like those of the voice, as in speaking, must be about the fifth of a second apart, in order to be heard separately. If we stand in front of a sufficiently large and dis- tant reflecting surface, such, for example, as a high wall, and speak in a loud voice, we will hear two sepa- rate sets of sounds, viz., 1st. Those produced on the ear by the direct sound, and, 2d. Those produced by the sound-waves which have been reflected by the distant object back again to us. These latter sounds, which are produced by the reflected sound-waves, are called echoes. If the time required for the sound-waves to move towards and from the reflecting surface is no shorter than one-fifth of a second, the reflected sound will be heard separately from the direct sound ; if the time be less than this, the direct and reflected sounds will blend with each other. S’ 0 UND. 149 At the temperature of 60° Fah., the velocity of sound is about 1120 feet per second. During the fifth of a second, the waves would travel 224 feet. If, therefore, a person is standing in front of a reflecting surface which is 112 feet distant, and is speaking at the rate of five syllables per second, the reflected sound could go and return so as to be heard before the next syllable would be pronounced, and thus both the direct and reflected sound could be heard. Were the surface 224 feet distant, the waves produced by tw'o syllables could go and return in such a time as to be heard separately from the direct sound. At 560 feet, flve syllables could be separately heard. Very sharp, quick souuds produce a less permanent effect on the ear, and can therefore cause distinct echoes, when the reflecting surface is less distant. Multiple Echoes . — Wliea the source of sound is be- tween two opposite walls, or other nearly parallel walls, the waves are thrown back and forth between the walls, thus repeating the original sound many times. These are called multiple echoes. If the reflecting surface is less distant than 112 feet, the direct and reflected sounds are blended, and but one sound is heard. If the direct and reflected sounds reach the ear at nearly the same time, the original sound is prolonged and strengthened, otherwise the two sounds produce confusion. In a properly proportioned room, the voice of a speaker is strengthened by the waves reflected from the walls and ceiling reaching the ear at nearly the same time as the direct sound. This effect is spoken of as produced by the resonance of the room. When, however, the hall is large, the reflected sound is apt to confuse the direct sound by the partial echoes so produced. The confusing effects caused in this way in large, empty halls, are greatly diminished when the halls are crowded, since the bodies of the people are bad reflectors. Curtains and drapery have a similar effect. 13 * 150 NATURAL PHILOSOPHY. 172. Whispering Galleries. — If two carved mir- rors, A and B, be placed facing each other, as shown in Fig. 76, and anj source of sound, as, for example, a watch, be suspended at a certain distance in front of the mirror A, the waves, after reflection from both mirrors, Avill collect at a point, a, in front of the mir- ! ror, B. From whatever part of the mirrors the waves I Fig. 76, — Eeflection of Sonnd-Waves. have been reflected, they will all reach this point, a, at the sarne time. The ticking of the watch can there- fore be distinctly heard by a person listening, as shown in the figure. Sometimes the ceilings or Avails of rooms are of such a shape that a person standing in certain posi- tions ean distinctly hear all that is said by a person at a distance in some other part of the room, and speaking in but a faint Avhisper. The shape of the ceiling or walls is such that the sound-Avaves reflected from different parts are all brought by reflection to the place Avhere the other person is standing. Eooms Avith dome-shaped ceilings frequently act in this way. The name whispering galleries is given to such rooms. SOUND. 151 I In the dome of St. Paul's, in London, persons standing on opposite j sides of the gallery can converse in faint whispers. The person talking places his mouth near the wall ; the one listening, holds his ear near i, that portion of the wall directly opposite. ' 173. Refraction of Sound. — The straight direc- tion in which sound-waves move is changed when the waves pass from one medium into another of different density. In this case, the direction of the waves is changed at the surface of the dense medium, but the waves, instead of being thrown off or reflected by the medium, pass through it in a direction somewhat dif- ferent from that in which they were moving before they reached the other medium. The change caused in this manner in the direction of sound is called the refraction of sound. Sound, for example, is refracted in passing from air to water, or from air through any solid, or the reverse. 174:. The Effect of Distance on Sound. — If we are walking towards a distant bell while it is sounding, we notice that the sound constantly grows louder and louder as we near the bell. It can be shown that the loudness, or the intensity of any sound, decreases as the square of the distance from the source of the sound. Thus, if, at the distance of ten feet from the bell, we hear the sound with a certain loudness or intensity at twenty feet, or twice as far from the bell, the intensity would be found to be but one-fourth as great, that is, as (if or = i. The speaking -tules lohich connect the different rooms in a building, enable a person talking in a moderate tone to be distinctly heard by a person listening at an- other part of the building. Here the intensity of the sound is not so greatly diminished, because the sound- waves are confined to the air in the tube, and so do not 152 NATURAL PniLOSOFIIY. spread outwards in all directions. The faint sounds, conveyed by the string or the wire in the string-tele- phone are distinctly heard at the other end, for the same reason. Experiment. — Speak faintly into an empty water-hose, and a per- son listening at the other end can hear distinctly all that is said. Syllabus. The word sound is used in two distinct senses, viz.: 1st. As the im- pression produced on the brain by sound-waves, and 2d. A.s the waves or shakings of the air themselves. In wave-motion, the particles do not have any continued onward motion ; they simply move backwards and forwards through compara- tively short distances. The waves themselves, however, move onwards often for considerable distances. A vibration, wave, or undulation consists in the backward and for- ward motion of a particle from one side of the position it occupies when at rest to the other side. The length of a wave is the shortest distance between any two sets of particles that are moving at the same time in the same direc- tion. The amplitude of a wave is the greatest distance any particle has been moved out of its position of equilibrium during the progress of the wave. The wave-period, or the time of vibration, is the time required for any particle to make one complete movement to and fro. The time of vibration is the same whether the amplitude is great or small. Sound is caused by the shakings or vibrations of some body ; there- fore all bodies producing sound are in vibration. Sound is conveyed to the ear by waves, called sound-waves, pro- duced in the surrounding air by the vibrating body. Sound-waves consist of alternate condensed and rarefied spaces in the air called waves of condensation and rarefaction. The vibrating body strikes the air near it and sets it into motion ; this air imparts its motion to the air next to it, and comes to rest, and this to the air beyond it, until the last particles reach the ear and cause sound. This motion is similar to that transmitted through a row of glass balls touching one another, when a ball is thrown against one of the end balls. QUESTIONS FOR REVIEW. 153 A medium is necessary to transmit sound. All sounds cease in a vacuum. Any elastic substance will transmit sound, as, for example, water and most solids. The velocity of sound in air at the temperature of freezing water is 1090 feet per second. Sound travels more rapidly in hot than in cold air. Its velocity is about four and one-half times greater in water than in air. Its velocity in most solids is greater than in either air or water. All ordinary sounds travel with the same velocity. When sound-waves strike any large, reflecting surface, they are thrown off, or reflected from it. The angle of reflection is equal to the angle of incidence. An echo occurs whenever sufficient time has elapsed between the reflected and the direct sound to allow us to hear the reflected sound separately from the direct sound. In whispering galleries, the walls or ceilings so reflect the sound- waves as to bring them from all parts of the room to nearly the same place at the same time. When sound-waves meet a medium whose density is different from that in which they have been moving, they are turned somewhat out of their direction on entering it. This is called the refraction of sound. The intensity of sound is inversely proportional to the square of the distance. Speaking-tubes, wires, and strings convey sounds distinctly to great distances, because the motion is then transmitted in only one direction. Questions for Review. In what two different senses is the word sound used ? Define a vibration, wave, or undulation. What is meant by the length of a wave ? By its amplitude? By its time of vibration ? Is the time of vibration affected by the amplitude ? In wave-motion, do the particles actually move forwards for any considerable distance? What is the true nature of their motion ? What is the cause of sound ? Describe any experiment which shows that a sounding body is in vibration. How is the motion of the vibrating body conveyed to the ear, in order to produce the sensation of sound ? Describe the nature of sound-waves. What name is given to these waves? In what respects is the motion of sound-waves through air, or any other body, like the motion sent through a row of glass balls touching one other, when one of the end balls is struck ? 154 NATURAL PHILOSOPHY. Why is a medium necessary to transmit sound ? Will any elastic medium transmit sound ? Describe any experiment which proves that strings or wires transmit sound. What is the velocity of sound in air ? How is the velocity of sound in air affected by the temperature ? Have all ordinary sounds the same velocity in air ? How can you prove this ? How does the veloc- ity of sound in water, or in elastic solids, compare with its velocity in air? How does the angle of reflection compare with the angle of inci- dence ? What IS the cause of echoes ? How far must the reflecting surface be from the source of sound in order to give an echo of a single syl- lable of articulate speech? What is the cause of multiple echoes? What are whispering galleries ? What is meant by the refraction of sound? What is the difference between the reflection and the refraction of sound ? What effect is produced on the intensity of sound by the distance from the source of sound ? Why can sounds be carried so much further through speaking-tubes, or through wires or strings, than through the open air ? CHAPTER 11. THE CHARACTERISTICS OF MUSICAL SOUND.— MUSICAL INSTRUMENTS. 175. Musical Sound and Noise. — When the vibra- tions of the sounding body are regular and periodical, that is, when the time of each vibration is the same, a musical sound is produced ; but when the time of vibration is irregular, a noise is heard. Noises may be momentary^ that is, may produce an effect on the ear of too short a duration to be measured, as, for example, the report of a gun ; or they may be continuous, such as those caused by the rolling of thunder, or the saw- ing of a board ; continuous noises are produced by a mingling of many discordant musical sounds. The unpleasant effect produced by noises on the ear has been com- pared to a similar effect produced by a flickering light on the eye. In both cases, the unpleasant feeling is most probably caused by the sudden and abrupt changes which these organs transmit to the brain. 176. Musical Sounds produced by Regular Im- pulses. — That regular impulses will produce musical sounds when they follow each other with sufficient rapidity, is a matter of common experience. As each tooth in a circular-saw strikes the board it is sawing, a momentary sound is produced ; but if the saw is mov- ing with sufficient rapidity, a more or less musical note is heard. 155 156 NATURAL PHILOSOPHY. Experiment — Draw the blade of a penknife over the milled edge of a large coin ; if the motion be sufficiently rapid, the separate taps will produce a musical tone. 177. The Characteristics of Musical Sounds. — A very little observation will convince us that all sounds are not the same. They differ from one another in a ' variety of ways. These differences may, however, all ^ be traced to three peculiarities or characteristics, viz., i the intensity., the pitch, and the quality. 178. Intensity. — By the intensity of a soxind we ■ mean the peculiarity that enables us to distinguish be- tween tones that are loud or feeble. The intensity of a i sound is dependent on the amplitude of the waves pro- ducing it, which, in sound-waves, depends upon the extent to which the air is alternately condensed and , rarefied by the vibrating body, or upon the degree of rarefaction and condensation that exist in the sound- waves which reach the ear. When a bell is struck vigorously, it gives a loud sound, because the distance through which its sides swing is comparatively great, and the air around the bell is considerably condensed or rarefied. Experiment Stretch a wire firmly between two stout hooks se- , curely fastened to the top of a table. Pluck the wire gently with the fingers, and a musical note will be heard. Now pluck the wire at the same point, but with more force, and a note will be heard louder than before, but neither higher nor lower. The two notes differ in their intensity or loudness. Caution . — The wire must be quite tight. If a stout screw can be obtained, the wire may be fastened to the hook at one end, and the other end be secured to the screw, and tightened by moving the screw in the proper direction. 179. The Speaking-Trumpet is used to allow the voice to be heard at great distances. It is conical in shape, and has a trumpet-shaped end. The small end is held to the mouth of the person talking. CHARACTERISTICS OF MUSICAL SOUND. 157 Experiment. — Roll a piece of stout pasteboard into a cone; place the mouth at the small end, and talk or sing into it. The voice will be greatly strengthened. Point the cone directly at a person standing in the far end of a room, and whisper ; he will he able to hear dis- tinctly all that is said, while those on either side of the instrument, but nearer it, are unable to hear. 180. The Ear-Trumpet is used to aid deaf per- sons in hearing. It acts bj concentrating the voice on the listener’s ear. Its shape is similar to that of the speaking-trumpet, only the small end is placed in the ear, and the person talks into the large end. Experiment. — Place the small end of the paper cone used in the preceding experiment, in a person’s ear and whisper into the large end, and if no defect of hearing exists, he will hear distinctly. Go to the far end of the room and again whisper, though somewhat louder, and he will still hear what is said. 181. Pitch. — By the ijitch of a musical sound we mean that peculiarity that enables us to distinguish between sounds that are high or ?ow, sharp or grave. The pitch depends on the number of vibrations per second imparted by the sounding body to the air. The greater the number of vibrations, the higher the pitch of the sound, that is, the shriller the note. Thus, when a circular-saw is in motion, the sound it causes is shriller the more rapid its revolution. A wheel furnished with teeth on its circumference, and supported on a suit- able frame, as shown in Fig. 77, may be set in rapid rotation by means of a cord wrapped around its axis. If a card be now held against the teeth, a musical sound will be produced, the Pig. 77.— Savait’s pitch of which will be shriller the Wheel, more rapid the rotation. As the wheel gradually 14 158 NA TURAL PHIL OSO PH Y. loses its motion, and moves slower and slower, tlie pitch, of the note changes in a marked manner. The shorter the length of a string or wire, the more rapid its vibration. The shrill treble notes of a piano are produced by the short, thin strings ; the grave base notes, by the long, thick strings. After the strings of a piano have been struck, the sound will grow fainter and fainter, because the amplitude of the vibration of the string grows less and less. The pitch of the note, however, will not change, since the time of vibration is the same, whatever may be the amplitude of the vibration. Experiment. — Cut out a piece of hard wood, with a flat base and a sharp edge, rather too high to go under the wire stretched across the top of the table, as described in a previous experiment. Lift the wire and place the piece of wood under it, so that the wire will press firmly against the sharp edge. Now, by sliding the piece of wood from one end of the wire to the other, and vibrating the wire between the wood and either end of the wire, it will be found that the shorter the portion of wire vibrated, the shriller will be the note it gives, that is, the higher its pitch. Experiment. — The shrill sound made on a slate or black-board by the pencil or chalk is caused by a series of taps rapidly following one another. The taps can be recorded on the black board as follows. Select a long crayon, and holding it loosely in the fingers at about the middle of the crayon, move it in an arc over the black-board, so as to cause it to emit a shrill sound. Examine the curved line, and it will be seen to consist of a number of separate marks made each time the chalk tapped against the board. By altering the pressure of the chalk on the board, sounds of different pitch may be obtained ; and on examining the line, it will be found that the shriller sounds were those produced by the greatest number of taps in a given time. Caution . — To succeed with this experiment, some little practice may be necessary. It is, however, so beautiful and instructive, that it should, if possible, be shown. 182. The Limits of the Ear. — Though the limits of hearing vary somewhat in different persons, none can hear sounds produced hy fewer than 16 vibrations per second, or more than about -±8,000 per second. If CHARACTERISTICS OF MUSICAL SOUND. 159 the vibrations are fewer than 16 per second, we hear each as a separate tap, or puff ; if they are more than about 48,000, the sound becomes too shrill to be audible. 183. The Siren, — -The exact number of vibrations necessary to produce a given note can be determined by means of an instrument called the siren. A sectional view of this instrument is shown in Fig. 78. The cylin- der, A, is mounted on a wind-chest. At the upper end of the cylinder is fixed a smooth, metallic plate, E. Immediately above the fixed plate, E, is a movable plate, C, attached to an axis, D, which permits it to move freely over the plate, E. Both E and C are pierced at regular intervals with small holes, extending through the plates in an oblique direction, as shown in the figure. The holes in E are equal in number to those in C, but are inclined in the opposite direction. When a current of air is forced through the cylinder, A. the plate, C, and the axis to which it is connected, are set in rapid rotation, and a musical note is produced, whose pitch increases as the speed of rotation becomes greater. This note is caused by the columns of compressed air that are allowed, at regular intervals, to escape through the openings of the plate, E, when they are not closed by the plate, C. The number of columns of compressed air that escape in this manner will, of course, depend on the number of openings in the plate E. and the speed of revolution of C. To determine the number of revolutions of C, an endless screw, H, attached to the axis, D, moves counters (like those on gas-meters), over graduated dials, by means of the toothed wheels, 0 and 1. In order to use the siren to ascertain the pitch of any note, wind is urged through the cylinder. A, until the pitch of the note given by the siren is the same as that of the note the number of whose vibrations is to be determined. The speed of the siren is now kept constant, and the number of revolutions of the plate during any second ascer- tained. This, multiplied by the number of openings in the plate, E, will give the number of vibrations per second required to produce the given note. t Fig. 78. — The Siren. 160 NATURAL PHILOSOPHY. 184:. The Quality. — When we sound the same note with equal loudness on two different musical instru- ments, as, for example, on a piano and on a flute, al- though the notes are of the same pitch and intensity, yet there is something which enables the ear to dis- i tinguish one note from the other. So, when two per- I sons are speaking in the same tone, we can recognize a ' difference in the sounds produced ; these peculiarities ! are known as the quality of the sounds. \ 185. Cause of Differences of Quality. — When sin- gle notes are produced by any musical instruments, we are generally able to hear but a single sound, that is, a sound of a given pitch. In nearly all cases, however, accompanying the sound of the note struck, are a number of other sounds of different pitch, but of so feeble intensity that, unless we listen attentively, we are unable to distina-uish them. These additional O tones are called overtones. Although we do not hear them as separate tones, yet, mingling with the tone of ‘ the note struck, they impart to it a peculiarity of tone which we recognize as its quality. Differences of quality are caused hy differences in the pitch of the overtones, or in their intensity. 186. Sympathetic Vibrations. — Partially raise the top of a piano, and place the foot on the loud pedal; lean over the instrument, and sing any note into it in a loud voice. On ceasing to sing, we wdll bear the same note given back by the piano. Now sing a dif- ferent note, and it will be found that the piano will give back this particular note, and so with others. The sound-waves produced by the voice have struck against all the strings of the piano, but have only set ' in vibration those particular strings that are capable CHARACTERISTICS OF MUSICAL SOUND. 161 of giving sounds of the same pitch as their own. Vibrations so produced are called syrapathetic vibra- tions. The cause of sympathetic vibration is as follows : The sound-waves strike all the strings, and give each a very feeble push. If, now, the time of vibration of any string be exactly the same as the time of vibration of the sound-wave, the next forward impulse which the waves give to the string will be received by it at exactly the time when it is beginning to move forward, and hence the motion it acquired by the first impulse is increased by the second impulse, and as the same is true of all the other impulses, the string at last acquires a consider- able motion, and emits an audible sound. If, however, the string has a rate of motion different from that of the sound-waves, it will some- times receive a forward impulse from the waves, when it is moving in the opposite direction, and its motion being thereby diminished, it can never acquire any very considerable motion. Experiment. — Suspend a heavy weight, of say 10 or 15 lbs., by a string, and let it swing as a pendulum. Note the time of its oscilla- tion. Now, while it is swinging very gently, blow a puff of air against it from the mouth just as it is moving away. Wait until it is again moving away, and give it another puff of air. Do this thirty or forty times, and the pendulum will acquire a considerable increase of motion. While the pendulum is swinging freely, give it the puffs of air when it is just beginning to move towards you, and the motion will be stopped sooner than otherwise. Caution. — As regular and somewhat forced breathing is apt to produce headache, a pair of hand-hellows may advantageously replace the breath. 187. Examples of Sympathetic Vibrations. — Stand near a table on which a number of goblets and glasses of different sizes are placed upright. Sing powerfully a number of different notes. If the proper note be struck, a goblet or glass will give it back again. Two clocks, whose pendulums are exactly of the same length, are hung on the same wall. One clock is started and the other stopped. The movement of the 14* L 162 NATURAL PUILOSOFIIY. pendulum of the one may, it is said, at last move the pendulum of the other sufficiently to start the clock. The gas-lights in a ball-room have been known to ] flicker when in the music of the orchestra certain j notes were sounded. ! 188. Resonance. — We have already seen how the * voice of a speaker is strengthened by the reflection i of sound-waves from the ceiling and walls, or, as it i is called, Ijy the resonance of the room. The effect of \ resonance, however, is more marked when the sound- j waves are enabled to set in vibration some elastic body whose time of vibration is exactly the same as j their own. The strings of a violin or guitar are too thin to set | much air in motion. The notes produced by these J instruments are really due to vibrations of the wood i forming the body of the instruments; and it is on the i elasticity of this wood, and its ability to accept from j the strings different rates of motion, that the value of j the instrument depends. It would be found on trial i that a wire stretched across the corner of a room to the i I walls, would give a much feebler sound than if it were attached to hooks in the top of a table, because the table can take up the motion of the wire much better than j the walls of the room. i ; A mass of air whose dimensions are such as to cn- j able it to vibrate in exactly the same time as a certain j sound, will be set in motion by the sound, and greatly || increase its intensity by its resonance. Resonance of jj this kind is, therefore, dependent for its action on li sympathetic vibrations. i ! In order to increase the intensity of the overtones | i of a note sufficiently to enable them to be distinctly CHARACTERISTIC S OF MUSICAL SOUND. 163 heard, instruments called resonators are employed. They consist of hollo^v spheres of brass, of the shape shown in Fig. 79, Avith openings at a and h ; one of these, n, is placed in the car, and at the other the sound-waves enter. If there be present in the tone an overtone whose l'ig'79'“E,esoiiator. rate of vibration is exactly the same as the rate in Avhich air contained in the sphere can best vibrate, the resonance of the sjDhere will cause the sound to be dis- tinctly heard. The resonant case on which a tuning- fork is mounted, should contain a column of air whose rate of vibration is exactly that of the fork. Across the end of the tube leading from the outer air into the ear is a tightly stretched drum-head, called the tympanum. The column of air contained within this tube is. capa- ble of resound- ing to and greatly strengthening cer- tain sounds by res- onance, as is par- tially illustrated by the following Experiment Tie two strings to a poker, or a bar of iron or steel, some little distance from the ends, as shown in Fig. 80. — An Experiment in Eesonance, Fig. 80. Hold the ends of the string over the end of one of the fingers of each hand, letting the poker hang in a horizontal position. 164 NATURAL P [I ILOSOrUY. Now insert in the ears the fingers holding the strings, being careful toj J avoid allowing the strings to rest against the body. Strike one end of the poker against the wall, or have some person strike it near the' middle, and a sound will he heard like that of a larrje hell, when you are very near it. Here the vibrations of the poker are transmitted through the strings to the columns of air within the ears, which by resonance strengthen the sound. The effect is to a great extent due to the vibrations being carried through the bones of the head directly to the ear. Caution. — Allow the whole weight of the poker to be supported by the string. Avoid having the string partly supported by the ear. ^ The air which fills any hollow body will respond to J some particular note. When a shell is held to the ear a sound is heard, which a pretty superstition of child- 1 hood regards as thef imprisoned sounds of | ocean waves beating * against a shore. The sounds are really caused by the air with- 1 in the shell strength- ' euing by its resonance *■ the feeble sounds that, are always present in i the air. Similar sounds i may be heard by hold- ing an empty pickle - 1 iar or tomato-can near | the ear. Experiment. — The follow- ^ ing piece of apparatus, well * known to most boys, mav be 1 Fig, 81,-Aa Experiment in Eesonance. follows:' Punch a| hole in the bottom of an empty tomato-can. Kun a stout stringl through the hole, and knot the string at the end to prevent its being J pulled out. Holding the can in the hand, as shown in Fig. SI. run the rosined fingers down the string, and a noise far from musical will be CHARACTERISTICS OF MUSICAL SOUND. 165 heard. The vibrations of the string are strengthened by the resonance of the air within the can. Try the same experiment with a shorter can, and it will be found to give a much shriller note. 189. The Interference of Sound-Waves. — When two notes of the same intensity and pitch are sounded together, they sometimes strengthen and sometimes com- pletely obliterate each other. That two sounds could he so put together as to cause silence.^ seems, at first thought, impossible ; but if Ave remember that sound is an effect produced by a Avave- motion, aa'C can easily see Iioav one sound could oblit- erate another, for if one sound-AA^ave is endeavoring to condense the air at the same moment the other is endeavoring to rarefy it, the air AV'ould neither be con- densed nor rarefied, and silence aa'quH result. If, hoAV- ever, each sound rarefied or condensed the air at the same time, the degree of rarefaction or condensation would be increased, and the sound Avould be louder. These effects are knoAAm as the interference of sound- icaves. 190. Musical Instruments. — There are three classes into Avhich musical instruments may be divided, viz., stringed instruments., icind instruments, and instruments in which the sounds are produced by the vibration of plates or membranes. 1. Stringed Instruments. — Examples of stringed in- struments are seen in the piano, harp, violin, violincello, guitar, and banjo. In the piano and harp, there is a separate string for each note. In each of the other instruments, the same string is made to give different notes, by touching it at different points AA’ith the fin- ger, and thus practically shortening its length. The tighter any string is stretched, the shorter its length, 166 NATURAL PHILOSOPHY. and the smaller its diameter the shriller the note which it will give. 2. Whid Instruments. — The sounds produced hv wind instruments are caused by the vibration of a column of air contained within the instrument. The pitch of the note is dependent upon the dimensions of the air column, especially its length, and also upon whether the tube containing the column of air is open at both ends, or at but one end. The column of air within the instrument may be set into vibration in a number of ways, of which we shall consider two of the most important, viz. ; 1st. By means of a mouth-piece, and 2d. By means of a vibrating plate called a reed. In the organ-pipe, the vibration of the air column is produced by the action of a mouth-piece. The organ-pipe is placed on a box called the wind- chest, supplied with air from a bellows. The air entering through the opening, a, Fig. 82, passes through a narrow slit, c, and escapes at the open- ing, 0 . The air does not flow out of this opening in a continuous stream ; for, on striking against the bevelled lip, r, it is broken into a flutter or series of puffs, the rate of which is controlled by the length and size of the column of air in the pipe. A reed is a thin, vibrating plate of any elas- tic material, which is moved backwards and for- wards by the air. The notes of reed-organs, accordions, and Jews-harps are caused b}' the vibrations of reeds. Fig. 82, — An Organ- Pipe. I ! 1 I' h Experiment. — Cut a stout wheaten straw into a length of about four inches from the knot. With a sharp penknife, cut a slit, a, down to the knot, h, as t shown in Fis;. S3. Ivow Fig. 83. — A Straw Seed. i b “ place the mouth completely over the cut part and blow, and a musical note will be heard. The CHARACTERISTICS OF MUSICAL SOUND. 167 pitch of this note will increase, if the length of the straw tube is short- ened by cutting a piece off the open end. Caution. — If the pipe will not give a note at once, do not throw it away. A certain pressure with which the air is driven through it is necessary in order to make the pipe sound. Experiment. — Prepare a straw reed as before, only use a longer straw, and cut holes in the side about one inch apart. Sound the reed when the holes are all open, and remember the pitch of the note. Now, with one finger close the hole nearest the knot end, and the pitch of the note jirodueed will he lower: close the next hole, still keeping the finger on the opening previously closed, the note will be still lower. These effects are the same as would be produced by lengthening the pipe. Hcnec, we may conclude that in any wind instru- ment, the length of the vibrating column is affected by openings in the side of the instrument. Caution. — Blow with as nearly the same force in each case as possi- ble, as different notes are produced in the same pipe by blowing with different degrees of strength. In the flute, flageolet, and fife, the different notes are produced by virtually altering the length of the air column by opening or closing holes in the side of the instrument. 3. Musical Instruments whose notes are due to the vibrations of plates or membranes. In the musical-box, the notes are produced by the vibrations of a series of steel plates of different lengths, which are set into motion by pins projecting above the surface of a revolving cylinder. In the xylophone, the notes are produced by the vibrations of plates of wood of different lengths. The notes of the cymbal are produced by the vibrations of brass plates ; those of the drum, by the vibrations of a membrane. Syllabus. The vibrations which produce a musical sound, follow one another at regular intervals of time ; those which produce noise, follow one another at irregular intervals. 168 NATURAL PHILOSOPHY. I The unpleasant effect produced on the car by noisy sounds is similar to that produced on the eye by a flickering light. There are three characteristics of musical sound, viz., intensity or loudness, pitch, and quality. The intensity of the sound is that peculiarity in virtue of which sounds are loud or feeble. The intensity of a sound is dependent on the amplitude of the vibrations causing it. By the amplitude of a sound-wave, we mean the degree of condensa- tion or rarefaction in the wave. The speaking-trumpet is used to enable tbe sound of the voice to be heard at a great distance. The ear- trumpet is used to aid deaf persons in hearing. By the pitch of a tone, we mean the peculiarity that enables us to distinguish between sounds that are high or low, sharp or grave. The pitch of a sound depends on the number of vibrations per second, that is, on the rapidity with which the sounding body is vibrating. We cannot hear sounds that are produced by fewer than 16 or more than 48,000 vibrations per second. By the qnality of a musical sound is meant that peculiarity which enables us to distinguish between two notes of the same pitch and in- tensity when sounded on different instruments. The differences in the quality of musical sounds are caused by the differences in the pitch and intensity of the overtones which accompany them. Sympathetic vibrations are vibrations of the same period that are produced in surrounding bodies by sound-waves. Sounds are often strengthened by the resonance of elastic substances in which the sound-waves produce sympathetic vibrations. The’ notes given by the strings of a violin or guitar are greatly strengthened by the resonance of the wood forming the body of these instruments. The column of air within the tube of the human ear strengthens certain sounds by its resonance. The confused murranrings heard when an empty shell is held to the ear are caused by the faint noises present in the air being strength- ened by the resonance of the air within the shell. By the interference of sound-waves, we mean the strengthening or weakening of the amplitude of one wave by another. Musical instruments may be divided into stringed instruments, wind instruments, and those in which the notes are produced by tbe vibration of plates or membranes. The piano, harp, guitar, violin, violincello, and banjo are examples of stringed instruments. The notes of wind instruments are caused by the vibrations of a column of air contained within the body of the instrument. This column may be set into vibration, 1st. By a mouth-piece ; 2d. By a reed. QUESTIONS FOR REVIEW. 169 An organ-pipe is a good example of a musical instrument where the air is set in vibration by a mouth-piece. In the accordion, melodeon, and Jews-harp, the sound is caused by a reed. The musical-box, xylo- phone, cymbal, and drum are examples of the third class. Questions for Review. What is the difference between musical sounds and noises ? What are the three characteristics of musical sounds ? Define intensity or loudness. By what are differences in the in- tensity or loudness of sounds caused ? Describe any experiment by means of which variations in the intensity of sounds can be shown. Describe the speaking- and ear-trumpets. For what is each used? How may some simple experiments with these instruments be shown ? Define pitch. By what are differences in the pitch of musical sounds caused ? What is the gravest sound the human ear can hear ? What is the shrillest ? How may the sound produced by rubbing a piece of chalk on a black-board be made to record its vibrations ? Describe the siren. Define quality. By what are differences in the quality of musical sounds caused? What is meant by overtones ? Explain in full what is meant by sympathetic vibrations. Give some examples of sympathetic vibrations. How may the strength of any sound be increased by resonance ? What has the body of a violin or guitar to do with the strengthening of the sounds of the strings ? Describe any experiment which shows the power of the column of air within the ear to greatly increase the intensity of certain sounds by its resonance. For what purpose are resonators used? What is meant by the interference of sound ? How can it be pos- sible for two notes sounded at the same time to produce silence ? Into what three classes may musical instruments be divided? Name some examples of each class of instruments. In what different ways may the column of air in wind instruments be set into vibration ? By which of these methods is the air in an organ-pipe set into vibration? By which in a melodeon or accor- dion? 15 CHAPTER III. THE NATURE OF HEAT.— THERMOMETERS AND EXPANSION. 191. The Cause of Heat. — The molecules of matter are never at rest, but are constantly moving towards and from one another. Heat is caused by these motions. In hot bodies the molecules are vibrating very rapidly through distances that are great when com- pared to tbe size of the molecules, wliile in cooler bodies they are not moving so energetically, or through as great distances. The cause of heat is, therefore, similar to the cause of sound, since each is produced by a vibratory motion of matter. The nature of this motion, however, is different. When bodies vibrate so as to cause sound, the whole mass of the body moves ; when they vibrate so as to cause heat, it is only the molecules that vibrate. 192. The Luminiferous Ether. — The bell shown in Fig. vd cannot be heard if struck when the vessel, A, is emptied of all the air rvhich filled it, since then there is nothing around the sounding bell which it can set into waves. But if we hang a hot body in ichat seems to us to he an empty sjoace, we find that the heat readily yiosses through this space. For this and other reasons we believe that all space, even though it appears to be empty, is filled with something which is set into wave- 170 THE NATURE OF HEAT. 171 motion by tbe vibrations of the molecules of heated bodies. This something which fills all space is called the luminiferous ether.^ and it is by vibrations., or waves in it, that heat is transmitted f rom one place to another. It is called the luminiferous ether because it also trans- mits light by its vibrations. The luminiferous ether fills all space, even that between the molecules of matter. The molecules of hot bodies, by their shahinys, cause waves in this ether, just as the shakings of the sides of a bell cause waves in air, and when these ether-waves strike us, we have the sensation of heat. Since the ether can pass through the spaces between the molecules, and as matter is porous, it is evident that v/e cannot mate a vessel ether-tight, that is, wo cannot prevent the ether from passing into or out of it. 193. General Effect of Heat. — Although we can- not see the shakings of the molecules of a body, yet we can see the efi:ect which these shakings produce. The hotter the body gets, the more energetic becomes the motion of the molecules ; and in this way the force of molecular attraction is partly overcome, and the body expands. When a body loses heat, the molecular motion becomes less energetic, attraction again draws the molecules together, and the body contracts. Whe7i, therefore, matter is heated, it expands, or grows larger, because the molecules are made to swing through greater distances. 194:. Temperature. — When two bodies can be placed in contact without any heat passing from one to the other, they are said to be at the same temperature. When heat passes from one to the other, the body which gives the more heat is said to be at the higher temperature. 172 NATURAL PUILOSOPHY. The temperature of a body is measured by the ther- mometer. 195. Thermometers. — The thermometer depends for ^ its operation on the expansion of a liquid contained in a glass tube. The liquid most commonly employed is mercury.^ though alcohol is sometimes used. The thermometer consists of a long, straight tube, A i?. Fig. 85, of small inter- nal diameter, closed at the upper end, and widened at the lower end into a bulb, C. The bulb, (7, and part of the tube con- tains mercury ; the space above the mer- cury in the tube is a vacuum. ^Vhen the thermometer is taken into a hot place, the mercury, becoming warmer, expands and its level rises ; but when c taken into a cold place, it loses heat, con- I®' tracts, and its level falls. Equal spaces, called degrees, are marked on the tube. 196. Construction of a Thermometer. — While the tube is open at the top, the bulb and part of the tube are filled with cold mercury. The bulb is then cau- tiously heated. As the mercury grows hotter, it ex- pands, and, filling the tube, drives out all the air, and begins to flow over the top. The heat is now taken away from the bulb, and at the same time the upper eiid is completely closed, by being melted in the flame of a blow-pipe. As the bulb cools, the mercury falls in the tube, leaving a vacuum or empty space above it. The tube is next yraduated^ or divided into degrees. For this purpose, the bulb is dipped into melting ice, and a mark made at the point on the tube to which the 212 ° 32° 100 ° 0 ° Fig. 84. Fig. Fahren- Cen heit. grai THE NATURE OF HEAT. 173 mercury sinks. Tlie bulb is then exposed to the steam rising from boiling water, and another mark made at the point to which the mercury rises. These two points are called respectively the freezing and the boil- ing points. The length of the tube betv:een these two points is then divided into a certain number of equal parts called degrees.^ and the rest of the tube, that is, the part below the freezing point and above the boil- ing point, is divided into degrees of the same length. There are two thermometer scales in common use, viz., the Fahrenheit and the Centigrade. These two scales are shown in Figs. Si, 85, In the Fahrenheit scale, Fig. ST, the length of the tube between the freezing and boiling points is divided into 180 equal parts called degrees. The freezing point is marked 32°, and the boiling point 212°. In the Centigrade scale, Fig. 85, the distance between the freezing and boiling points is divided into 100 equal parts. In this scale the freezing point of water is marked 0°, or zero, and the boiling point 100°. De- grees of the Fahrenheit scale are indicated by an F. or Fah. ; those of the Centigrade scale by a C. Thus, the freezing point of water is 32° F, or 0° C. 197. Uses of Thermometers. — We cannot entirely rely on our sensations to determine the difference between the temperature of two bodies. If, Avhen the hand is very warm, Ave plunge it into a basin of tepid AVciter, the AAmter Avill feel cool ; but if the hand is quite cold Avhen Ave plunge it into the same tepid Avater, the Avater Avill feel warm. Again, if in Avinter we come from the cold air outside into the entry or hall, the entry or hall feels Avarm ; but if Ave go into the entry from the Avarmer parlor, the entry then 15 * 174 NATURAL PBILOSOPUY. feels cold, Tlie indications of a thermometer are not open to these objections. So, also, in a room where there is no fire, and all things are at the same temperature, the marble mantle feels cold to the hand, while the hearth-rug feels warm. W e see, also, from these remarks, that the words hot and cold are but relative, since the same body may he hot when compared with one body, and cold when com- pared with another. j 198. Expansion of Solids. — Solids expand le.'JS j than liquids, and liquids less than gases. Different i solids expand differently in amount when heated one | degree. Zinc, lead, and tin are among the most ex- j pausible of the metals, and steel and platinum among ' the least. Ice is more expansible than zinc. Most ^ crystalline solids expand more in one direction than I in another ; while most bodies that are not crystallized, expand equally in all directions. 199. Examples of the Expansion of Solids. — When solid bodies expand or contract, they exert con- siderable force. The tires of wheels are made of such , a size that, Avhen cold, they will not go on the wheel. ' They are heated until they are large enough to slip on easily, and when cold they contract and fit very tightly to the Avheel, and firmly hold its parts together. The rails of railways are laid with some little dis- tance between their ends, so as to leave room for ex- pansion ; but for these spaces, the force of expansion would twist the rails from the Avooden ties. The snapping and crackling of a stOAm when sud- denly heated, as by building a fire, or cooled as by opening the door, is caused bj' the unequal expansion of the metal at different parts. THE NATURE OF HEAT. 175 Thick glass-ware is apt to break when suddenly heated or cooled, because the unequal expansion or contraction overcomes the cohesion of the particles. Suppose, for example, hot water be poured into a thick glass tumbler, then the inside of the glass expanding- before the outside becomes warm, is very apt to break the tumbler. Experiment. — A glass vessel can be cut into any desired shape as fol- lows ; Suppose it is desired to cut off the top of a broken bottle. Start a crack in the edge of the bottle by heating and suddenly cooling it. From the end of this crack, draw a chalk line in the direction in which it is desired to cut the bottle. Heat the end of a thin poker red hot, and hold the heated point firmly against the glass, on the chalked line, and a little distance from the crack. In a moment a click will be heard, and the crack will extend to the point of the poker. Lift the poker from this point, and place it again on the chalked line a little beyond the crack, and so on until the crack has extended all round. Caution . — When the poker has become too cold, reheat. With some little practice, broken glass vessels can be given a variety of useful shapes by this method. 200. Expansion of Liquids. — Most liquids expand when beated and contract when cooled, and this is true whatever the temperature. Water, however, presents some curious exceptions to this general statement. When water which is at the temperature of melting ice, or 32° Fah., is heated, it contracts and gets denser, and this continues until it reaches the temperature of 39°.2 Fah.; when heated above this point, however, it expands like most other substances. The temperature of 39°.2 Fah. is called the temperature of the maximum or greatest density of water. When water is at the temperature of its maximum density, it will expand whether it be heated or cooled. The general effect produced in all matter by an increase of temperature is to cause it to expand, while a decrease of temperature causes it to contract. 176 NATURAL PHILOSOPHY. Experiment. — Fit a narrow glass tube, A, Fig. 86, tightly into a cork, B, and, inserting the cork into the neck of a large bottle, C, fill the bottle and tube with water to the point A. Stand the bottle in a pan, D, and sur- round it by layers of broken ice and salt, or snow and salt. As the water is cooled, the column of water in the tube will fall, thus proving that the water is contracting. But when the water is cooled to 39°. 2 Fah., the column will cease falling, and will begin to rise, al- though the water is still growing colder. Caution. — This experiment will be more successful, the larger the bottle and the Fig. 86.— Expansion of Water smaller the diameter of the tube. To enable those at a distance to see the water in the tube, it may be slightly colored with bluing. 201. Effect on the Freezing of Large Bodies of Water. — Let A, Fig. 87, be a deep lake of fresh water, in which the water throughout the whole lake is at Fig. 87.— Effect of Maximnin Density on Freezing. the temperature of 50° Fah. If, now, the air grows cold, the water at the surface of the lake grows cold and falls, because it becomes heavier than the other water. When, however, all the water in the lake reaches the tempera- ture of 39°.2 Fah., it is as heavy as it can yet by loss of heat. If the water near the surface now continues to lose its heat, it will grow lighter, and will remain at the surface until changed into ice. Since water con- ducts heat poorly, the water at the bottom remains unfrozen. Were it not for this peculiarity in its ex- pansion, the water throughout the entire lake would continue to cool until the Avhole mass became solid, Avhen not even a summer’s heat would entirely melt it. THE NATURE OF HEAT. 177 202. Expansion of Gases. — When our atmos- phere is lieated, it expands ; when cooled, it contracts. When any mass of air is heated it expands, and, be- coming lighter than the surrounding air, rises, while the cooler air on the sides blows in towards the place from which the heated air has risen. Winds are caused in this way. The draught in a chimney is caused b}'- the air within the chimney being heated and rising, and the cooler air rushing in through the fire to take the place left by the rising air. Experiment. — A large bottle, A, of thin glass, is pro- vided with a narrow glass tube fitted to a cork, and inserted in the neck of the bottle. Hold the bottle for a few minutes near a gas flame or hot stove, and then quickly place it, as shown in Fig. 88, with the end of the tube dipping below the surface of some colored liquid placed in a vessel, £. As the air in the bottle cools, it will contract, and the pressure of the outside air will cause a column of water to mount in the tube. If, how- ever, the air in the bottle be again heated, as by holding the hand against it, the expansion of the air will drive the column of colored liquid down. This apparatus will, therefore, act like a thermometer ; only the column of colored liquid falls when the air is warm, and rises when it is cold. It also in part acts like a barometer. Caution. — Do not hold the bottle too long by the fire, or else, when placed in B, the liquid will rise and completely fill the tube. Fig. 88. — Ex- pansion and Contraction of Air. Syllabus. Heat is caused by the vibrations of the molecules of matter. Heat is transmitted by means of waves produced by the vibrations of the molecules in a medium called the luminiferous ether. This medium is believed to fill all space, and even to penetrate the invisible pores be- tween the molecules. M 178 NATURAL PHILOSOPHY. We cannot make a vessel ether-tight because we do not know of any substance whose molecules have no spaces between them. The general effect of heat upon matter is to cause it to expand. It expands because, when heated, the molecules vibrate through greater distances. We measure temperature by means of an instrument called a ther- mometer. The thermometer depends for its operation on the expan- sion of a liquid contained in a tube. The space above the mercury in a thermometer is vacuous. The tube is graduated into degrees by dividing the length between the freezing and boiling points into a given number of parts. In the Fah- renheit scale this length is divided into 180 parts, and in the Centi- grade scale into 100 parts. We cannot entirely rely on our sensations to distinguish between hot and cold bodies, since bodies at the same temperature may some- times feel hot and sometimes cold. Solid bodies exert considerable force in expanding and contracting. Tires are shrunk firmly on wheels by heating the tires until they are large enough to slip on the wheels, and then cooling. When water at the temperature of 32^ Fab. is heated, it contracts until it reaches the temperature of 39.2° Fah., which is called the temperature of its greatest density. This peculiarity of water prevents large bodies of fresh water from becoming frozen throughout. Winds are caused by the expansion of air by heat. When any mass of air becomes heated, it expands, grows lighter and rises, while the cooler air blows in from all sides. The draught of a chimney is caused by the expansion of the air within the flue. A simple thermometer may be made by fitting a tube tightly in the neck of a large bottle, gently heating the bottle, and then dipping the end of the tube into a vessel filled with water or colored liquid. Questions for Review. What is the cause of heat? By what means is heat transmitted from one place to another ? How do the vibrations which cause heat differ from those which cause sound ? What is meant by the luminiferous ether ? Why do we think that it exists ? Define expansion. Why should bodies expand when they are heated ? QUESTIONS FOR REVIEW. 179 When are two bodies said to be at the same temperature? How is temperature measured ? Describe the construction of a thermometer. What method is gen- erally adopted for filling the bulb ? How are the freezing and boiling points obtained ? How is the length of a degree for any thermometer determined? Explain the difference between the Fahrenheit and the Centigrade scale. Why can we not entirely rely on our sensations to determine the difference of temperature between any two bodies? Show that hot and cold are relative terms. Name some of the most expansible metals ; name some of the least expansible. Give any instances of the practical application of the force exerted by the expansion or contraction of solids. How may a broken glass vessel be cut into any desired shape? Name any example of the expansion of a liquid. What exception to the general statement that bodies expand by heat does water present? What effect has this fact on the freezing of large bodies of water ? Why ? Describe an experiment by which it can be shown that water some- times contracts on being heated. Explain the cause of winds. Why does air rise when it is heated? What is the cause of draught in chimneys? Whj’ is the draught stronger when the fire is hotter ? Describe in full the construction of a simple thermometer from a bottle, tube, and vessel of liquid. What causes the liquid to fall in the tube ? What causes it to rise ? CHAPTER IV. THE COMMUNICATION OF HEAT. — THE SURFACE ACTION OF BODIES. 203. The Communication of Heat. — Heat may i be communicated or transferred from one body to another in three ways, viz., hy conduction^ by convec- tion^ and hy radiation. Heat is mainly communicated or distributed through solids by conduction ; through liquids and gases mainly by convection and radiation. 204:. Conduction of Heat. — If one end of a bar | of iron, copper, or any other metal, be placed in a fire, j the other end will after awhile become too hot to be \ held. The heat of the fire has been communicated to i the rod, and carried through it by conduction. When the molecules at the heated end of the bar are set into motion by the heat of the fire, they grad- ually impart their motion to the molecules beyond them, and in this way the heat is conducted through the body. The rapidity of the conduction from one end of a bar to the other is the same whether the bar be straight or bent. I 205. Differences of Conductivity. — The rapidity j with which heat is conducted varies greatly in differ- j ent solids. If two rods of the same length and thick- i ness, one of copper and the other of iron, be put to- 180 THE COMMUNICATION OF NEAT. 181 Fig, 89. — Unequal Conduction of Copper and Iron. gether, witli one end of each in the fire, the heat will be conducted through the copper rod much more rapidly than through the iron one. Again, if a rod of glass be used in the same way, it will be found to be a very bad conductor of heat. Experiment. — Get two bars — A, of copper, and B, of iron — of the same length and thick- ness. By means of wax, stick a number of marbles or buck-shot to each at equal distances, as shown in Fig. 89. Now expose one end of each bar to the same source of heat, as, for example, the flame of an alcohol-lamp, and it will be observed that more balls will fall in the same time from the copper bar than from the iron, thus showing that it is the better conductor of heat. Experiment — An alcohol-lamp can be easily made as follows : Bore a hole through a fine-grained cork, and insert in it a small piece of glass tubing, or a bit of tin rolled in the shape of a tube, and through this pass a piece of wick. Place the cork and wick in a bottle filled with al- cohol. Experiment. — Where illuminating gas is at hand, a more convenient source of heat is furnished by the Bunsen burner, one of which can be easily made as follows : Take a piece of tin in the shape of a rectangle, and cut out small pieces from one of its shorter ends, so that, when the tin is rolled in the form of a hollow tube, as shown at B, Fig. 90, there will be openings provided as at A. Fit the end, C, loosely over a gas-burner. Turn on the gas, and light it from above. If the burner has been properly made, the flame will be bluish and almost non-lumi- nous, and will not soot articles heated in it. Air enters at the openings below, and burns the gas more thoroughly than an ordinary gas-burner. This flame is very hot; glass tubes held in it may be softened and bent in any desired shape. Caution. — If the burner is not well made, the gas will often ignite at the gas-burner, and burn with a luminous flame. The fault will 16 Fig. 90.— A Simple Bunsen Burner. 182 NATURAL PHILOSOPHY. generally be found to be due either to the holes below not being large enough, or the tin tube not being of sufficient length. 206. Applications of the Conductivity of Solids. — When a hot body is placed in contact with a cold body whose conducting power is good, or if a cold body be placed on a hot conductor, the cold body may become of the same temperature as the hot body. If we wish to keep a body at the same temperature for any time, we must surround it by some substance that conducts heat very ^worly. Ice is wrapped in blankets to keep from melting, because the blankets are poor conductors of heat, and therefore prevent the heat from entering. We wear thick woollen clothes in winter to keep the heat of our bodies from escaping rapidly. Thin muslin or linen clothes are comfortable in summer because they readily permit the heat of the body to escape. Ice-hoQses are made with thick, double walls, filled with shavings or saw-dust, to keep the outside heat from entering. Fire-proof safes have double or triple walls filled with some poor conductor for the same purpose. The interior of the earth is believed to be very hot; very little of the heat, however, reaches the surface, owing to the low conducting power of the materials of the crust of the earth. 207. Conduction of Fluids. — Liquids and gases are very bad conductors of heat. When heated from below, they grow hot by a process called convection; but when heated from above, they conduct but very little heat downwards. The poor conducting power of water may be shown by the following THE COMMUNICATION OF HEAT. 183 Experiment. — Insert the tube of the simple thermometer, described in a previous experiment, in a cork, and place it in the small end, A. of a funnel, the neck of which has been cut off. Fill the funnel with water, so as to cover the bulb, B, to the depth of about one-fourth of an inch, as shown in Fig. 91. Pour common ether on the water and set fire to it, and the heat, although rather intense, will scarcely be conducted downwards through the water enough to cause the level of the column of colored water at h to fall. Caution . — Select a funnel of as thin glass as pos- sible, so as to avoid its being broken by the heat. 208. Convection, — • Wlien a vessel containing a liquid is placed over a source of heat, the liquid at the bottom pig.gi. — vTater of the vessel, by touching the hot bot- tom, is heated, and expanding becomes lighter and riseS) its place being filled by some of the cooler portions of liquid falling. These in turn be- come heated, and are replaced by other portions, until at last the whole mass of the liquid is at the same temperature throughout. Gases are heated in the Poor Conductor of Heat. same way. Liquids and gases, therefore, when heated are so stirred about by the heat, that all parts are brought into contact with the sides of the vessel. This method of communicating heat is called convection. Liquids are cooled in a similar manner. Experiment. — Place a lump of ice in a tumbler of tepid water. As the water which touches the ice is cooled, it be- comes heavier, and, sinking, pushes up the warmer and lighter water from below. By watching the water closely, these currents can be seen, especially if there are any little specks of dirt in the water. Their general direction is shown in Fig. 92 by the arrows. During convection, the warmer por- Fig. 92. — Convec- tion Currents. 184 NATURAL PHILOSOPHY. tions of tlie liquid or gas always move towards the cooler portions, and tlie cooler portions towards the warmer portions. Winds are huge convection currents caused by the unequal heating of different portions of the earth. The constant currents which occur in the ocean are convection currents in the water, and are caused by 1 differences of temperature between the equator and j the poles. I 1 209. The Radiation of Heat. — If a hot body be | held above the hand, its heat will be felt by the hand. Now, since gases are very poor conductors of heat, ; this heat cannot have been communicated to the hand by conduction ; nor can the hand have received it by convection, since the hot currents of air rise. It must, , therefore, have been communicated in some other way. The cause of heat, as we have seen, is the vibra- tions of the molecules of the heated bod}-. These molecules are surrounded by the luminiferous ether, and by their motion cause waves in this ether. Heat communicated m this tvay, hy the luviinif erous ether., is called radiant heat, and the process is called radia- tion. The heat of the sun crosses the otherwise empty space between the earth and the sun, by means of waves in the luminiferous ether, that is, hy radiation. When these waves strike against a body, if they cause its molecules to vibrate, the body is thus made warm. 210. Heat Radiated in all Directions. — If we stand at the same distance from a stove, but on different sides, we will find, if the stove-door is shut and the THE COMMUNICATION OF HEAT. 185 i stove is of the same material all around, that the in- i tensity of the heat thrown out or radiated is the same i in all directions. If a tin vessel be filled with boiling 1 water, a delicate thermometer held anywhere at the I same distance from the sides will receive the same 1 amount of heat. j The motion of the molecules of heated surfaces is ! communicated through the ether equally well in all ; directions. Heated bodies^ therefore^ radiate their heat I upwards^ downwards, or in any direction equally well. 211. Heat Radiated in Straight Lines. — If the medium through which the heat is passing be uniform throughout, the heat which is radiated from a body passes off from it in straight lines. If a screen, through which heat cannot pass, be interposed between a ther- ' mometer and a source of heat, so that any straight line drawn from the source of heat to the edge of the screen will just pass the bulb of the thermometer, the thermometer will, by its indications, show that it is no longer receiving heat from the source. If, how- ; ever, the screen, be lowered, the thermometer will j rise in temperature. On hot days it is cooler in the shade than in the sunshine, because the heat which accompanies the light moves, like it, in straight lines. A single line of heat is called a ray. When a ray of heat passes from one medium to another of different density, it is bent out of its straight course, or is refracted as with a ray of sound. A common burning-glass owes its power of heating things placed at a certain distance in front of it, be- cause it is so shaped that the rays of heat from the sun are so bent by refraction on passing through it 16 * 186 NATURAL PHILOSOPHY. that when they pass out they all come together at nearly one point. 212. Intensity of Radiant Heat. — The hotter a I body, the more intense the heat radiated from it. As I the molecules of a hot body have a greater amount of ! motion than those of a cooler body, the motion the : hot body communicates to the surrounding ether must , be greater than that communicated by the cooler body, i The intensity of radiant heat is inversely propor- tional to the square of the distance. The further we go from a hot body, the less is the intensity of the heat we receive. If we are twice as far from the hot body at one time as at another, the intensity of the radiant heat will be four times less. This decrease in i the intensity of heat, with the increase of the distance, is similar to the decrease in the intensity of sound. 213. Luminous and Obscure Heat. — The heat which is radiated from a luminous source is called i luminous heat., and that radiated from a non-luminous source obscure heat. Luminous heat almost always contains in addition obscure heat. 214. Effects which Occur at the Surface of Bodies. — When the ether-waves strike against the surface of a body, they are either reflected.^ absorbed, or tram- ■ mitted. 215. Reflection of Heat. — When the ether- waves ■ strike against any suitable surface, they are, like the waves in any elastic medium, reflected from it. The reflection of heat is like the reflection of sound — the angle of reflection is equal to the angle of inci- dence. Bodies vary greatly in their power of reflecting heat. THE COMMUNICATION OF HEAT. 187 I Some, like smootli, polished silver, will reflect nearly all the heat which falls upon them ; while others, like a rough, soot-covered surface, will reflect but little. ; As a rule, bright polished metallic surfaces are good reflectors of heat. Experiment. — Hold a brightly polished flat piece of tin in front of an open fire, so as to catch the light and heat. Notice where the patch of light reflected from the tin falls. This spot will be found to be warmer than the space around it, thus showing that the heat of the fire is reflected as well as the light. Hold the tin so that the light of the fire is thrown into the face, and an increase of temperature will be felt. 216. Absorption of Heat. — When the ether-waves ! strike against the surface of a body, those which are not reflected pass into the body. If, in passing through the body, the waves give up their motion to the mole- cules.^ the body becomes hot, and we say that the heat is absorbed', if, however, the ether-waves in passing through the body simply set in motion the ether that ! occupies the spaces between the molecules, Avithout moving the molecules, the radiant heat simply passes through the body without warming it, and Ave say that the body is diathermanous, or transparent for heat. When a body radiates heat, it gives the motion of its molecules to the ether outside it ; Avhen it ab- sorbs heat, its molecules are set into motion by the ether-waves striking against them. ..If a body is a good absorber of heat, it must also he a good radiator or emitter of heat. Lamp-black is both a good absorber and radiator of heat. If a body is a good reflector of heat, that is, if it throws off most of the ether-Avaves from its surface, it of course cannot be a good absorber. 188 NATURAL PniLOSOPHY. Good reflectors of heat are had ahsorhers of heat, and good ahsorhers are had reflectors. Aiiytliing which increases the reflecting power of a body, therefore, must decrease its absorptive and radiat- ing power, and anything which increases the absorp- tive or radiating power must decrease the reflecting power. Thus, smoothing and polishing diminishes the absorptive and radiating, but increases the reflecting power. The opposite effects are produced by rough- ening or dulling the surface. 217. Applications of the Absorptive, Emissive, and Reflecting Powers. — Coftee and tea are brought to the table in brightly polished pots, which, being good reflectors of heat, are bad radiators, and the contents of the pots Avill therefore keep Avarm longer than if the outside Avas rough and dull. Meat-roasters are so arranged that the radiant heat of the fire is reflected from the surfaces of brightly polished tin on to the meat to be cooked. If the outside of a stOA'e be too brightly polished, much less heat Avill be radiated into the room than if the outside is rough and dull. 218. Selective Absorption. — Not only do different substances Amry in their poAver of absorbing heat, but even the same body vmries in the ability it possesses of absorbing the heat from different sources. Thus, a surface covered Avith Avhite lead will absorb nearly all the heat radiated to it from a copper vessel filled Avith boiling water ; Avhile it Avill only absorb about half of the heat radiated from an oil-lamp. In other words, the same body may vary in its power of ahsorhing lu- minous and non-luminous heat. TEE COMMUNICATION OF HEAT. 189 219. Cause of Selective Absorption. — We have ! seen that sound-waves striking against the strings of a piano may give part of their motion to the strings, pro- vided they can vibrate at the same rate as the sonnd- I waves which strike them, or, in other words, the sound-waves excite sympathetic vibrations in the strings. The same is true with the ether-waves ; if they strike against a body whose molecules are able to vibrate at the same rate that they do, then the heat : is absorbed, since the molecules are set into vibration ; otherwise the ether-waves simply pass through the body by setting the ether between the molecules into motion. 220. Diathermancy. — When the ether-waves pass through a body without heating it, that is, when they pass through without giving their motion to its mole- cules, we say that the body is diatliermanous.^ or trans- parent to heat ; when it will not let the heat so pass through, it is called athermanous.^ or opaque to heat. Clear rock-salt is very diathermauous to all kinds I of heat. Dry air is very diathermauous, but when it contains water vapor, it is rendered less diathermauous. It is mainly, therefore, to the vapor of water which our atmosphere contains that it owes its power of ab- sorbing a part of the sun’s heat. Diathermancy is independent of transparency. Thus, alum, which freely allows light to pass through it, stops obscure heat ; while smoky quartz, which is almost opaque to light, allows heat to pass through it. A solution of iodine in bisulphide of carbon is opaque to light, but very transparent to heat. Experiment. — Hold a piece of common window glass between tlie face and the open door of a fire, and it will be found that the face will feel less hot than when not shielded by the glass ; the glass, there- 190 NATURAL PHILOSOPET. fore, is atliermanous to the heat of the fire. Now hold the glass be- tween the face and the sun, and no perceptible difference will be felt in the heat, whether the face be shielded by the glass or not; the glass, therefore, is diathermanous to the sun’s heat. Syllabus. Heat is communicated in three ways; viz., by conduction, by con- vection, and by radiation. When heat is communicated by conduction, the molecules impart their motion to the molecules beyond them, and in this way the heat is conducted from one part of the body to anotlier. Substances vary greatly in their power of conducting heat ; some are excellent conductors, while others are very poor conductors. When we wish to keep a body at a constant temperature, we gen- erally surround it with some substance that is a poor conductor. Clothes keep the body warm by keeping the heat in. Blankets keep ice from melting by keeping the heat out. Ice-houses and fire-proof safes are made witli double or triple hollow walls, filled with some poor conducting substance, so as to preserve their contents from the effects of external heat. Gases and liquids are very poor conductors of heat, but very little heat being conducted downwards through them. They are generally heated by convection. When a liquid or a gas is heated, the difference of density between the hot and the cool parts causes currents called convection currents, which thoroughly stir the liquid or gas. AVhen a lump of ice is put in a tumbler of water, the warm water is brought into contact with the ice by means of convection currents. When the molecules of a hot bod_v impart their motion to the ether outside them, they are said to radiate heat. Bodies radiate or give off their heat equally in all directions. The intensity of radiant heat is directly proportional to the tem- perature of the body from which it is radiated, and inversely propor- tional to the square of the distance from the radiating body. Heat is radiated in straight lines as long as it passes through a uniform medium. When it passes from one medium to another of different density, it is turned out of its course or refracted. Bodies held at a certain distance from a burning-glass are heated because the rays of the sun are so refracted by passing through the glass that when they come out they all collect at nearly one point. QUESTIONS FOR REVIEW. 191 Heat from a luminous source is called luminous heat ; that from a non-luminous source, obscure heat. Luminous heat is generally ac- companied by obscure heat. When the ether-waves strike against the surface of a body, they are either reflected, absorbed, or transmitted. When heat is reflected, the angle of reflection is equal to the angle of incidence. Polished metals are good reflectors. The incident beat which is not reflected passes into the body. If, in passing through it, the ether-waves give their motion to the molecules of the body, the body is warmed ; and we say that the heat is absorbed. If the waves pass through a body without giving their motion to the molecules, the body remains cold, and the heat is transmitted. The body is then said to be diathermanous. Substances which possess high reflecting powers have but feeble powers of radiation or absorption, while those which have high ra- diating or absorptive powers possess but feeble reflecting powers. The absorptive and diathermanic powers of the same substance vary with different kinds of heat. Many substances which will ab- sorb or transmit obscure heat very well, absorb or transmit luminous heat either very slightly or not at all. Clear rock-salt is diathermanous, or transparent to all kinds of heat. A solution of iodine in bisulphide of carbon is opaque to luminous heat, but transparent to obscure heat. Questions for Review. In what three ways may heat be communicated ? By which of these is heat communicated in solids? By w'hich are liquids and gases heated ? Name some good conductors of heat ; name some poor conductors. How may poor conductors be employed to keep the temperature of a body uniform? Describe any experiment showing the difference in the conducting power of different substances. Explain how a simple Bunsen burner may be constructed. Are liquids good conductors of heat ? How can this be shown ex- perimentally ? Define convection. By what are convection currents caused? Give an example of huge convection currents in air and in water. What is meant by the radiation of heat? By what means do bodies radiate their heat ? How does the sun’s heat reach the earth ? 192 NATURAL PHILOSOPHY. Is heat radiated any better in one direction than in another ? Upon what does the intensity of radiant heat depend ? How is the intensity of heat affected by an increased distance from ; the body radiating the heat ? Is heat radiated in straight or curved I lines ? How can you prove this ? I Distinguish between luminous and obscure heat. Define reflection ' of heat. Is the reflection of heat similar to the reflection of sound? \ When are the ether-waves which strike the surface of a body ab- 1 sorbed? When are they transmitted? ^ What name is given to a substance which transmits heat? What ' is it called when it does not transmit heat? Why are good reflectors of heat poor radiators and absorbers ? Why are good radiators and absorbers poor reflectors ? Will coffee and tea keep hot longer in brightly polished pots, or in dull, tarnished ones? Why? What should be the nature of the - surface of a stove in order that the heat may readily escape into the room ? What is meant by selective absorption ? By what is it caused ? What effect is produced in the reflecting, absorptive, and emissive powers of substances by polishing them? What by roughening them? , Name any diathermanous substance. Is dry air diathermanous or 1 athermanous? Is moist air diathermanous or athermanous ? CHAPTER V. CHANGE OF STATE. — LATENT AND SPE- CIFIC HEAT. — MECHANICAL EQUIV- ALENT OF HEAT. 221. Fusion and Solidification. — The force of heat is opposed to that of cohesive attraction^ so that, if suf- ficient heat is added to a solid, it becomes changed into a liquid ; while, on the other hand, all liquids, when caused to lose sufficient heat, are changed into solids. These processes are called /wsfow and solidification. The amount of heat in a body, as indicated by a thermometer, that is, by the temperature, is gener- ally spoken of as the sensible heat of the body. Dur- ing a change of state, such, for example, as the change from a solid to a liquid, or from a liquid to a gas or vapor, considerable heat passes into the body without changing its temperature. This heat, therefore, is dif- ferent from sensible heat, and, since it does not affect i the temperature, is called latent heat. I 222. Laws of Fusion.- — -When a solid has been liquefied or melted by the action of heat, we say that it has been fused. It can be shown, experimentally, , 1st. That under the same pressure every substance capable of fusion has a fixed temperature at which it begins to fuse. ; 17 N 193 194 NATURAL RHILOSOPHT. 2d. That when any substance begins to fuse, the tem- perature remains the same until the whole of the sub- stance has been fused. Thus, ice begins to melt or fuse at the temperature of 32° Fall. If a quantity of ice be placed in a vessel over any source of heat, a thermometer plunged in the water which comes from the melted ice, will remain at the same temperature, viz., at 32° Fah., until all of the ice is melted. 223. L-aws of Solidification. — When bodies which have been fused by heat are sufficiently cooled, they solidify. As solidification is merely the reverse of liquefaction, the laws of solidification are similar to those of fusion, viz.: 1st. Under the same pressure every substance solidifies | at a certain temperature, which is the same as that at which it fuses. j 2d. When the solidification has begun, the temperature | remains the same until all the substance has solidified. Thus, water begins to freeze at 32° Fall., which is exactly the same temperature as that at which ice begins to melt. When a body of water which has been cooled to 32° Fah. comniences to freeze, no matter how intense the cold, the temperature will remain at ! 32° Fah. until all the water is frozen. | 224. Latent Heat. — Heat must be passed into ice * before it can be melted, and yet the water formed by | its melting has exactly the same temperature as that i of the ice from which it came, viz., 32° Fah. It is, ! therefore, evident that all the heat ichich was absorbed < during the melting of the ice has been rendered latent. In order to cause water heated in an open vessel to ! 212° Fah. to continue boiling, it is necessary to pass ' LATENT AND SPECIFIC HEAT. 195 into it considerable beat; and yet tlie pressure remain- ing the same, the steam produced by the boiling of the water has exactly the same temperature as that of the water from which it came, viz., 212° Fah. It is, therefore, evident that all the heat which was absorbed during the boiling of the water, that is, during its change from liquid to vapor, has been rendered latent. /^Latent heat is that which does not increase the tem- perature, and cannot, therefore, be detected by the ther- mometer.^' 225. Latent Heat of Water at 32 ° Fah. — The amount of heat that is rendered latent during the melting of a mass of ice is sufficient to raise the tem- perature of one hundred and forty-two times as much water 1° Fah. This fact is often expressed by saying that 142° of heat have been rendered latent. This can be shown as follows ; A pound of ice and a pound of water, each at 32“ Fah., are placed in exactly similar vessels, over the same source of heat, so that in any time each shall receive as much heat as the other. When all the ice has melted, the temperature of the water formed therefrom is only 32“, but the water in the other vessel will he at 174“ ; that is, it has gained 142“ of heat. We infer, therefore, that the ice required 142“ to melt it ; but since the temperature of the water so produced is only 32“ Fah., then 142“ must have been rendered latent. 226. Change of Latent into Sensible Heat. — A definite amount of heat is required to raise the tem- perature of a body through any number of degrees, and when the body so heated is again cooled to its original temperature, the same definite amount of heat is removed. Whenever, therefore, heat has become latent in producing a change of state in any substance, when the substance returns to its original state the same quan- tity of heat is again evolved. Heat is rendered latent in changing a solid into a 196 NATURAL PHILOSOPHY. liquid, or a liquid into a vapor or gas. Latent heat | becomes sensible, that is, heat is evolved, when a liquid ‘ solidifies, or a vapor or gas is condensed. I 227. Effect of Freezing on the Temperature of j the Air. — A large body of water cannot, in general, : be cooled below the temperature of 32° Fah. until its entire mass has been changed into ice. During the change from water to ice, a large quantity of heat is | given out which was before latent in the water, conse- quently, the cold air in contact with the water, while causing the cooling of the water, itself becomes heated. | Therefore, the freezing of large bodies of water tends to raise the temperature of the air. Freezing and melting are both gradual processes, on i account of the heat which disappears in the one case and reappears in the other. 228. Freezing Mixtures. — Considerable heat is rendered latent during the change of a solid into a liquid by solution. Flence, when solids are rapidly dissolved in a liquid, a marked coolimj occurs. In some cases the liquid is unarmed ; here, however, a chemical combination has occurred between the solid and the liquid, and more heat has been produced than was absorbed during the solution. Advantage is taken of the cooling produced by solu- tion to obtain low temperatures artificially. Freezing mixtures consist of various proportions of dift’erent solids, or of different solids and liquids, which, when mixed, will dissolve, and so cause a reduction of tem- perature. By their use very low temperatures can be obtained. One of the simplest freezing mixtures consists of one part of salt, and two parts of ice or snow, spread in alternate layers. ith this LATENT AND SPECIFIC HEAT. 197 mixture, a temperature as low as — 5° Fah., that is, 5° below the zero of Fahrenheit’s scale, can be obtained. Freezing mixtures are used in the preparation of ice-cream. The material to be frozen is put in a tin vessel placed inside a larger vessel of wood. Salt and ice are packed in layers between the two vessels. The salt causing the ice to melt rapidly, the heat necessary for the melting of the ice is taken from the cream, which is thus frozen. The outer vessel is made of some bad conductor of heat, such as wood, so as to prevent the ice obtaining any heat from the air. 229. Increase of Volume during Solidification. — Many metals, such as mercury, contract on solidifying; the freezing of the mercury in the bulb of a ther- mometer does not, therefore, burst the bulb. Other metals, such as antimony, are believed to expand on solidifying, and hence fill the moulds into which they have been poured, and so take sharp casts. Water also expands on freezing, and does so with considerable force. Wash-tubs filled with water are often burst daring a cold night ; even iron bomb-shells have been burst by filling them with water, and exposing them to a freezing temperature. ' When water is absorbed by porous rocks, or runs into the crevices of those of more compact structure, on freezing, it expands with sufficient force to break the rocks into fragments. It is in this way that water I acts to degrade or break down the rocks, and so aids the rivers in carrying the mountains towards the ocean. ‘ 230. Vaporization. — When most liquids are suffi- ciently heated, they become changed into vapors. ; When the vapor is given off from only the surface, : the liquid is said to evaporate ; but when the vapor is : given off from below the surface also, the liquid is said to hoil. Some solids pass into a state of vapor ! without appearing to first become liquids ; they are then said to be sublimed. Arsenic is an example of such a solid. 198 NATURAL PHILOSOPHY. 231 . Formation of Vapors in a Vacuum. — If any volatile liquid be put into a vacuous space, it 'vvill rapidly pass into the state of a vapor without the addition of external heat. If a drop of any volatile liquid be passed up into the empty space above the mercury in a barometer tube, we will notice not only that the liquid will dis- appear and turn into vapor, but that at the same time the vapor zvill more than fill the vacuum., that is, that it will depress the mercury column, thus showing that it possesses tension. If, now, a little more liquid be passed into the tube, it will also evaporate, and the column of mercury will be further depressed ; but after a certain amount of vapor has been formed, the amount of which depends on the temperature, the mercury column will remain stationary, and no more liquid will be evaporated ; and if it be passed into the tube, it will simply float on top the mercury. The vapor in the tube is then at its maximum or greatest tension, and the space it occupies is saturated icith vapor. If, now, the tube be heated, more of the liquid will evaporate, and the mercury will be further depressed. 232 . Circumstances Influencing Evaporation. — The rapidity with which a liquid evaporates into the air depends 1st. On the extent of surface, becanse evaporation takes place at the surface. 2d. On the quantity of the same vapor already pres- ent in the air, because,' when the air is saturated, no more of the liquid can evaporate. 3d. On the removal of the air. Evaporation ceases when the air over the liquid is saturated; if, however, fresh air is brought to the surface, more liquid evap- orates. LATENT AND SPECIFIC HEAT. 199 4111. On the temperature^ because warm air can hold more vapor than cold air. 5th. On tlie pressure on the surf ace. The greater the pressure the greater the evaporation. In a vacuum all vaporizable liquids rapidly vaporize. 233. Laws of the Boiling of Liquids. — The laws for the boiling of liquids are similar to those for the i fusion of solids. They may be expressed as follows : 1st. The pressure remaining the same, there is for every I liquid a certain temperature at which it hoils. 2d. When a liquid has been heated to the boiling-point, the temperature remains the same until all the liquid has been vaporized. All the heat a liquid receives, after it has once reached its boiling-point, is rendered latent in converting it into vapor. 234. Circumstances Influencing the Boiling-Point. — An increase of pressure raises the boiling-point. Before a liquid exposed to the air can boil, its vapor must i have a tension sufficient to enable it to overcome the pressure of the air. Hence, the tension of the vapor es- caping from a boiling liquid is equal to the pressure of the atmosphere. If, therefore, the pressure on a liquid be increased, the tension of its vapor must be increased I before the liquid can boil ; that is, the temperature ! of the liquid must be increased. j A liquid can never be raised above the temperature of its boiling-point, if its vapor is allowed to escape ! into the air. If the vapor be confined, the pressure 1 on the surface is increased, and the temperature can I be increased to any extent, provided the vessel is suffi- I ciently strong. To extract glue from bones, they are I boiled at very high temperatures, in water placed in closed vessels provided with safety-valves for the es- 200 NATURAL PHILOSOPHY. cape of the steam, should the pressure become too great. Experiment. — That a diminished pressure lowers the boiling point may be shown by an experiment that is sometimes called the culinary paradox. Water is boiled in a suitable glass flask, A, and after a few minutes of vigorous boiling, so as to permit the steam formed to drive all the air out of the flask, the source of heat is removed, and the neck is closed by a tightly-fitting cork, which has been previously steeped in melted wax or paraffin, so as to fill all its pores. The vessel is now inverted below a water-sur- face, C, to prevent the entrance of air. For a few moments the water will continue to boil ; but the increased pressure on the sur- face produced hy the confined vapor soon raises the hoiling-poini and stops the hailing. Fig. 9^ Igj. goi-fie cold water fall on the bottom The Colinary Paradox. ...in. ^ -n- no t-u of the flask, as shown m Fig. 93. The va- por will then be condensed, and the pressure being diminished, the liquid bursts into vigorous boiling. Pour hot water on the flask and it will again stop boiling. The vapor which escapes into the air from boiling liquid, has at the ordinary pressure of the air always the same temperature — viz., 212° Fah. Fig. 94. Water Boiled in a Paper-Bag. That the temperature of the liquid does not rise above the boiling-point may be shown by the following equally curious operation. Experiment . — To boil water in a paper-hag. Take a square piece of paper and fold it so as to form a conical bag. A, as shown in Fig. 94. Suspend the bag by strings, and, pouring water into it, allow the flame of an alcohol-lamp or Bunsen burner to fall on the bag, being careful to prevent the flame from touching the paper in any place where there is no water. The water can now be heated until it boils, without the paper being burned, because the paper LATENT AND SPECIFIC HEAT. 201 cannot be heated much more than 212° Fah., and this is not sufBcient to burn it. Caution . — Select good writing-paper, moderately free from sizing. Do not crease the paper in folding. Avoid letting the flame play too much on the point of the cone. InfMence of adhesion on the boiling-point. — The press- ure remaiuing the same, the boiling-point varies slightly with the nature and shape of the vessel in wdiich the liquid is boiled ; because, before the vapor can escape, the adhesion of the liquid to the vessel must be over- come. Solids dissolved in a liquid increase the tem- perature of the boiling-point for the same reason ; be- cause the vapor which escapes does not carry the solid with it. Distillation is a process by which a liquid can be separated from a solid dissolved in it. We boil the solution and condense the vapor as it escapes. 235. Latent Heat of Vapors. — The amount of heat rendered latent during the change of a given quantity of water at 212° into steam at 212°, is sufficient to raise the temperature of about one thousand times as much water 1° Fah. This is often expressed by saying that the water has absorbed 1000° of heat, or that 1000° degrees have been rendered latent. When vapor loses heat and condenses, the latent heat again appears as sensible heat. 236. Reduction of Temperature Caused by Evap- oration. — We are cooled by fanning, because the warm air thus brought into contact with the skin causes a rapid evaporation of the moisture on the skin, and thus lowers the temperature. If water be placed in a vacuous space, and the vapor which escapes from it removed as rapidly as it is formed, the water will at last be frozen by its own 202 NATURAL PHILOSOPHY. evaporation. Ice macliines are constructed on this principle. The water must have heat in order to evaporate, and this heat is taken from the rest of the water, which is thus frozen. During the change of water into vapor bj the heat of the sun, more than 1000° of heat are rendered latent. This heat again becomes sensible as the water condenses. Since a large amount of this condensation takes place in cool or cold countries, we can see that such countries must be made warmer bj means of the rain or snow which falls in them. 237. Specific Heat. — Equal quantities of different substances are not raised through the same number of degrees of temperature, by the addition of the same quantity of heat. Thus, if we add the same amount of heat to a pound of mercury and to a pound of water, we will find that if the water has been heated one de- gree, the mercury will have been heated thirtv-three degrees. We therefore conclude that water has thirty- three times the capacity for heat that mercury has, or that thirty-three times more heat is necessary to in- crease the temperature of a given weight of Avater one degree than is required to increase the temperature of an equal weight of mercury one degree. By the specific heat of a substance Ave mean the amount of heat required to raise the temperature of a given quantity of that substance through a certain number of degrees, as compared AA'ith the amount of heat required to raise an equal quantitv of some other substance through the same number of degrees. W ater is the substance generally adopted as the unit of com- parison. Specific heat, like specific gravity, is merely a ratio. LATENT AND SPECIFIC HEAT. 203 • 238. The Calorimeter. — The thermometer only ! indicates the change -which has taken place in the temperature of a body, that is, it only gives the sen- ! sible heat. To obtain the specific heat, -we must kno-w f the latent as -well as the sensible heat. There are ^ various methods of doing this, one of the best of ( which is by means of the caloriraeter. I I I I i We can ascertain the total amount of heat given out by a body in cooling, from any given temperature to that of melting ice, by seeing how much ice it will melt. The calorimeter consists of three hollow vessels, M, A, and £, placed inside one another, as shown in Fig. 95. The vessels A and B are packed with dry ice. The substance, whose specific heat is de- sired, is placed in M, and in cooling melts the ice in A, the quantity melted being inferred from the water which runs into D. The ice in B prevents the heat of the outer air from melting any of the ice in A. Suppose one pound of a given substance is heated to 212° and placed in M, and in cooling to 32° only melts half as much ice as a pound of water would in cooling from 212° to 32° — then the specific heat of the body is | = .5. Fig. 95. The Calorimeter. j 239. The Specific Heat of Water. —Water has a [ higher specific heat than almost any other common sub- j stance, that is, it takes in more heat in being warmed, i and gives out more in cooling, than any common sub- stance. Since about three-fourths of the earth’s sur- j face is covered with water, we can see that the high i specific heat of water must exert a great influence in I preventing extremes of temperature, since water can absorb or emit considerable heat without much change I in temperature. j 240. Mechanical Equivalent of Heat. ^ ^ Energy can never be destroyed. When it disappears in one j form it reappears as another. Heat is one of the com- 204 NATURAL PHILO SOPHY. monest forms into which mechanical motion can he i changed; for, since lieat is an effect produced by tbe i vibrations or shakings of the molecules of bodies, we can easily change mechanical motion into heat. This fact was first discovered by an American named Ben- jamin Thomson, but afterwards called Count Eumford. We know, as the result of many accurate experi- ments, the exact amount of mechanical force neces- I sary to produce a given quantity of heat. j The force necessary to •produce sufficient heat to raise j the temperature of one pound of water 1° Fah., is equal to that produced by a weight of T12 lbs. falling through j the space of one foot. Or, conversely, one pound of j water in cooling through 1° Fah. gives out a quantity i of heat which is capable of exerting a mechanical force I sufficient to raise 772 lbs. through the space of one , foot. These figures were first determined by an Eng- ■ lishman named Joule, and are called Joule's equivalent. \ The fact that heat can be produced by mechanical ! force is seen in the heat developed by tbe friction of ' one surface on another, and also in the heat developed i by percussion. A stout copper wire, if rubbed with a i piece of stiff paper, may be made hot enough to set fire ! to a match; or a soft iron nail may be made sufficiently hot, by rapid hammering, to char a piece of paper. 241. The Steam-Engine. — The method most com- monly adopted for the change of heat into mechanical work, is by means of the steam-engine. The heat is employed to change water into steam. The water is placed in a suitable vessel, generally made of iron, called the boiler.^ the construction of which varies with the character of the steam-engine with which it is to be used. LATENT AND SPECIFIC HEAT. 205 The steam passes from tlie boiler through a pipe leading to a box, Fig. 96, called the steam-chest, through which, by a con- trivance, B, called the slide-valve, it is admitted alternately to different sides of a piston, C, so ar- ranged as to move freely in the cylinder, D. By the pressure which x the steam exerts on the piston, it is moved back- wards and forwards from one end of the cylinder to the other. The mo- tion of the piston is com- municated by means of Fig. 96. The Low-Pressnie Steam-Engine. the rod, E, to a beam, F, moving on a hearing, G. By means of the connecting-rod, H, attached to the other end of the beam, R, the motion is carried to a large heavy wheel, I, called the fly-wheel. The alternating motion of the beam is converted into a steady rotary motion by means of the crank, J. In this manner the backward and forward motion of the piston is caused to produce a continued rotary motion of the fly-wheel. To the axis of the fly-wheel pulleys are generally at- tached, from which, by means of belting and shafting, the motion is carried to the machinery to be moved. By means of the slide-valve the steam in the steam- chest is alternately cut off from one side of the piston and admitted to the other side, and at the same time an opening provided through which the steam can escape from that side of the piston from which the steam has been cut off. In the low-pressure steam- 18 206 NATURAL rniLOSOPHT. engine this steam passes through a pipe, into a chamber, called the condenser. In this chamber a jet of cold water is allowed to play. By this means j the steam which passes from the steam-cylinder into i the condenser is condensed, thereby lowering the ! pressure on that side of the piston from which the : steam has been cut off. A pump, J/, called the air- pump, worked by the rod, 0, removes the water from the condenser. The slide-valve is moved by means of a bent lever moved by the eccentric rod, P. In the high-pressure engine, the steam escapes at once into the air, after it has moved the piston to either end of the cylinder. The puffs of steam which escape from such engines denote the speed with which the piston is being driven backwards and for- wards. When the piston is at the farthest part of its stroke, that is, when it is at one end or the other of the cyl- inder, the crank, J, and connecting-rod, H, are in the same straight line. In such a position the tendency to motion of the beam, F, will not be carried through the connecting-rod and crank so as to produce a rota- tion of the fly-wheel, but will simply produce a strain on those parts. These two positions are called the dead points of the engine. The fly-wheel, by its inertia, continues to move and carries the crank past these points. 242. Other Sources of Heat. — Besides the sources of heat already mentioned, we have heo.t of the sun and fixed stars, and that produced by chemical combination and by electricity. The way in which heat is produced by chemical com- bination is well illustrated by the case of a body burn- SYLLABUS. 207 ing in the air. As the combustible body combines with the oxygen of the air, the oxygen, in rushing towards the combustible materials in the body, sets its molecules into the vibratory motion necessary to cause heat. Syllabus. When a solid is changed into a liquid by the addition of heat, it is said to he fused. When the fused liquid is allowed to cool, it again becomes solid. Sensible heat is the heat a body possesses as indicated by its tem- perature, or by the thermometer. Heat which does not raise the tem- perature is called latent heat. In order to cause a mass of ice at 32° Fah. to melt, a quantity of heat is rendered latent, which, were it acting as sensible heat, would be suf- ficient to raise the temperature of 142 times as much water 1° Fah. Heat is rendered latent in changing a solid into a liquid or a liquid into a gas. Latent heat becomes sensible, or heat is evolved, when a liquid solidifies or a gas is condensed, j Freezing mixtures consist of mixtures of various solids, or solids and j liquids which, when brought together, melt or dissolve rapidly. The ! reduction of temperature they cause is due to the heat which is ren- ; dered latent. ^ Some substances, like water and antimony, at the moment of solidi- ij fying expand with considerable force. [ A liquid is said to evaporate when it gives off vapor from the sur- I face only ; it is said to boil when it gives off vapor both at and from I below the surface. All vaporizable liquids vaporize at once when placed in a vacuum, j: When a given space holds as much vapor as it can be made to hold f without change of temperature, it is said to be saturated. [ The rapidity of evaporation is influenced, 1st. By the extent of sur- face ; 2d. By the amount of the same vapor already in the air ; 3d. E By the renewal of the air ; 4th. By the temperature, and, 5th. By the 1 pressure on the surface. When a liquid begins to boil, the temperature remains the same I until the whole has been vaporized. The vapor which escapes from of the liquid. 208 NATURAL PUILOSOPEY. The boiling-poiut is increased by an increase of pressure. It i.o also increased by adhe.sion, whether between the liquid and the walls of the vessel or between the liquid and solids in solution. The reduction of temperature produced by evaporation is caused by the large amount of heat that becomes latent when a liquid is changed into a vapor. By the specific heat of a substance, we mean the amount of heat re- quired to raise the temperature of a given quantity of the substance through a given number of degrees, as compared with the amount of heat required to raise the weight of an equal quantity of water through the same number of degrees. The sensible heat of a body is determined by a thermometer ; the specific heat by a calorimeter. Water possesses the greatest specific heat of any common substance. By the mechanical equivalent of heat we mean the amount of me- chanical force which a given quantity of heat is capable of exerting. 772 lbs. falling through the space of one foot, represent a force ca- pable of heating 1 lb. of water 1° Fah. Heat may be caused to give up its energy in the form of mechanical work by means of the steam-engine. The principal sources of heat are, 1st. Mechanical ; 2d. Heat of sun and fixed stars : 3d. Chemical combination ; 4th. Electricity. Questions for Review. What is the difference between latent and sensible heat? State the laws for the fusion and for the solidification of solids. How many degrees of heat are rendered latent during the melting of ice ? Explain fully how this can be determined. Under what circumstances is sensible heat rendered latent ? Under : what circumstances is latent heat rendered sensible ? WTiat are freezing mixtures? Explain the manner in which they | act. Name any simple freezing mixture in common use. i , Name any substances that increase in volume on solidifying. Name | i any substance which decreases in volume on solidifying. How does | ' water cause the degradation or breaking down of rocks? j When is a liquid said to evaporate? When is it said to boil? |i When is a solid said to be sublimed? j What happens when a volatile liquid is put into a vacuum? De- 1 1 scribe an experiment which shows, 1st. That vapors possess tension, ; i QUESTIONS FOR REVIEW. 209 and 2d. That when a space is saturated with any vapor, no more of the liquid will evaporate. Name the circumstances which influence the rapidity of evapora- tion. Explain why each acts in the manner it does. State the laws for the boiling of liquids. How much heat is ren- dered latent during the change of water at 212° to steam at 212° ? Name the circumstances which affect the boiling point. Describe the culinary paradox. How can water be boiled in a paper-bag? Why does not the paper burn ? Name some examples of the reduction of temperature caused by evaporation. Define specific heat. How may the specific heat of a substance be determined ? Describe the construction of the calorimeter. What effect is produced on the climate of the earth by the very high specific heat of water ? Why should this effect be produced ? What do you understand by the mechanical equivalent of heat? What is the value of Joule’s equivalent? Who discovered the rela- tion that exists between heat and mechanical force ? Give any examples in which heat is produced by the exertion of mechanical force. Name the principal sources of heat. 18* O Part IV. Light and Electricity. CHAPTER I. LIGHT. — ITS NATURE AND SOURCES.— AC- TION OF MATTER ON LIGHT. 243. The Nature of Light. — Light is caused by a vibratory motion of tbe luminiferous ether. 1 As we have already seen, hot bodies radiate or give j off their heat by means of waves imparted by their j molecules to the surrounding ether. When this ether- motion is sufficiently rapid, it becomes visible as light The invisible ether-motion constitutes, in most cases, radiant heat, although the visible rays also possess heating power. Neither the visible mr the invisible ether- motion, however, is to be confounded with temperature, which in all cases is caused by the vibration of the mole- | cules of the body, and never by the vibrations of the ether. 1 It is only the atmospheric vibrations between certain rates that are j able to excite the sensation of sound. These limits, as we have seen, I extend from 16 to about 48,000 per second. Atmospheric vibrations i less rapid than 16, or more rapid than about 48,000, fail to affect the j ear as sound, although they differ from those which can so affect it in ; no other respect than as to their wmve length. The same is true of the ether-waves ; only those between certain 210 LIGHT— ITS NATURE AND SOURCES. 211 rates are able to cause the sensation of light. When either too slow or too rapid, they fail to affect the eye, and are then invisible. 244. Sources of Light. — The principal sources of light are the sun and fixed stars, chemical combina- tion, and electricity. Nearly all our light comes from the sun. Artificial light is generally obtained by some form of combustion, such as that produced by burning gas or oil in air. 245. Luminous and Illuminated Bodies. — A body which produces the light it gives off is called a lumi- nous body. A lighted candle is a luminous bodjr, since it produces or causes the light it gives off A body which shines by throwing off light it has received from a luminous body, is said to be illuminated. The sun is a luminous, the moon an illuminated body. Nearly all visible objects are illuminated ; w'e see them by means of the light they receive from luminous bodies. I 246. Transparent, Translucent, and Opaque Bod- : ies. — When a body allows light to pass through it so ! as to enable us to see clearly the outlines of other bod- I ies through it, it is said to be transparent ; when it allows the light to pass through it so as not to permit , us to see the outlines of other bodies through it, it is I said to be translucent ; when it will not allow the light to pass through it at all, it is said to be opaque. The difference between a transparent and a translucent body is not so much in the amount of light which can pass through each, as in the manner in which the light q^asses i through. W ater is transparent ; oiled paper is trans- ! lucent ; and iron and wood are opaque. ! I Many substances that are ordinarily opaque are partially trans- i parent when in very thin films ; thus, a thin film of gold allows yel- 11 lowish-green light to pass through it, and a thin film of silver a bluish I light. 212 NATURAL PHILO SOPHY. 24:7. Ray, Beam, and Pencil. — A ray is a single i line of light, taken in the direction in which the light | is moving ; a beam is a number of parallel rajs ; a j pencil is a number of rays from any luminous point; | the pencil is converging when the rays are all moving I towards the same point ; and diverging when they are all i moving yVom the same point. The particles of air in a sound-wave vibrate in the same direction in which the wave is moving; but in light and heat the ether particles vibrate at right angles to the direction in which the wave is moving. 24:8. Direction in which Light Moves. — Like sound and heat, light moves in perfectly straight lirm, if the medium through which it is passing continues ' of the same kind and density throughout. A beam of light coming into a darkened room lights up the dust particles floating in the air, and we can then see that the light moves in straight lines. 24:9. Shadows. — When light falls on an opaque body, the space immediately behind the body into which the light cannot penetrate is called a shadow. Shadows result from the fact that light moves in straight lines, and is not per- ■ ceptibly bent on Fig. 97. — Umbra or Complete Shadow. passing the edge of a body. If a luminous point, s, Fig. 97, be placed near an opaque body, A, the light falling on the opaque body will illumine the parts nearest it, but grazing the edges of the body, as at a and h. will continue moving in sensibly the same straight lines, s a and s h, in which it came from the luminous point. I LIGHT— ITS NATURE AND SOURCES. 213 ' If the luminous point come nearer the opaque body, as at s', the shadow becomes larger, being now bounded ^ by the lines s' a a' and s' h h' . The shape of the shadow, therefore, is dependent on the shape of the opaqxie body, and I the size of the shadow on tlte distance of the luminous point { from the opaque body. I* When the luminous body has any extent, as for 1 example, S, Fig. 98, the light from its central part ! meets the opaque j body, which casts , a shadow of the ■ same shape as be- j fore. Onlya part ! of this shadow, however, is complete, viz., that lying ; within the conical space a o b. The shadow is less and less complete as we pass at P P outside this space, since these portions are illumined by the light coming from the edges, c d, of the luminous body. That part of the shadow lying within a o b, into which no light pene- I trates, is called the nmbra or complete shadow ; that lying j without these lines, as far as and b g, is called the I penumbra or partial shadow. ! Experiment. — Hang a wet sheet from the ceiling, to act as a curtain i or screen. Where convenient, it is preferably hung in an open door- i way leading into an adjoining room. Place a lighted candle on the floor back of the sheet, and then walk backwards and forwards between I the candle and the sheet, and very curious and grotesque shadows appear to those on the other side of the sheet, without their being able to see how they were caused. While walking towards the candle I the shadow rapidly increases in size, and while walking away from it, ; rapidly decreases. By stepping over the candle, the shadow appears I to be leaping through the ceiling. i 250. Velocity of Light. — Light moves with the almost inconceivable velocity of about 185,000 miles a second. This velocity could take it more than seven 214 NATURAL PHILOSOPHY. times around the earth at the equator, in a second. For all distances on the earth at which objects are visible, we may regard the transmission of light as instantaneous. The velocity of light has been determined by various astronomical observations. It has also been measured by different instruments es- pecially contrived for the purpose. 251. Actions which Take Place at the Surfaces of Bodies. — When light falls on a body, it either passes into the body, or is thrown otf from its surface. The light Avhich is thrown oft' from the surface is either diffused or reflected. The light which passes into the body may pass through it, if the body be transparent or translucent; if in this case the direction of the light is changed on entering the body, the light is said to be refracted. When the light which enters the body does not pass through it, the light is then said to be absorbed. Ab- sorbed light is generally converted into heat ; but it sometimes renders the body phosphorescent. 252. Diffusion of Light. — When the light which falls on the surface of a body is ihroivn off from it in all directions., it is said to be diffused. Illuminated bodies shine by means of dift'used light, which they throAV oft' in all directions. 253. Reflection of Light. — When light falls on the surface of a body, and is thrown off from it at equal and opposite angles to. that at which it struck the sur- face, it is said to be reflected. Since the ether-waves, which are the cause of light, are elastic, they are reflected from the surface of hard bodies in the same manner as any other elastic body. 254. Laws of the Reflection of Light. — 1st. The angle of reflection is eipaal to the angle of incidence. LIGHT— ITS NATURE AND SOURCES. 215 Let a ray of liglit, A B, Fig. 99, fall on a reflecting surface at the point B. At this point draw the perpen- dicular, D B, then A B D the angle of incidence, and D B 6' is the angle of reflection. 2d. The light will he reflected in the same plane as that in which the incident ray and the perpendicular at the point of in- cidence lie. Thus, if the incident ray, A B, and the perpendicular, D B, lie in the plane of the paper, the reflected ray, B C, will also lie in the plane of the paper, and not above or below it. 255. Amount of Light Reflected. — The amount of light reflected at any surface depends, 1st. On the kind of material forming the surf ace, and its degree of polish. 2d. On the angle at which the light strikes the sur- face. Highly-polished metals and glass are excellent reflect- ors of light. Most light is reflected from the surface of a transparent substance, like glass or water, when the light falls obliquely on the surface. When light falls on such surfaces nearly at right angles to the surface, most of the light passes through the body. When the sun is nearly overhead, we can look at his image in a water-surface without being dazzled, because but little of the light is reflected ; but when the sun is nearly setting, the image is too dazzling to be looked at stead- ily. Most light is -reflected from opaque surfaces, like 216 NATURAL PHILOSOPHY. those of polished metals, when the light falls the most directly on the surface — that is, at right angles to it. 256. How Bodies become Visible. — Only those bodies are visible which throw off light in all directions. Both luminous and illumined bodies do this, and hence are visible. Illumined bodies are visible on account of the diffused light they throw off'. A body which regularly reflects light cannot be seen. A fine 2 Dolished mirror is invisible, and is often mistaken for an open doorway. When a mirror is tarnished or covered with dust, it then diff'uses a part of the light which falls upon it, and thus becomes visible. A ray of light is invisible unless some light from it enters the eye. We do not see the rays which pass from the stars to the earth. The path of a raj' through a dusty room is visible because the particles of dust scatter or diff'use the light. 257. Absorption of Light. — AYhen the ether-waves fall on a body, and are neither transmitted through it nor are thrown off from its surface, they pass into the body and are absorbed. Absorbed light generally causes the molecules of the body to vibrate so tis to jwoduce heat. It sometimes causes the molecules to vibrate rapidly enough to produce light, when the body is said to be phosphorescent. When a surface absorbs most of the light which falls on it, it appears black or dark, because it diffuses but little light. No surface absorbs all light which falls on it, since we know of no bodies so black as to be invisible. 258. Phosphorescence. — Phosphorescent bodies are those that, when exposed to a bright light, will con- tinue to shine for some time after they are taken into LIGHT— ITS NATURE AND SOURCES. 217 the dark. The ether-waves in being absorbed im- part their motion to the molecules, and cause them to vibrate so as to give out light or become luminous. The term phosphorescence is sometimes also applied to the faint light emitted by glow-worms, fire-flies, and jelly-fish, or by decaying animal and vegetable substances. This is quite different from the phosphores- cence just described, and is due to a slow oxidation of a substance pro- duced by the animal, or which results from the decomposition of decay- ing animal or vegetable matter. 259. Refraction of Light. — When light passes from one transparent substance to another, it is bent out of its straight course, or refracted., as it enters the other substance. While passing through this substance, how- ever, it moves in straight lines. Thus, if a ray of light, D A, Fig. 100, pass through the air and fall on a water-sur- face, S B, at the point A, a part will be reflected in the direction A E ; but the part which enters the Avater does not continue in the same direction., A F, it had while in the air., but is bent at the point where it strikes the surface.. A, and takes the direction A G. If the light is refracted, it must either pass on the side of A F, which is farther from the perpendicular, A H, or on the side AA'hich is nearer it. When light passes from a rare to a dense medium, as from air into Avater or glass, it is bent towards the perpendicular ; when it passes from a dense to a rare medium, as from Avater or glass into air, it is bent from the perpendicular. 19 218 NATURAL PHILOSOPHY. 260. Laws of Refraction of Light. — 1st. The inei dent ray, the perpendicular at the point of incidence, and the refracted ray, all lie in the same pilane. 2d. Between the same ivjo media, the sine of the anyle of refraction bears a constant ratio to the sine of the anyle of incidence, whatever may he the anyle of incidence. In Fig. 101 let a circle be described about the point of incidence, I, by the radius, D I, and let lines, D N and P S, be drawn from the ends of the radii, ID and I S, at right angles to N I P, the perpen- dicular at the point of incidence ; then these lines, D N and P S, will be respectively the sines oi D I N and SIP, the angles of inci- dence and refraction. Now, between the same two media, the sines of the angles of incidence and refraction always hear the same ratio to each other, no matter what may be the angle Fig. 101. Index of Refraction. incidence. This ratio is called the index of refraction, and may he represented as D V follows, viz. ; the index of refraction ==-p~^. The index of refraction varies for different media, hut is constant for the same two media. The refraction ivhich occurs ivhen light passes from air to water may be shown as follows : Experiment. — Place a coin, a. in the bottom of an empty howl, ..4, Fig. 102, and stand in such a position that the coin is just invisible, as at c. Let some one quietly pour clear water -.X into the basin, and the ray of light which just passes the edge of the basin will be bent as it passes out of the water, and, taking the direction h c, will enter the eye of the observer, who Fig. 102.— An Effect of Refraction, will see the coin in the position a'. Caution. — Place the eye so as just to see the coin over the edge of the empty basin ; then move the head back until the coin just disappears; this will be the best position in which to stand. LIGHT— ITS NATURE AND SOURCES. 219 261. Effects Caused by Refraction. — To an ob- sorver at c, the coin appears to be raised from its true position, a to a'. This effect is produced when Ave are looking at things in water, and is caused by refraction. In Fig. 102, the light is represented as coming from one point of the coin. The diA^erging pencil of light, which enters the eye, appears to diverge from the point a', and consequently the eye sees the image of this point of the coin at a', and not at a. The coin and bottom of the vessel appear raised, and the water appears less deep than it really is, a circumstance Avhich often causes errors of judgment in entering the Avater. A stick partly immersed in water appears bent at the surface, from the refraction of the hght at that point. 262. Intensity of Light. — The amount of light re- ceived by any surface is smaller the greater its distance from the luminous source, or in other words, the inten- sity of the light it receives decreases as the distance increases. The rate of this decrease may be expressed as follows, aTz. : The intensity of the light received hy ciny surface is inversely proportional to the square of its distance from j the luminous source. L If an opaque body be placed in the path of a beam j; of parallel rays, the intensity of the light it receives will be sensibly the same at all distances ; but when I illumined by a diverging pencil, such as is given off ; from all luminous points, the intensity decreases as the square of the distance from the source. ; Let A, in Fig. 103, be a luminous point, and A an opaque screen one inch square ; then A Avill receiA^e a 1 certain amount of light, depending on its size and its 1 distance from S. Let its distance from S be one foot, i J \ 220 NATURAL PHILOSOPHY. then, if a screen be placed at i?, two feet from /S', it will receive a shadow four times as large as ; at three feet from *S', or at (7, it will receive a shadow nine times as large as A, and at four feet, or at D, sixteen times as large. If, now, the opaque body. A, be placed at £, the quantity of light it vdll receive will be ' but one-fourth that which it received at | A, and consequently the intensity of the | light it receives vdll be but one-fourth its ^ intensity at J. ; at C the intensity of the ^ light will be but one-ninth, and at D but one-sixteenth. If the eye be placed at A, the amount of light which it receives will be limited by the size of the pupil or the opening through which light enters the eye. IVhen ■ placed at .S, the size of this opening re- maining sensibly the same, the amount of light the eye receives is but one-fburth of that received at A. 263. Photometers. — -lYe can measure the intensity of different lights by means of instruments called j)ko- tometers. If a faint grease spot be made in a sheet of paper, it becomes visible when held between the eye and a source of light, because more light comes through where the paper is greased than elsewhere. If it he held between two sources of light., so as to be equally ilhrmined, the grease spot will disaqjpear, since it will then be no brighter than the rest of the paper ; but if it be moved towards either light, the spot will be more illumined than the rest of the paper, and will again appear. This Pig. 103. The Intensity of Light. LIGHT— ITS NATURE AND SOURCES. 221 is the principle of Bunsen's Photometer.^ in which a faint grease spot is made in a sheet of paper supported in a frame. The paper is placed between two lights, and moved backwards and forwards until a position is ob- tained at which the spot disapj^ears. Call the two lights A and and suppose the screen be one foot from A and two feet from .6, then B has four times the inten- sity of A. 261:. Images Formed by Small Openings. — If the rays of light from brightly illumined objects be allowed to come through a small opening into a dark room in which a white screen, ri, Fig. 101, is placed in front Fig. 104, — Images Formed ty" Small Apeitnies. of the opening, they will form on the screen an accurate image of the objects from which they came. The image Avill have the same colors as the object, but will be in- verted, that is, turned upside down, and from right to left. The size of the image will depend on the distance of the screen from the opening. The diffused light, coming from all points of the ob- ject, enters the opening and produces on the screen an 19* 222 NATURAL PHILOSOPHY. exact representation of tliose parts from wliicli it came. As, liowever, the rays cross at the opening, the light from the top parts of the object will be received on the lower parts of the screen, and those of the lower parts of the object on the top parts of the screen; the image must therefore be inverted. Experiment. — Allow the sunlight to pass through a hole in the shutter of a darkened room, and let the image fall on a piece of white paper, held at right angles to the direction in which the light is enter- ing the room. A round disc of light will be seen on the paper. This is the image of the sun. Experiment. — Paste a flat, smooth piece of tin-foil over the hole in the shutter, and punching a hole in the foil with a large pin or needle, allow the diffused light from the trees, houses, or other objects outside, to fall on a screen held opposite the pin-hole. An inverted image of the things outside will be seen on the paper. Experiment. — •Unsolder the top from an empty tomato-can, by | holding it in the flame of a Bunsen burner, and punch a hole in the i bottom with a nail. Paste a piece of tin-foil over the nail-hole, and | make a pin-hole in it. Cover the open end of the can with a piece of | oiled paper. If now a lighted candle be brought near the pin-hole, ' an inverted image of the caudle will be seen on the oiled paper. , Caution. — Round, smooth-edged holes are preferable. Large holes give brighter, but less distinct and sharp images than small holes. 265. Mirrors and Specula. — A .liigbly-polished ! body, having a regular surface and capable of reflect- | ing most of the light which falls upon it, is called a • mirror or a spectdum. A reflector made of glass, cov- ered on the back with some good reflecting surface, is called a mirror ; a highly-polished metallic reflector is called a speculum. Mirrors or specula may be either plane or curved. 266. Images Seen in Plane Mirrors. — M^hen an object is placed in front of a plane mirror, an image the same size as the object will be seen as far back of the mirror as the object is in front of it. LIGHT— ITS NATURE AND SOURCES. 223 We always see an image in the direction in wliicli a ray of light coming from the object enters the eye. If, therefore, an object, such as a can- dle, A B. Fig. 105, he placed before the plane mirror, C B, the image will be seen by an eye at P, as though it were at M B' back of the mir- Pormed by Plane Mirrors. ror. Every point of the object, as sends a cone of rays to the mirror, a part only of which, however, enters the eye after reflection. This poiot of the object will appear to be situated back of the mirror wdiere those rays apparently meet. It can be proved by geometry that this distance, C fl.', back of the mirror is eqnal to the distance, C A., of the luminous point in front of it. The image appears as far back of the glass as the object itself is in front of it. The image which appears back of the glass, and which is formed by rays which do not come directly from the object, is called the virtual image. If the eye looked at J. P it Avould see a real image; that is, one formed by rays coming straight from the ob- ject. Plane mirrors cause the image to appear inverted from right to left. Experiment. — Write on a sheet of paper, and before the ink dries press a clean piece of blotting paper on the writing, and on removing the blotter it will have a copy of the writing, inverted from right to left, just as a mirror appears to invert objects ; for, hold the blotter in front of a looking-glass, and the writing on the blotter can easily be read in the glass. B D B' 224 NATURAL PHILOSOPHY. The fact that the eye sees an image in the direction in which the rays enter it can be amusingly shown as follows ; Experiment. — By placing four small pieces of looking-glass at a, h, c, and d, as shown in Fig. 106, a ray of light from a distant object will, after reflection from the mirrors, enter C the eye at C, in the same direction as that in which it came from the ob- ject; the eye, therefore, will see the object, although an opaque object, such as a brick, be held at B, between the eye and the object, thus making it appear as though the person was seeing through the brick. The mirrors may be concealed in a suitably shaped box, with openings at A and C. Fig. 106. — Looking throngli a Brick. When any object is placed between two plane mir- 1 rors inclined at any angle to eacli other, a number of images will be seen, which Avill be greater as the inch- j nation between the two mirrors is less. This multi- , plication of the image of an object is seen in the kalei- < doscope. , Experiment. — Place two looking-glasses, or pieces of glass, at any i angle with each other, and observe the images of an object placed I between them. Now change the inclination of the mirrors, and note j the change in the number of images. ! 267. The Visual Angle. — The rays of light which i come from the extremities of an object meeting at the i eye, form an angle called the visual angle. ^Ve judge ! ^ of the size of an oh- ; ject mainly by means i of the Ausual angle: the larger the visual angle the larger does : the object appear. Thus, in Fig. 107, the A'isual . angle under Avhich the eye sees the object J. J. is | Fig. 107. — The Visual Angle. LIGHT— ITS NATURE AND SOURCES. 225 A 0 A. If, now, tlie object be carried to A' A', it will then be seen under tlie smaller visual angle A' 0 A'., and ■will, therefore, appear smaller. The smaller object, (7(7, is seen under the same visual angle as A' and there- fore appears of the same size. Any cause which alters the value of the visual angle, changes the apparent size of the object. 268. Curved Mirrors. — Curved mirrors may be of a variety of forms. We will consider two of these forms, "viz., concave and convex mirrors. Concave mir- rors are curved like the inside of a watch crystal. Con- vex mirrors are curved like the outside of the crystal. When rays of light from illumined objects enter the eye after reflection from curved mirrors, their direction is generally so changed, that the ■visual angle under which the eye ■views the image, is different from that under which it would have -viewed the object, had it observed it directly. The apparent size of the image, therefore, is different from the apparent size of the object. Whenever a number of rays collect at a single point, that point is called a focus. 269. Concave Mirrors. — Parallel rays of light fall- ing directly on a concave mirror, collect, after reflec- Fig. 108. — Action of Concave Mirror on a Beam of Light. tion, at a point in front of the mirror. This point is called the principal focus of the mirror, and is situated 226 NATURAL PHILOSOPHY. midway between the centre of the mirror and the centre of the sphere of which the mirror may be conceived to be a part. Thus, in Fig. 108, the principal focus is shown at F, midway between the mirror and the point (7, called the centre of curvature. If a luminous point be placed at Z, more distant from the mirror than the centre of curvature, the diverging rays which it casts on the mirror Avill, after reflection, converge to a focus at S. Conversely if the luminous point be placed at S, the focus vdll be at L. The points L and S are called respectively the longer and shorter conjugate foci. If the source of light be placed at (9, between the principal focus and the mirror, the rays vdll, after re- flection, appear to come from a point, ZT, back of the mirror, called the virtual focus. 270. Images Formed by Concave Mirrors. — If an object be placed before a concave mirror, between tlie mirror and the principal focus, the image vdll ap- pear loch of the mirror erect and larger than the object. The manner in which the visual angle is changed by reflection, is seen in Fig. 109, in which a person is represented as looking at his magnified image in a concave mirror. The rays Fig. 109. of light coming from anv Virtual Image in Concave Mirror. fall on the mirror, being reflected from it as shovTi by the arrow, and enter the eye of the observer as though they came from a point, o', back of the mirror, so, also, those coming from the point b are so changed LIGHT— ITS NATURE AND SOURCES. 227 in tlieir direction by reflection as to appear to come from the point b'. The observer, therefore, sees an en- larged erect image at a' V. If the object, such as a candle, be placed before a concave mirror, at a shorter conju- gate focus, an in- verted and mag- nified image will be seen at the longer conjugate focus, as shown in Fig. 110 at : but if the object be placed at the longer conjugate focus, the image will be inverted and smaller than the object, and will be seen at the shorter conjugate focus. The foci of convex mirrors are similar to those of concave mirrors, but are formed on opposite sides of the mirror to what they are formed in concave mirrors. J Syllabus. |. Light is caused by a vibratory motion of the luminiferous ether. I When the motion imparted to the luminiferous ether by the molecules ; of heated bodies is sufficiently rapid, it becomes visible as light. [; Invisible ether-motion constitutes, in most cases, radiant heat. The I visible ether-motion also possesses heating powers. Neither the visible nor the invisible ether-motion is to be confounded with temperature, which, in every case, is caused by the vibrations of the molecules of ; the body, and never by the vibration of the ether. I The principal sources of light are the sun, stars, chemical combina- I tion, and electricity. i A body which gives off the light it produces, is called a luminous 228 NATURAL PHILOSOPHY. body. One which gives off the light it receives from other bodies, is called an illuminated body. Transparent bodies allow us to clearly see other bodies through them. Translucent bodies allow light to pass through them, but will not allow other bodies to be seen through them. Opaque bodies do not allow any light to pass through them. A single line of light is called a ray; a number of parallel rays is called a beam ; a cone of light is called a pencil ; pencils may be either converging or diverging. Light moves in straight lines. When it meets an opaque body, a shadow is formed back of the body. The umbra or complete shadow receives no light; the penumbra or partial shadow is partially illu- mined. The velocity of light is about 185,000 miles per second. When light falls on the surface of a body, it is either thrown off from the surface or it enters the body. That which is thrown off from the surface is said to be diffused when it passes off in all directions ; but to be reflected when it passes off in only certain directions. The light which enters the body either passes through it, and is bent out of its course or refracted on entering the body, or it is absorbed by the body. When absorbed it causes heat, or renders the body phos- phorescent. The laws for the reflection of light are the angle of incidence is equal to the angle of reflection; and the incident ray, the perpendicular at the point of incidence, and the reflected ray, all lie in the same plane. The amount of light reflected at any surface depends on the kind of material forming the surface, its degree of polish, and the angle at which the light is incident. Bodies become visible by means of the light which they give off in all directions. When a ray of light passes from a rare to a dense me- dium, it is refracted or bent out of its original course towards the per- pendicular at the point of incidence; if it come from a dense into a rare medium, it is refracted from the perpendicular. Whatever be the angle of incidence, the angle of refraction bears a constant ratio to the angle of incidence, provided the light is passing through the same two media. The incident ray, the perpendicular at the point of incidence, and the refracted ray all lie in the same plane. The refraction of light, in passing from water into the air, causes the water to appear less deep than it actually is. The intensity of light decreases as the square of the distance from the source. QUESTIONS FOR REVIEW. 229 The relative intensity of different lights is measured by means of instruments called photometers. Rays of light passing through a small aperture in the wall of a darkened room form an inverted image of the object from which they came. Mirrors and specula are polished bodies with regular surfaces, that are capable of reflecting most of the light which falls on them. They are either plane or curved. Images formed by plane mirrors are of the same size as the objects, and appear to be as far back of the mirror as the object is in front of it. The visual angle is the angle formed by the rays of light from the extremities of an object meeting at the eye. The apparent size of an object depends on the visual angle under which it is seen. Curved mirrors, by altering the visual angle under which objects are seen, cause them to appear either larger or smaller than they actually are. An object placed before a concave mirror, between the principal focus and the mirror, appears enlarged, erect, and back of the mirror; if placed at either conjugate focus, it is seen at the other focus, in- verted. Questions for Review. By 'what is light caused ? How does it differ from heat ? Name the principal sources of light. Distinguish between a lumi- nous and an illumined body. Define transparency, translucency, and opacity. Are any of the metals transparent? Distinguish between a ray, a pencil, and a beam. What two kinds of pencils are there ? How can you prove that light moves in straight lines ? How are shadows caused? Define umbra and penumbra. Upon what does the shape of a shadow depend? Upon what does its size depend ? Describe any experiment in shadows. What is the velocity of light? Upon what does the amount of light reflected from any surface depend ? Define diffusion, reflection, refraction, and absorption. State the laws for the reflection of light. How do bodies become visible ? When only are rays of light visible ? Is there any substance known which can absorb all the light which falls on it ? 20 230 NA TURAL PHIL OSO PHY. In wliat two different senses is the word phosphorescence used? What do you understand by the refraction of light ? When is the ray bent towards and when is it bent away from the perpendicular at the point of incidence ? State the laws for the refraction of light. Why does clear water appear less deep than it really is ? Why should the intensity of light decrease as the square of the dis- tance ? Describe Bunsen’s photometer. Why are the images formed by light passing through small aper- tures inverted? Describe any experiments proving that the images are inverted. Distinguish between mirrors and specula. In wha.t direction is an object seen by the eye ? Describe the experi- ment of seeing through a brick. How far back of a plane mirror does the image of an object appear? What effect is produced by placing an object between two inclined plane mirrors ? Define visual angle. What effect has the visual angle on the ap- parent size of the object? What two kinds of curved mirrors are there? Define principal focus, conjugate foci, and virtual focus of a concave mirror. Describe the position of the image of an object placed before a con- cave mirror, between the principal focus and the mirror ; when placed at the longer conjugate focus ; at the shorter conjugate focus. CHAPTER 11. LENSES. —OPTICAL INSTRUMENTS AND VISION. 271. Effect of Refraction on Apparent Direction. — When rays of light pass through a prism, the refraction they undergo, both on entering and leav- ing the prism, causes them to emerge con- siderably out of their original direction. If the rays of light from a candle pass through a prism and enter the eye of an observer placed as Fig. lll. — An Effect of Eefraotion. shown in Fig. Ill, the candle will appear out of its real position. 272. Lenses. — Lenses are generally made of pieces of transparent glass bounded by two surfaces, both of which are curved ; or one of which is curved and the other plane. Eays of light on entering and leaving a lens, are refracted or bent out of their course. Objects seen through lenses will, therefore, as in the case of a prism, 231 232 NATURAL PHILOSOPHY. appear out of their true position. The amount of the displacement of an image depends on the shape of the lens, and the kind of material of which it is made. 273. Forms of Lenses. — Lenses may he divided into two classes, viz., converyimj and diveryiny. Converyiny lenses cause the diverging rays which come from a luminous point to converge or collect at one point, after passing out of the lens. Diveryiny lenses cause the rays from a luminous point, after passing through the lens, to diverge as though coming from some other luminous point. The luminous point may be so placed before either converging or diverging lenses as to cause the rays to emerge from the lens parallel to each other. The three forms of converging lenses are sho^vn at A B and (7, Fig. 112, and the three forms of diverging lenses at D E and F, Fig. 113. It will be noticed that the converyiny lenses are all thicker in the middle than at the edyes.^ while the diveryiny lenses are thinner in the middle than at the edyes. These lenses are named as follows : .4 is a double convex leris, or more frequently a convex lens ; £ is a, plano-convex lens ; (7 is a converging concavo-convex lens, or sometimes a meniscus; Z> is a double concave lens, or more frequently a concave lens; H is a plano-concave lens ; and F a diverging concavo-convex lens. ABC Fig. 112. Converging Lenses. D E Pig. 113. Diverging Lenses. light collect. 274. Foci of Lenses. — The foci of lenses are the points at which the rays of In what is about to be said concerning the foci of lenses, it must be distinctly understood that the curvature of the opposite faces, when both are curved, is supposed to be the same, and that the values given for the foci are only approximately true for ordinary glass. The kind of glass used is that commonly employed in making small lenses. LENSES— OPTICAL INSTRUMENTS, VISION. 233 The principal focus of a lens is its focus for parallel rays. The principal focus of a convex lens is situated at about the centre of curvature of the face at which the light is in- cident. Thus, in the lens shown in Fig. lid, the principal focus is shown at F. The principal focus of a concave lens is situated at about the centre of curvature of the face, at which the light is incident. Thus, in the lens shown Fig. 114, — Principal Focns of Convez Lens. in Fig. 115, the principal focus is at F. The con- cave lens causes the light, after passing out of the lens, to appear to diverge. Principal Focus of Concave Lens. as though it came from the point F. The Conjugate Foci. — If a luminous point be placed before a convex lens, at a distance from it equal to twice its radius of curvature, the rays, after emerging from the lens, will be converged to a point at an equal dis- tance on the opposite side of the lens. But if the lu- minous point be situated farther from the lens than twice the radius of curvature, as at (7, Fig.^116, the rays will collect at a focus, C", on ' C' the other side of the lens, somewhere Fed. between the centre of curvature of the lens and a point situated at twice the distance of the centre of curvature in the same side. These foci, G and O', are called re- spectively the longer and shorter conjugate foci. The Virtual Focus. — If a luminous point be placed before a convex lens, at 0, Fig. 117, somewhere between 20 * Fig. 115. 234 NATURAL PHILOSOPHY. the principal focus and the lens, the rays, after emerg- ing from the lens, will diverge as though coming from a point, F, on the same side of the lens as the [ luminous point, and at a | distance from it greater | than the distance of its ; principal focus. This is called the virtual focus. The foci of concave lenses are all virtua l. ! 275. Images Formed by Lenses. — Whenever the \ visual angle under which the eye views an image formed hy a lens.! is different from that under which the eye would '• view the object directly.^ the apparent size of the object is ' different from its real size. If an object, Fig. 118, be held between the prin- cipal focus and the lens, an eye placed on the other side ' A of the lens will see ^ an erect and masni- fied image, apparent- ^ ly situated farther \J from the lens than Fig. 118 . —Virtual Image of Convex Lens. the objcct. If we examine any object with a magnifying-glass, we will find, if we move the object towards and from the lens, \vithout ever taking it farther from the lens than the principal focus, that the size of the image i\dll vary, but that the image ivill be most distinct when the object is i held at a certain distance from the lens. It can be shown that this position lies very near the principal, focus. The image will then appear at the distance at which the eye sees objects the most distinctly, and which is therefore called the limit of distinct vision. The limit of distinct vision varies with different persons, and therefore each Fig. 117. —Virtual Focus. [ I j i I I LENSES— OPTICAL INSTRUMENTS, VISION. 235 person must hold the glass at such a distance from the object that the image formed shall be at his limit of dis- tinct vision; that is, he must focus the glass to suit his eyes. 276. Images at Conjugate Foci. — If an object be placed before a convex lens, any- where between the principal focus and twice the principal focus, — that is, at the shorter conju- gate focus,— an en- . conjugate Focus. larged and inverted image will be formed at its longer conjugate focus ; but if the object be placed at the longer conjugate focus, an inverted and diminished image will be formed at the shorter conjugate focus. In Fig. 119, the object, A B, is placed at the longer conjugate focus of the convex lens, 0, and an inverted image, smaller than the object, is seen at A' B' . As this image is real, it may be received on a screen, as shown. 277. The Eye. — The human eye consists of a nearly spherical chamber, dark- ened on the inside and provided withtwoopenings — one in trout ® ■' for the admission of light, and one at the rear. Fig. 120, for the admission of the nerve called the ogotic 236 NATURAL PHILOSOPHY. nerve^ which conveys the imj^ressions of light to the brain, and thus enables us to see. On its entrance to the chamber of the eye tTie optic nerve is spread out into a delicate net-work of nerve filaments, called the retina., seen in the figure at K. The retina acts as a curtain, or screen, to receive the image formed by the lenses of the eye. At the opening in the front of the eye there is a transparent substance, called the cornea, seen at A, more convex than the ball of the eye. Be- hind the cornea, and forming the colored part of the eye, is a circular curtain, called the iris, seen at D. A circular aperture in the middle of the iris, C, called the pupil, forms the opening through 'which light enters the eye. Immediately back of the iris is a convex lens, E, called the crystalline lens. The space, B, be- tween the cornea and the crystalline lens is filled with a liquid called the aqueous humor. The large cavity, i L, back of the crystalline lens is filled with a clear, ! jelly-like substance, called the vitreous humor. ' All these transparent portions of the eye act lihe one con- I vex lens, and form an inverted and diminished imaye of ] a distant object on the retina. If this image is sufficient- j ly distinct, is properly illuminated, and remains long { enough on the retina, the object is seen distinctly. If too j much light enters the eye through the pupil, the image is not seen distinctly. The iris, however, is so affected by the light that, if too much light passes through the 1 pupil, the pupil contracts or grows smaller ; while if | too little enters, the pupil enlarges and allows more ; light to enter the eye. ! Experiment. — Hang a small mirror immediately under a gas-light. Look in the mirror at the image of your eye, and note the size of \ the pupil; now snddenly turn the light down, leaving only sufficient j light to see the image of the eye. The pupil will now he seen to slowly LENSES— OPTICAL INSTRUMENTS, VISION. 237 dilate. Turn the gas on bright again, and the pupil will he seen to contract. Caution. — Do not cause the pupil to dilate and contract too sud- denly, as weak eyes might thereby be injured. 278. Near-sightedness and Long-sightedness. — Objects are not seen distinctly unless the lenses of the eye cause the images to fall directly on the retina. In many eyes, the lenses converge tlie rays so much as to cause the images of distant objects to be formed in front of tbe retina. Sucb people are near-sighted, and cannot see minute objects distinctly witbout bring- ing them very near tbe eye. Near-sightedness can be partly remedied by the use of concave spectacles. In other eyes, tbe lenses of tbe eye cause tbe images of near objects to fall back of tbe retina. Sucb people i are long-sighted, and cannot see near objects distinctly; i thus, in reading, tbe book must be beld at some dis- I tance in front of tbe eyes. Long-sightedness can be \ partly remedied by the use of convex spectacles. 279. Optical Instruments. — By an optical instru- ment we mean any combination of lenses, or of lenses and mirrors, wbicb will enable us to examine tbe images of distant or near objects. In optical in- struments, a number of lenses are generally used, yet they act, as far as tbe production of tbe image is concerned, as if there %vere but a single lens present ; for example, tbe simple microscope, the pjhoto graphing camera, the magic lantern, o.nd the camera obscura. 280. The Simple Microscope. — Fig, 121 , In tbe simple microscope, a single The Simple Microscope. lens or combination of lenses is placed as shown at 238 NATURAL PHILOSOPHY. il, in Fig. 121. The object to be examined i.s placed on a stage, (7, at a distance from the lens, rather less than its principal focus. An eye placed above A sees an enlarged and erect image of the object. A screw, V, is provided for focussing. 281. The Photographing Camera. — The photo- graphing camera. Fig. 122, consists of a lens placed in a tube at A, inserted in the camera-box, (r. The image of an object, for instance,a person, placed in front of the tube. A, at the longer conjugate focus, is received on a screen of ground glass, A, as an inverted and Fig, 122, -The Photographic Camera, diminished image. This image is sharply focussed on the. screen by means of the screw, D. The screen is then removed and replaced by a plate covered ivith chemicals sensitive to light. The image now falls on the sensitive plate, and is im- pressed upon it by the light causing certain changes in the chemicals on its surface. 282. The Magic Lantern. — In the magic lantera. Fig. 123, a transparent picture, A, is placed in an inverted position, before a lens, B. at its shorter conjugate focus, and is received as an enlarged and erect image on a screen placed in front of the lantern. For The Magic Lantern. the purpose of stronglv illu- mining the picture, a mirror is placed at J/, back LENSES— OPTICAL INSTRUMENTS, VISION. 239 of the source of light, L, and a lens, (7, called the con- denser, is placed in front of the light, and between it and the picture. 283. The Camera-Obscura. — The camera-obscura is an arrangement by which a distinct and well illu- mined image of any object can be thrown on a sheet of paper for convenience in sketching. A mirror, i/. Fig. 12d, is placed in an inclined position above a lens, L. Fig. 124. — Camera Obscnra. The light from a distant object is reflected from the mir- ror to the lens, which forms an image of it on a sheet of paper placed on a table, B, at a suitable distance below. Experiment. — Place a small convex spectacle lens in a suitable hole in the top of a hox; attach a piece of looking-glass directly over the lens, and inclined at an angle of about 45°. On holding a sheet of paper inside the box, at the proper distance from the lens, the side of the box having been removed for that purpose, an image of any object in front of the mirror, as, for example, a tree or person, will be seen on the paper. 284. The Compound Microscope. — In the com- pound microscope and the telescope, at least two sets of lens are used. 240 NATURAL PniLOSOPHY. In the compound microscope, Fig. 125, instead of looking directly at the object, the magnified image formed by one set of lenses is viewed through a second leus, which still farther magnifies it. The lens which is placed Pig. 125. -The Compound Microscope, nearer the object, is called the object-glass; the one nearer the eye, the eye- lens. The object, h a, is placed before the object-glass, 0, at its shorter conjugate focus^ and an inverted and enlarged image formed by it, at a' V. This image, a' U, lies nearer to the eye-lens, (7, than its principal focus, and the eye at D, therefore, sees a greatly enlarged image at A B. 285. The Telescope. — The telescope, Hke the mi- croscope, has only two sets of lenses ; the set nearer the object is called the object-glass, and that nearer the eye the eye-glass. Telescopes may be constructed either with both object- and eye-lens of glass, in which case they are called refracting telescopes ; or a concave mir- ror may be used in place of the object-lens, when they are called reflecting telescopes. In the refracting telescope, shown in Fig. 126, 0 is the object-glass and E the eye- lens. Since the tel- escope is u.'^ed forvievdnadis- Fig. 126. — The Eefracting Telescope. tautobject^ the object is at the longer conjugate focus. Its image, there- LENSES— OPTICAL INSTRUMENTS, VISION. 241 fore, seen at a h, is inverted and diminished. As in the microscope, this image falls within the principal focus of the eje-lens, and is therefore viewed by the eye placed at as a magnified image. A' B' . 286. Reflecting Telescopes employ a mirror for the object-glass in- stead of a lens. In Fig. 127 we have a representation of Herschel’s reflecting telescope, in which ,1 • nr c Fiff. 127. — The Eeflecting Telescope, the mirror, ii, forms & r an image, a Zi, of a distant object, which, viewed by the eye at 0, through the eye-lens, A, appears as a magni- fied image, a' V . The penetrating power of a telescope is the distance at which it can collect and transmit to the eye suffi- cient light from a distant object to enable it to be dis- tinctly seen. The telescope owes its great penetrating power to the size of the object-glass. In the unaided eye, the amount of light which can enter the eye from a dis- tant object is limited by the size of the pupil ; but by 1 the use of a telescope, all the light which falls on the i object-glass is swept into the eye. The penetrating I power of the telescope will, therefore, be as much [ greater than that of the eye as the area of the object- glass is greater than the area of the pupil of the eye. In a large reflecting telescope, made by an Englishman named Lord Eosse, the area of the concave mirror, [ which acted as the object-glass, was 518,400 times 1 greater than the pupil of the eye. This instrument i collected, even after allowing that half the light was i 21 Q 242 NATURAL PHILOSOPHY. 3 lost by reflection, rather more than 250,000 times as much light as the unassisted eye. 287. Dispersion of Light — The Spectrum. — We have seen that sounds of a single pitch are seldom pure, being always accompanied by a number of sounds of some other pitch : the same is true of light. Sun- light, though apparently pure, — that is, of one color, — in fact contains a very great number of different-col- ored lights. Idle different colors that are present in sunlight may be separated from one another so as to be visible as separate colors, by passing the light through a prism. If sunlight coming through a narrow slit, in the shutter of a dark room, in the direc- tion -FA", be allowed to pass through a prism, P, as shown in Fig. 128, the light will not only be bent out of its course by Fig. 128.— Formation of a Spectrnm by a Prism, refraction, but it will also be separated into a number of differently colored rays, which, if allowed to fall on a screen, ivill be spread out in the form of a brightly colored band, R T , called a spectrum. There are almost an infinite variety of colors in the spectrum of sunlight ; but, for the sake of simplicity, we may distinguish seven well-marked regions of color, which are called violet, indigo, blue, green, yellow, orange, and red. The order of these colors maybe easily remembered by the word vthgijor, which is formed by putting together the first letter in each of the colors. LENSES— OPTICAL INSTRUMENTS, VISION. 243 It will be observed in Fig. 128 that the different colors of the spectrum are turned out of the direction in which the sunlight was moving, in different degrees ; thus, the red is the least and the violet the most turned out of its course; that is, the different colors of the spec- trum differ in their refranrjihility. The separation of light into different colors, by its passage through a prism, is called dispersion. The different colors of the spectrum differ in the lengths of the ether-waves which cause them. The wave-lengths of the red are the longest, and those of the violet the shortest. These waves are so exceed- ingly small that 37,640 of the red, or 59,750 of the violet, would only equal one inch in length ! 288. The Re-combination of the Colors of the Spectrum. — If all the colors of the solar spectrum be again mixed together, they will produce white light like that of the sun. This may be done by causing ; the spectrum to fall on a convex lens, and allowing the light which passes through to fall on a screen placed at the focus. The spot of light formed on this screen , will no longer be colored, but will be pure white. Experiment. — Allow a narrow beam of light entering a darkened room through a hole in the shutter to pass through one of the glass. ' prisms taken from a chandelier. The spectrum formed may be received ' on a sheet of white paper held near the prism. \ Experiment. — Cut out a disc of stiff paste- i board, and paint on it, in the spaces shown in ^ Fig. 129, seven colors, as near as can he ob- ' tained to those seen in the solar spectrum or in the rainbow. Stick a pin through the centre I of tlie disc, and whirl it rapidly around with ; the fingers. When it is turning fast enough the disc will appear to be painted grayish white, from the mingling of the different colors. Caution. — Observe the size of the spaces shown in the figure. 244 NATURAL PHILOSOPHY. 289. The Cause of Color. — When sunlight falls on any colored body, as, for example, on a piece of red cloth, all the colors of the light but the reds are ab- sorbed, and these colors only being given off from the cloth, it appears red ; so in a green leaf all the colors but the greens are absorbed, and the greens only given off. The colors of bodies, then, are due to the light which falls on them. In the dark, no body has color. ! If a body of any color, such, for example, as pure j red, be illumined by light which contains no red, then ' the body will appear black. Thus, if a piece of bright i red flannel be held in the pure green light of the solar | spectrum, it will appear black. j Experiment. — Eoll a piece of lamp-wick into a loose ball, and soak it for a few moments in very strong solution of salt in w'ater. Then ; place it in a saucer and pour some alcohol (spirits of wdne) over it, and set fire to the wick. It will hum with a pure yellow light. If, now, , different-colored objects, such as zephyrs, cloths, or silks, be examined by means of this light, in an otherwise darkened room, they will all appear to have lost their color, except those which were yellow. Now bring a lighted candle near any of the colors, and, since the lighted candle gives off light of all colors, the color of the fabrics will again appear. Caution . — Unless the room can be darkened very thoroughly, this i experiment should be tried at night. 290. The Rainbow. — When sunlight passes through falling rain-drops, it is separated into its : different colors, the rain-drops acting like prisms. In i entering the drops the light is reflected from the sur- faces farthest from the sun, and passes out separated , into its prismatic colors. This light entering the eye ! of an observer standing with his baek to the sun, causes him to see a band of colors called the rainbow. Eainbows are largest when the sun is near the horizon, : as when nearly setting, or shortly after rising. SYLLABUS. 245 291. Relations of Light and Radiant Heat. — Both light and radiant heat are effects produced by one and the same cause, viz., hy vibrations or waves in the lumi- niferous ether, and differ only in the fact that the vibra- tions which 'produce light are more rapid, and the waves shorter than those which produce radiant heat. When any solid body, as, for example, a piece of platinum, is heated, its molecules vibrate backwards and forwards, and by imparting their motion to the ether outside of them, give off’ or radiate heat. As the body becomes hotter and hotter, its molecules vibrate more and more rapidly, and give off shorter and shorter waves to the ether outside the body, until at last these ether-waves thus radiated from the body become short enough to aff’ect the eye, and the body is red hot and radiates red light, the color produced by the longest ether-waves. If now we continue heating the platinum, and exam- ine, by a prism, the light it radiates, we will find, as it grows hotter, that the next color given off will be the orange, the next longest wave-length, and then the yel- low, the green, the blue, the indigo, and the violet ; at this moment the body will be white hot, and, like the sun, will give off' all the colors of the spectrum. Syllabus. Objects seen through a prism appear out of their real position, on account of the light being refracted on entering and leaving the prism. Lenses are pieces of transparent glass bounded by two surfaces, both of which are curved, or one of which is curved and the other plane. Lenses are either converging or diverging. The converging lenses are the doubly convex, the plano-convex, and the converg- ing concavo-convex. The diverging lenses are the double concave, the plano-concave, and the diverging concavo-convex. 21 * 246 NATURAL PHILOSOPHY. The foci of lenses are the points at which they cause the rays pass- ing through them to collect. The principal focus of a double convex lens is situated at the centre of curvature of one of its curved faces. The shorter conjugate focus is situated between the principal focus and a distance twice as great as that of the centre of curvature. The longer conjugate focus is situated farther from the lens than twice its centre of curvature. Whenever the visual angle under which the eye views an image formed by a lens, is different from that under which the eye would view the object directly, the apparent size of the object is different from its real size. When we look through a convex lens at an object placed between the lens and its principal focus, an erect magnified image is seen on the same side of the lens as the object, but farther from it than the object. If the. object he placed at the shorter conjugate focus, a magnified and in- verted image will appear at the longer conjugate focus. If the object be placed at the longer conjugate focus, an inverted and diminished image will appear at the shorter conjugate focus. The eye consists of a dark chamber containing a number of lenses, which cause images of objects in front of the eye to fall on a screen. This screen is at the baek of the eye. The cause of near-sightedness is the too great converging power of the lenses of the eye. The cause of long-sightedness is their too feeble converging power. In near-sightedness the image falls in front of the retina, and in long-sightedness it falls back of the retina. Near-sight- edness may be partly remedied by the use of concave, and long sight- edness by the use of convex spectacles. The simple microscope contains a single convex lens or set of lenses. The object to be examined is placed rather nearer to the lens than its principal focus ; the images it forms are erect. In the photographing camera, the object is placed before a single convex lens or set of lenses, at the longer conjugate focus. The image is received by a sensitive plate placed at the shorter conjugate focus. In the magic lantern, a strongly illumined picture placed in an in- verted position at the shorter conjugate focus of a single lens or set of lenses, appears as an erect, magnified image on a screen placed at the longer conjugate focus. In the camera obscura, a lens forms an image of any distant object, which image is received on a sheet of paper placed at its shorter con- jugate focus. In the compound microscope and in the telescope there are two lenses or sets of lenses. The one near the object is called the object-lens ; the one near the eye, the eye-piece. I QUESTIONS FOR REVIEW. 247 la the compound microscope, the object is placed at the shorter con- jugate focus of the ohject-lens, and forms a magnified image near the principal focus of the eye-lens. The eye sees the magnified image of this image through the eye-lens. In the telescope, the object is placed before the object-lens at its longer conjugate focus, and forms an inverted and diminished image near the principal focus of the eye-lens. The eye views this image through the eye-lens, by which it is magnified. The telescope collects as much more light than the unassisted eye as the area of the object-glass is larger than the area of the pupil. When sunlight is passed through a prism it is separated into a great number of colors called a spectrum ; this separation is called dispersion. There are seven well-marked colors in the spectrum, viz. ; violet, in- digo, blue, green, yellow, orange, and red. The red is the least and the violet the most refrangible. If all the colors of the solar spectrum are again mixed together they will form white sunlight. The cause of color is due to certain of the rays of sunlight being ab- sorbed by the bodies on which the light falls, and only the rest of the colors being given off. The rainbow is caused by the dispersion of sunlight by rain-drops. Light and radiant heat differ from one another only in that the vibrations of the ether-waves that cause light are more rapid than those which cause radiant heat. I Questions for Review. How is the position of an object affected by viewing it through a prism ? What are lenses ? Name the three forms of converging and t the three forms of diverging lenses. I What is the position of the principal focus, shorter conjugate focus, j longer conjugate focus, and virtual focus of a convex lens ? Why do lenses cause the apparent size of objects seen through them to differ from their real size ? I What will be the position of the image formed by a convex lens, ! when the object is placed between the principal focus and the lens; at j the shorter conjugate focus; and at the longer conjugate focus? Describe the general structure of the eye. Name the situation and [ use of the following parts of the eye, viz. : the cornea, the aqueous i humor, the iris, the pupil, the crystalline lens, and the vitreous \ humor. ! 248 NATURAL PHILOSOPHY. What is the cause of near-sightedness ? What is the cause of long- sightedness ? How may each be partially remedied ? What are optical instruments ? Describe the construction and oper- ation of the following optical instruments, viz. : the simjile micro- scope, the photographing camera, the magic lantern, and the camera obscura. How does the compound microscope differ from the simple micro- scope ? What two kinds of telescopes are there? To what does the tele- scope owe its great penetrating power ? How does the refracting telescope differ from the microscope ? How can you prove that the light of the sun contains a great num- ber of different-colored lights ? Name seven of the most prominent of these colors. Define spectrum ; dispersion. Which of the colors of the spectrum has the greatest refrangibility ? Which has the least refrangibility? Which has the greatest wave- length ? Which has the least wave-length ? What color results from the mixing of all the colors of the solar spectrum? How may this mixing be effected? Explain in full the cause of color. What is the cause of the rain- bow ? What is the only difference between light and radiant heat? CHAPTER III. ELECTRICITY. — ELECTRICAL CHARGE, OR ELECTRICITY OF HIGH TENSION. 292. The Nature of Electricity. — But little is kuown as to the real nature of electricity. It is, however, a form of energy, and all other forms of energy can readily be converted into it. 293. Varieties of Electrical Energy, — Electricity manifests its presence in a variety of ways ; these, however, may all be arranged under two heads : viz., 1st. As a charge. 2d. As a current. 294. Electrical Charge or Electricity of High Tension. — If a dry rod of glass or stick of sealing- wax be briskly rubbed with a piece of silk or flannel, the portions rubbed will acquire, in addition to the properties they originally possessed, the power of at- tracting or repelling light objects. By means of the friction, the glass or wax has become electrified, that is, has acquired an electrical charge. Bodies may acquire an electrical charge in a number of ways, one of the commonest of which is by means of friction. An electrical charge is most manifest when it is 249 250 NATURAL PHILOSOPHY. electricity of high tension, so named from its ability to leap through distances in order to overcome ob- stacles placed in its path. Electricity of high tension is seen in its greatest development in lightning. 295. Current Electricity. — When a body contain- ing an electrical charge is brought into contact with another body through which electricity is capable of passing, a current of electricity ensues. Such cur- rents, however, are but momentary. If a piece of copper and a piece of zinc, or two pieces of any different metals joined by a wire, be dipped into an acid solution that acts chemically on one of the metals, a continuous current of electricity will flow through the wire as long as the metal is acted on by the liquid. The current flowing through the wire will cause it to acquire, besides the properties it originally possessed, a number of additional properties, among which may be men- tioned its power of attracting or repelling other wires through which electrical currents are flowing, or of attracting or repelling certain bodies called magnets. 296. Effects Produced by Electrical Charge. — -If a body containing an electrical charge, such, for ex- ample, as a rubber comb, which has been briskly rubbed with a silk handkerchief, be brought near the face, a creeping sensation will be experienced, as though cob-webs were touching it. If the electrified body be brought near a blunt metallic body or the knuckle of the hand, a faint bluish spark will pass to the metal or the hand, with a slight crackling sound. Whenever any body receives an electrical charge by means of friction, both the body rubbed and the thing rubbing it are electrified. O O The high-tension electricity produced b}'' friction is ELECTRICITY OF HIGH TENSION. 251 sometimes caWed. f rictional electricity. Since, however, it can be obtained in a variety of other ways, the name is inappropriate. 297. Conductors of Electricity. — Bodies differ very considerably as to the resistance they offer to the passage of electricity through them. Some, like the metals, are very good conductors.^ while others, like resins or hard rubber, are very poor conductors. In the following table a number of common sub- stances are arranged in the order of their conductivity, beginning with the best conductors and ending with the poorest. 1. Metals. 2. Charcoal. 3. Graphite. 4. Acids. 5. Water. 6. Vegetables. 7. Animals. 8. Linen. 9. Cotton. 10. Dry wood. 11. Paper. 12. Oxides. 13. Caoutchouc. 14. Dry paper. 15. Silk. 16. Glass. 17. Wax. 18. Eesins. 19. Shellac. The conducting power of the substances near the end I of the list is so slight that they are sometimes called non-conductors. When a conductor is supported on a I non-conductor it is said to be insulated.^ and will then, if the air be dry, retain its excitement for a long time. Moist air is a partial conductor of electricity, while dry air is a non-conductor. All experiments in high- tension electricity should therefore he tried in clear., cold, dry weather, as in warm, damp weather the electrified body rapidly loses its charge. 298. Attractions and Repulsions by Electrified Bodies.— The attractions and repulsions of light bod- ies by electrified bodies can be conveniently shown by 252 NATURAL PHILOSOPHY. means of a pith-ball suspended by a silk thread from any suitable support, as shown in Fig. 130. When an Fig. 130. — Electrical Attractions and Eepnlsions. electrified body, such, for example, as a rod of glass which has been rubbed with a dry silk handkerchief, is brought near the pith-ball, the latter is attracted to the glass, as shown at A. As soon as the pith-ball touches the glass rod it is repelled from it, as shown at A, and if not allowed to touch the ground, or any conducting body in connection therewith, will continue to be repelled. If, however, it touches such a body, it will again be attracted to the rod, and again repelled. If any other electrified body, as, for example, a piece of sealing-wax rubbed with flannel, be brought near the pith-ball while it is quietly hanging from its support, it will be attracted and repelled the same as the glass. If, however, while the pith-ball manifests repulsion for the electrified glass, the electrified seal- ing-wax be brought near it, it is at once attracted ; or if it is repelled by the wax it is attracted by the glass. If any other substance be electrified by friction, we will find that it acts either like the glass or like the wax ; and we therefore conclude that there are but two kinds of electrical charge or excitement, viz. ; one like ELECTRICITY OF HIGH TENSION. 253 tliat excited in glass rubbed with silk, and one like that excited in wax rubbed with flannel. The former is called positive and the latter negative electricity. Posi- tive electricity is generally represented by a -f-; nega- tive by a — . The law of electrical attractions and repulsions may be stated as follows : Bodies charged ivith the same hind of electricity repel one another ; those charged with different hinds of electricity attract one another. This law is the same as that of magnetic attractions and repulsions. 299. Electroscope. — An electroscope is an instru- ment used to determine the kind of electricity with which a body is charged. The pith-ball shown in Fig. 130 is an electroscope. A better form, however, con- sists of two small strips of gold-leaf, n n, Fig. 131, at- tached to a metal rod ter- minating in a metal ball, c. The gold-leaves and rod are placed in a glass jar, B., in- side of which the air can be kept free from moisture. If the ball be touched by an electrified body, the gold-leaves receive a charge of the same kind as that in the electrified body, and are therefore repelled. To determine, by means of the electroscope, the kind of electricity with which any bodj^ is charged, the gold- leaves are repelled by a known kind of electricity, say positive, which can readily be done by holding a body whose electrical charge is known to be positive, near 22 254 NATURAL PHILOSOPHY. the ball, c. ISTow, while the leaves are thus diverged, bring the body, the kind of whose charge is unknown, near the ball, c, and watch the gold-leaves. If they divercye still farther., the charge of the body is posi- tive ; if, however, the leaves are attracted, its charge is negative. Experiment. — Cut out two pieces of gold or silver paper, A and B, about two inches square. Tie two pieces of sewing-silk of the same length to holes at a i and c (f, as shown in Fig. 132, and hang tliem to a rod or other support, C, so that they shall be directly opposite each other, and they will form an excellent electroscope. If the leaves be touched by an elec- trified body, they will be repelled ; and if the air be dry, will continue to stand apart for a long time. 300. Positive and Negative Charge, — The kind of electricity produced by rig, 132.— A Sim- friction, that is, whether positive or pie Electroscope, negative, depends on the bodies that are rubbed together. In the following list of substances suitable for pro- ducing electricity by friction, the different substances are so arranged that each will be positively electrified if rubbed by any body which it, but negatively electrified if rubbed by any body which precedes it. 1. Cat’s skin. 2. Woollea &brics. 3. Glass. 4. Cotton. 5. Silk. 6. The hand. 7. W ood. 8. Sealing-wax. 9. Hard rubber. Thus, if glass be rubbed with cat’s skin or flannel, it becomes negatively electrified ; but if rubbed with cot- I ton or silk it becomes positively electrified. | The rubber is always oppositely electrified to the I thing rubbed. Glass rubbed with silk is positively 1 electrified, while the silk is negatively electrified. I ELECTRICITY OF HIGH TENSION. 255 301. Hypotheses of Electricity. — A number of different hypotheses or suppositions as to the nature of electricity have been proposed, none of which, how- ever, are quite satisfactory ; we shall consider two of the most prominent, viz.: the single-fluid hypothesis.^ and the double-fluid hypothesis. The Single- Fluid Hypothesis was first proposed by Franklin, an American philosopher. It ascribes all electrical phenomena to the presence of an imaginary fluid, the particles of which are self-repellent, but are attracted by all matter, and are, therefore, supposed to be present in all bodies. When a body contains a cer- tain amount of this fluid, we cannot detect the presence of the fluid; but if by any means the quantity of the fluid be either increased or diminished., then the fluid manifests itself. When a body has more than its natural amount of the fluid, it is positively excited or charged ; when it has less than its natural amount, it is negatively excited. When glass is rubbed by silk, the glass is supposed to take some of the fluid from the silk, thus becoming positively, and leaving the silk, negatively electrified. The Double-Fluid Hypothesis of Symmer and Du Faye, supposes the existence of two different fluids, viz., the positive and the negative. The particles of either of these fluids will repel particles of its own kind, but will attract particles of the other kind. When both fluids are present in the same body, they mask or neu- tralize each other, so that it is only when they are separated that they can produce an electrical charge. When a body is electrified by any means, these fluids are supposed to be separated from each other. Thus, when glass is rubbed with silk, the neutral electricity of each is decomposed, the glass gives its negative to 256 NATURAL PUILOSOPHY. tlie silk, and tke silk its positive to the glass, and tlius eack becomes electrified. It should be borne in mind that these are merely hypotheses. We do not know whether electricity is a fluid, or whether, like light and heat, a wave-motion of the ether or of the atoms of bodies. The hypotheses are of value on account of the aid they afford in connecting a number of phenomena. Either the single- or the double-fluid hy- pothesis will explain nearly all the facts. For most purposes the sin- gle-fluid hypothesis is preferable to the double-fluid hypothesis. 7mi 1 A A 302. Induction of Electricity. — If an insulated con- ductor, as, for example, the cylinder, A jB, Fig. 133, be brought near an elec- trified body, as the in- ^sulated excited sphere, C, the cylinder will become electrified by induction,andthepith- balls hung thereon will manifest repulsion. ® This repulsion, how- ever, will be unequal, the pith-balls near the ends, A and B, showing the strongest excitement, while those at the point, J/, show none. If C be charged with posi- tive electricity, then the end A will be negative and the end B positive, as may be proved by means of an electroscope. The cause of induction can be explained by either the single- or the double-fluid hypothesis. Taking the sin- gle-fluid hypothesis, the free fluid in C disturbs the elec- trical equilibrium in the cylinder, and repels its elec- tricity to the end, i?, farthest from it. The cylinder now has a deficiency of fluid at which end is there- fore negative, and an excess at -B, which is therefore positive. It contains, however, no more electricity than ELECTRICITY OF HIGH TENSION. 257 it did at first, and if C be now removed, the fluid will flow from B towards C, and restore the electric equi- librium. If, however, anj part of the cylinder be touched while within the influence of C, the repulsion of C will drive some of the electricity from the cylin- der to the ground, and when the sphere, C, is removed, it will have less than its natural amount of fluid or will be negatively charged. When a body, therefore, is permanently electrified hy induction, its electricity is of the opposite name to that of the exciting body. When a body is electrified hy contact, it is electrified by electricity of the same name as that of the exciting body. 303. Cause of the Attractions and Repulsions of Excited Bodies. — Alternate attractions and repulsions of light bodies by electrified surfaces are the result of induction. If an un- electrified pith-bail, B, be brought near a conductor. A, Fig. 134, charged with positive electricity, it is at once electrified by induction, and the side nearer A, being of the opposite Fig. 134. Cause of Eleotrioal Attraction and Eepulsion. electricity to A, the ball is attracted to the conductor. As soon as it touches the conductor it receives a charge of free positive electricity, the same as that of the conductor, and is therefore repelled from it. If, now, it should touch a body, G, in connection with the ground, it will again be attracted and repelled as be- fore. 22* E 258 NATURAL PHILOSOPHY. 304. Electrical Tension. — All the phenomena of electricity result from the tendency of electrified bodies to regain their electrical equilibrium. According to the single-fluid theory, the body having an excess en- deavors to give off some of its fluid to the body having a deficiency. According to the double-fluid theory, the positive fluid of one body endeavors to combine with and neutralize the negative fluid of another bod3^ The condition of strain produced by these tendencies is called the electrical tension. 305. Distribution of an Electrical Charge. — When an insulated conductor is electrified, the electricity is all apparently on the outside of the body. An insulated hollow sphere, provided with a hole in the top, is charged with electricity. To determine the distribu- tion of the charge, a proof plane which consists of a small metallic disc attached to the end of a glass rod, may touch the outside of the sphere at any point, and will always take away a small charge of electrieity, as may be proved by an electroscope. But if touched to any part of the inside of the sphere, care being taken not to allow it to touch the edge of the hole in insert- ing or removing the proof plane, it will be found to contain no electricity, all the charge of the sphere resid- ing on its surface. It must be remembered, however, that this is only true for an electrical charge. When the electricity is in motion, or is an electrical current., then it passes through the whole substance of the conductor. A hollow wire will not conduct electricity any better than a solid wire of the same weight and length. 306. The Influence of Points. — In an insulated excited sphere the depth of the electrical fluid, as ELECTRICITY OF HIGH TENSION. 259 shown by the tension, is the same over all parts of the surface ; but if the excited conductor be egg-shaped, then the tension is greatest near the point of the egg, and this will be found to increase the sharper the point, so that when the end is very pointed, the tension may become sufficiently powerful to enable the electricity to escape into the air. Conductors intended to retain an electrical charge are, therefore rounded so as to avoid the presence of points. Fig. 135. The Electrophorus (charging). 307. Electrical Machines. The Electrophorus. — The simplest form of electrical machine is the electro- phorus. It consists of a plate, J., of brass or some other metal, attached to a glass rod, and a plate of resin, i?, placed in a me- tallic dish. The resin is excited with negative electricity by rub- bing it briskly with a piece of cat-skin. The plate, J, is then held by the glass handle, and placed over the resin, as shown in Fig. 135, and be- comes electrified by induc- tion, the side nearest the resin being positively, and the side farthest from it neg- atively, charged. If A is now raised from the resin without previously touching it to any conductor, it will be found to possess no charge, since the electricity on the positive side of the disc would flow towards the negative side, and neutralize it. If, how- Fig. 136. The Electrophorus (discharging). 260 NATURAL rillLOSOPHT. ever, the disc be touched hy the finger, it will, when raised from the resin, contain free positive electricity. Experiment. — An excellent electrophorus may be made as follows: Place in a tin pie-plate equal parts of rosin and gum-shellac, sufficient in quantity to nearly fill the plate when melted ; place the dish over a fire, and very gradually melt the rosin and shellac, at the same time stirring with a stick to break air bubbles. When melted, set the dish on a flat support to cool. Now cut a disc of wood smaller in diameter than the rosin-plate; bore a hole in the middle of the wood, and cement in it a glass rod or tube. Paste the tin-foil over the wooden disc, covering it completely, and remove any rough ends by smoothing the foil with the finger-nail. Now rub the rosin plate with a bit of silk or cat-skin, and place the tin-foil disc on the rosin, and touch it with the hand. On removing the disc a spark of positive electricity may be taken from it by the knuckle. When the rosin-plate is once charged by rubbing, an indefinite number of sparks may be obtained by placing the disc each time on the rosin, and touching with the finger, as before. When the air is dry and cold, as in winter, and the glass handle clean and dry, sparks of considerable length can be obtained. | Caution. — Do not overheat the mixtures of rosin and shellac, or j bubbles will form and spoil the surface. Before using the electro- ! phorus, be sure that everything is dry. I A very great number of electrical macliines have been I devised. We will describe, however, only one, viz. : i 308. The Plate Electrical Machine. — This machine i consists of a circular plate of glass. A, Fig. 137, mounted | on an axis, B, on which it can be turned by means of | an insulated handle, C. At D, a rubber made of piano i felting or chamois skin is pressed by brass springs firmly i against the plate. The rubber has generally a mixture I of tin and mercury, called an amahjam, spread over its ; surface, and is in electrical contact with an insulated j conductor, A, called the negative conductor. A series i of metallic points. A, connected with an insulated con- ductor, 0, called the positive or prime conductor, is I placed near that part of the plate diametrically opposite | the rubber, as shown. On turning the handle, the fric- | ELECTRICITY OF HIGH TENSION. 261 tion of tlie rubber on tbe glass causes the glass to become positive and tlie rubber negative. The negative con- ductor is now charged by the rubber, while the electric- ity on the glass, coming opposite the points, charges the Fig. 137. — Plate Electrical Machine. conductor connected with them positively. The lower half of the plate is loosely covered by a bag of silk, S. When only positive electricity is desired, a chain, IF, connects the negative conductor with the ground. ' 309. The Condensation of Electricity. — When an insulated con- ductor, Fig. ' 138, is brought into contact with the con- ductor of an electrical ma- chine, it can- not receive a charge hav- ing a tension Fig. 138. — The Condenser of ilpinns. higher than that of the machine. If, however, it be 262 NATURAL PHILOSOPHY. placed near a second insulated conductor, B, separated from it by a plate of glass, (7, and tbe two conductors moved towards each other until they touch the glass plate, and the plate, i?, be connected with the earth, then A can receive a charge whose tension is greater than that of the electrical machine. This apparatus, shown in Fig. 138, is called the condenser^ and the process by which A and B receive this high-tension charge, the condensation of electricity. The manner in which the charge is given to A and B is as follows : Suppose A positive ; then by induc- tion B becomes negative, and its free positive electricity is repelled to the ground. The positive and negative | electricities in A and B neutralize each other’s effects, | thus leaving the plates free to receive a new charge, i A can therefore receive more electricity from the ! machine, which again acts by induction through C, | and allows more electricity from A and i? to be i neutralized. In this way the tension of the charge : in the condenser may become very great, and in- ! deed often becomes sufficiently intense to pierce the i glass. \ To discharge the condenser.^ it is only necessary to j join the plates A and B by any conductor, when i the attractions of the opposite electricities cause them to flow through the conductor and neutralize each j other. 310. The Leyden Jar. — The condenser is generally i made in the form of the Leyden jar, which is shown in Fig. 139, where the lower part of a glass jar is coated on the inside and outside with tin-foil. The inside coating is in connection with the brass knob. A, by means of the chain, E. To charge the jar it is held ELECTRICITY OF HIGH TENSION. 263 in tlie hand, which grasps the outer coating while the knob, 4, is brought near the conductor of an electrical machine, and a number of sparks passed into the jar. If now a person place one hand on the outer coating, and the other on the knob, J., the opposite electricities pass through his body and give a more or less severe shock. A battery of Leyden jars consists of a number of jars having their inner and outer coatings respectively connected with each other. The shock from a Leyden jar may be passed through a number of persons joined hand to hand, the person at one end of the line touching the outside coating of the jar, and the person at the other end the knob. The shock from a very large battery may prove fatal. Fig. 139. The Leyden Jar. 311. Effects of the Electric Discharge. — The ef- fects produced by the electric discharge are physio- logical.^ luminous., thermal, mechanical, and chemical. The physiological effects are partly seen in the shock produced by the passage of the discharge through the body. The luminous effects are seen in the bright spark that always accompanies the discharge. The spark is more brilliant the greater the tension of the electricity. Its color depends on the kind of conductor between which the spark passes, and on the kind and density of the gas through which it passes. The thermal or heating effects are seen in the heat produced whenever the passage of the discharge is re- sisted as by passing it through a very thin wire or gold-leaf, which may be volatilized by the heat. The 264 NATURAL PHILOSOPHY. fact tbat the spark gives off light proves that it is itself hot. A spark from ah electrophorus will ignite a gas flame. The mechanical effects are seen in the violent frac- tures or tearing produced when a powerful discharge is sent through any poor conducting substance. Thick glass plates may be pierced, and boards fractured by the discharge from a powerful Leyden battery. The chemical effects are seen in the combinations or decompositions produced by the discharge. A small quantity of the nitrogen and oxygen in the air com- bine and form nitric acid, on passing a number of dis- charges through moist air. A spark of electricity sent through a piece of paper moistened with iodide of po- tassium stains the paper brown where the spark pierces it, by a deposit of free iodine. 312. Atmospheric Electricity. — The atmosphere almost always contains a charge of positive electricity, though it sometimes changes rapidly to negative on the approach of clouds or in stormy weather. Its in- tensity is least near the earth’s surface, and increases with the altitude. 313. Lightning. — The moisture in clouds enable them to collect the free electricity of the air ; for, since damp air is a conductor, the clouds collect the elec- tricity of the air through which they are moving, and allow the electricity to pass through and collect on their surfaces, until considerable tension is acquired. When such clouds come near the earth, the ground below them becomes oppositely electrified by induc- tion ; and when these opposite charges acquire suffi- cient tension, they discharge into each other through the air : the flash which accompanies the discharge is ELECTRICITY OF HIGH TENSION. 265 called lightning. An electrified cloud sometimes dis- charges into another cloud which is oppositely elec- trified. The thunder which follows the lightning is caused by the violent disturbance in the air produced by the electricity moving rapidly through it, or by the rapid formation and condensation of vapor on the passage of electricity through drops of rain. 314. Lightning-Rods. — Lightning-rods, first pro- posed by Franklin, consist of stout rods of iron, or preferably of copper, attached to the outside of the building to be protected, and extending some little distance above its highest point. The upper end of the rod should be pointed, and its lower end should extend deep into the ground.^ until it meets permanently damp earth, or some conductor of electricity. If under- ground water- or gas-pipes are in the neighborhood, it is well to connect the rod to them. If the roof of the building be of metal, such as tin or copper, it should he connected with the rod. The rod should be of sufficient thickness to conduct to the earth, without being melted, the heaviest discharge that may strike the building. A solid rod is to he preferred.^ as an electrical current passes through the whole mass of the conductor.^ and not only over the surface. A lightning-rod does not always protect the build- ing by conducting the discharge from the cloud to the earth. More generally it acts by quietly discharging the cloud ; becoming, oppositely electrified by the cloud, it then quietly discharges this opposite elec- tricity into the cloud, thus neutralizing its charge. A lightmng-rod not well electrically connected with the earth, is more a source of danger than of protection, 23 266 NATURAL PHILOSOPHY. since it attracts the discharge without being able to safely conduct it to the earth. 315. Experiments in Frictional Electricity. — Quite a number of entertaining and instructive electrical experiments may be shown with easily contrived ap- paratus. A few of these will be mentioned. The student is earnestly advised, as far as possible, to make the information his own, by trjdng and verifying the facts by experiment. Experiment — Cut a piece of elder pith into a ball, tie it to a piece of silk, and suspend it from any support. Try the effect of rubbing different bodies together, and hold them near the pith-ball, to see whether they are electrified. Experiment. — Use the gilt paper electroscope, described in a pre- vious experiment, and see what kind of electricity you have produced. As far as you can, test the correctness of the table given in par. 300. Experiment. — Cut two discs of gilt paper, a and h, as large as silver quarter dollars, and connect them, as shown in the Ex. on page 254, but with linen or cotton thread. Stick a wire through a cork of a wide-mouthed bottle, and tie the threads to the end of the wire, leav- ing the discs hanging by about inch of thread. Solder a smooth metal button. A, to the end of the wire, and put the cork in the bottle, so that the pieces of paper shall be inside the bottle, as shown in Fig. 140, first, however, being sure that the bottle is perfectly dry by heating it on a warm stove. The cork also must be perfectly dry. Run melted sealing- wax over the top of the cork, so as to prevent any moist air from afterwards getting into the bottle. This apparatus will now serve as an electroscope. Experiment. — Attach a long metallic wire to the button, A, of the electroscope just made. Excite the electrophorus described on p. 260, and touch the far end of the wire with the tin-foil disc. The leaves, a and b, will at once diverge, showing that the wire has conducted the electricity to them. Try the same with a dry silk thread, and prove that it is a poor conductor. Experiment. — In the corners of a square piece of wood bore four holes, large enough to insert the necks of four porter or ale bottles. Stand a person on this insulated stool, and charge him with electricity Pig. 140. An Electroscope. ELECTRICITY OF HIGH TENSION. 267 from the electrophorus by giving him 15 or 20 sparks from the tin-foil disc. If, now, the knuckle be approached to any part of his body, an electrical spark will pass from it to the knnckle. Experiment. — Place a person on the stool and charge him as be- fore. He can now light the gas with the spark from his finger. Experiment. — Place a person on the stool, and let him hold the electroscope. Fig. 140, in his hand, with his finger on the knob, A. Then strike him on the hack with a piece of cat-skin ; at every stroke the leaves, a and 6, will be seen to diverge. Experiment. — Suspend by silk threads, a and 6, as shown in Fig. 141, a tomato-can, free from sharp edges ; attach at the lower end, by linen or cotton thread, pith- halls, c and d, and you have an insulated conductor. Experiment. — Make another snch conductor, and try the effects of induction as described in par. 302. Experiment. — Excite the electrophorus, p. 260, and convince yourself that it is working and giving sparks. Get some one to hold the point of a pin near it when you are trying to get a spark from it, and the pin will discharge the tin-foil disc, and no spark will pass from it to the hand. Experiment. — To make a Leyden jar, select a candy jar with a wide mouth, preferably with glass of a greenish hue, and paste tin-foil on the inside and outside, as explained in par. 310. Insert a dry cork in the mouth, and run a wire through the cork down into the jar, until it touches the inner coating. Attach a smooth metal button to the top of the wire. Charge the jar from the electrophorus, and take a shock. Experiment. — Get a smooth pine board, larger than a sheet of letter paper. Heat the board and sheet of paper before a fire. Then place the paper on the board, and stroke it briskly with a piece of India- rubber, such as is used for erasing lead-pencil marks. The paper will become strongly electrified. Now lift it from the board by one of its edges, and bring it near the wall, and it will be at once attracted to the wall, and will cling to it for several minutes. Experiment. — Electrify the paper as before, and while it is on the board, cut it into strips. Take hold of all the strips at one end, and lift them from the hoard. Their lower ends will be repelled, and will stand out from one another in a very amusing manner. Experiment. — Electrify a piece of paper as before. Eemove it from the hoard, and lay a pith-ball on it. The ball will either be thrown off the paper at once, or will run to the lower side of the paper, and will then be shot off from it. Fig. 141. An Insulated Conductor. 268 NATURAL PHILOSOPHY. Experiment. — Place a metal waiter on top a dry glass goblet. Electrify the paper and place it on the waiter. Apply the knuckle to the edge of the waiter, and a spark will pass to it. Eemove the paper by the edge, and another spark can be taken from the paper. Experiment. — Support a clean dry pane of window-glass, one inch or so above the surface of a table, by resting the edges on sticks of wood. Place three or four pith-balls under the glass, and rub the top of the glass briskly with a piece of silk or cat-skin. The balls will move about in a curious manner, and some will probably stick to the glass. Now stop rubbing, and hold the finger near the glass above the balks, and they will at qnce fall. Since the glass is not a conductor of elec- tricity, and only the top is rubbed, the pith-balls must be electrified by induction. Syllabus. Electricity manifests its presence either as a charge or as a current. A body that possesses an electrical charge is said to be electrified. An electrified body acquires the property of attracting and repelling light bodies. A body may be electrified in a number of different ways, one of the simplest of which is by friction. An electrical charge, like that produced by friction, possesses the ability of leaping through short distances, in order to overcome ob- stacles placed in its path ; it is, therefore, commonly called electricity of high tension. Good electrical conductors are such as readily permit the passage through them of an electrical current. Substances that will not readily permit the passage through them of such a current are called non-con- ductors. A body is said to be insulated when it is supported by a poor con- ductor. There are two kinds of electrical charge, viz., positive and negative. Electricities of the same kind repel each other ; those of opposite kinds attract each other. The kind of electricity with which a body is electrified is determined by means of the electroscope. Glass rubbed with cat-skin becomes negatively electrified; but when rubbed with silk, positively electrified. A body which opposes or offers but little resistance to the passage of electricity through it is called a conductor. Substances differ greatly in their conducting power for electricity ; some are very good conductors, while others are very poor conductors. QUESTIONS FOR REVIEW. 269 According to the single-fluid electrical hypothesis, a ‘body is posi- tively excited when it has more than its natural quantity of electrical fluid, and negatively excited when it has less than its natural quantity. According to the double-fluid hypothesis, there are two kinds of electrical fluids, — positive and negative, — which are generally com- bined with each other. Electrical excitement is produced by sepa- rating the two fluids from one another. When an insulated conductor is approached to a positively excited conductor, it becomes electrified by induction. Its end nearest the excited conductor becomes negative, and the end farthest from it posi- tively excited. If it be now touched, it loses positive electricity, and becomes permanently excited negatively. The attractions and repulsions of light bodies by electrified surfaces are caused by induction. An electrical charge resides only on the surface of a conductor ; an electrical current passes through the entire mass of the body. A point attached to an electrified conductor acquires such a high electric tension that it will quietly discharge the conductor. In the electrophorus, the electricity is produced in the resin by fric- tion ; but in the metallic disc by induction. Condensers of electricity enable the electricity collected in a conduc- tor to acquire a higher tension than that of the electrical machine by which they are charged ; they operate by induction. The condenser is generally made in the form of the Leyden jar. The effects of the electric discharge are, 1st. Physiological ; 2d. Lu- minous ; 3d. Thermal ; 4th. Mechanical ; and, 5th. Chemical. The atmosphere always contains free electricity, which is generally positive. Lightning is caused by the discharge of a cloud to the ground, or to a neighboring cloud. The thunder is caused by violent disturb- ance in the air, produced by the lightning passing through it. Lightning-rods protect the buildings on which they are placed, either by conducting the discharge to the earth, or by quietly neutralizing the electrified cloud by discharging opposite electricity into it. Questions for Review. Name the principal varieties of electrical energy. Define electrical charge. When is a body said to be electrified ? Name some of the effects produced by an electrified body. What is meant by current electricity? Name any source of current electricity. 23* 270 NATURAL PniLOSOPHY. Distinguish between conductors and non-conductors of electricity. Name some good conductors. Name some poor conductors. When is a conductor said to be insulated ? Describe the attractions and repulsions produced by an electrified body. State the law of electrical attractions and repulsions. Describe the electroscope. For what is it used? Describe the con- struction of a simple electroscope. What kind of electricity will be produced in glass by rubbing it with silk? With cat-skin ? With cotton? Describe in full the single-fluid electrical hypothesis; the double- fluid electrical hypothesis. Explain what is meant by induction of electricity. Has a body when electrified by induction any more electricity than it originally contained? How may a body be permanently electrified by induction? Define electrical tension. Prove that an electrical charge resides only on the surface of a conductor. Is this true also of an electrical current? Explain in full the cause of the attractions and repulsions of light bodies by an electrified surface. What effects have points on electrical tension ? Describe the construction and operation of the electrophorus and of the plate electrical machine. Explain fully the construction and operation of the condenser. What is meant by the condensation of electricity ? Describe the Leyden jar. How is the Leyden jar charged? How is it discharged ? Enumerate the effects produced by the electric discharge. What is meant by a Leyden battery ? How does the electrical charge accumulate in a cloud ? By what is lightning caused? What causes thunder? Describe the construction of a lightning-rod. How do lightning- rods protect the buildings on w'hich they are placed ? What precau- tions are necessary in order to obtain an efficient lightning-rod ? CHAPTER IV. CURRENT ELECTRICITY. 316 . Sources of Current Electricity. — There are various sources of electrical current, the principal of which, however, may he arranged under three heads, viz. : 1st. Currents produced by chemical action, or voltaic currents. 2d. Currents produced by the action of heat, or thermo-electric currents ; and 3d. Currents produced by the motion of magnets, or magneto-electric currents. We will consider in this chapter the first two of these sources. 317 . Voltaic Currents. — We have already seen that if a piece of zinc and copper joined by a wire be dipped into any liquid which will act chemically on either metal, a current of electricity will be produced. An electrical current will be produced when any two dissimilar metals are used in this manner, but it will be found by trial that certain metals used in connec- tion with certain acid liquids will produce the greatest current. Any two metals that are used together for this pur- 271 272 NATURAL PHILOSOPHY. pose form wliat is called a voltaic couple.^ and the entire arrangement a voltaic cell. All chemical action is attended with a disturbance of electrical equilibrium. Of different kinds of chemical action, that which occurs between metals and acid liquids is found to be the best suited for the development of electrical current. 318. Galvani and Volta. — The production of elec- tricity by chemical action was first noticed by Galvani, an Italian physiologist, who erroneously ascribed the effects it caused to a vital fluid. He was making experiments, in which he used the legs of recently killed frogs. Hanging them against an iron balustrade, he noticed that whenever the metal touched a large nerve in the frog, and so brought it into contact with the muscles of the leg, that the legs were violently twitched, as though in pain. He thought that these movements were caused by a vital fluid which came out of the nerve, and flowed through the iron to the muscles. Volta, a distinguished physicist, showed that these movements of the frogs’ legs were due to electricity, and constructed an arrangement, called a pile or bat- tery, by means of which electricity could be readily produced. Voltaic electricity was so named after its discoverer. It is sometimes called galvanic electricity, though not so properly, since this might imply a belief in Galvani’s idea of its being the vital fluid. 319. The Simple Voltaic Cell. — A simple voltaic cell consists of two plates of different metals immersed in a liquid which can readily act on one of them, and connected outside of the liquid by a wire of some good conducting substance. One of the simplest forms given to the voltaic cell is seen in Fig. 1-12, where a plate of zinc and a plate CURRENT ELECTRICITY. 273 of copper are immersed in water, rendered sour by sulpliuric acid, and connected outside the liquid by means of a copper wire, M. If the zinc be pure, no action be- tween the liquid and either metal occurs as long as the metals do not touch eo.ch other. If, however, they ■ are made to touch each other, either in or out of the liquid, then an action takes place between the liquid and the zinc, and hydrogen gas, produced by the decomposition of the water, is A Simple Voltaic Cell, seen to escape in minute bubbles from the copper, and a current of electricity continues to flow from the zinc to the copper as long as the chemical action continues. The contact of the plates outside of the liquid may be made either directly, by allowing them to rest against one another, or by means of a wire of some good con- ducting substance, as copper. This wire may be many miles long, but as soon as the ends are brought to- gether, the liquid acts on the zinc, bubbles escape from the copper, and an electrical current is produced. 320. The Voltaic Circuit. — The direction of the current of the simple battery cell just described, is from the zinc through the liquid to the copper, and through the conducting wire outside of the liquid back again to the zinc, thus completing a circuit, called the voltaic circuit. We make or complete the circuit when we connect the zinc and copper by means of the conducting wire, and we break the circuit when we disconnect them by breaking or separating the conducting wire at any point. No current flows when the circuit is broken. S 274 NATURAL PHILOSOPHY. The current immediately begins to flow when the circuit is completed. No matter liow long the connecting wire may be, if the circuit be broken at any point of its length the current at once ceases. 321. The Polarity of the Battery — Electrodes. — In any voltaic cell the current always flows through the liquid from the metal most acted on to the metal least acted on.^ and out of the liquid in the opposite direction. In a voltaic cell, in which zinc and copper are em- ployed, the zinc is positive in the liquid and negative outside of it, and the copper is negative in the liquid and positive outside of it. If the wire connecting the plates be broken at any place, positive electricity accumulates at one of the ends, and negative at the other. These ends are called electrodes. That connected with the zinc, or with the metal most acted on, is called the negative electrode, and that with the copper, or with the metal least acted on, the positive electrode. 322. Amalgamation of the Zinc. — Ordinary zinc is impure, and is violently acted on by the liquid when the circuit is broken. This both wastes the zinc and the liquid, and weakens the strength of the current when the circuit is completed. It may be remedied by cleaning the zinc by dipping it in acid water, and then rubbing some mercury over its surface : the zinc is then said to be amalgamated. The zinc is partially dissolved by the mercury, and brought in a pure state to the surface of the plate. A number of voltaic cells, arranged so that the direction of the current is the same in all, is called a voltaic battery. 323. Electro-Motive Force — Resistance. — By the electro-motive force of a voltaic battery we mean the CURRENT ELECTRICITY. 275 force wliicli produces or causes the electric current, or the force which urges it forward. The electro-motive force varies with the hind of metals and liquids em- ployed. The electro-motive force of voltaic electricity is very much less than that of frictional electricity ; so that the power of voltaic electricity to jump across any non-conducting material separating the electrodes — as, for example, air- — is almost nothing. The quantity of electricity which flows through a conductor in any given time is called the current. Anything which opposes the passage of the current is called a resistance. The poorer the conducting power of any substance, the greater the resistance it opposes to the passage of the current ; the greater the conducting power, the less the resistance. In conductors of the same material, the resistance increases with the length of the conductor, and de- creases with the area of its cross-section. Thus, a copper wire of the same thickness, but twice the length of another, has twice the resistance. A cop- per wire of the same length as another, but twice the area of cross-section, has but half the resistance. Copper is about six times a better conductor than iron ; an iron wire of the same length and thickness as one of copper would, therefore, have six times the resistance as the copper, and placed in the circuit of a battery would allow less current to flow through it than it would were it of copper. A copper wire could I be six times as long as an iron wire of the same thick- I ness, and yet have no greater resistance. I Liquids are extremely poor conductors of electricity. { Acidulated water is several million times a poorer conductor than pure copper. The resistance of the 276 NATURAL PUILOSORHY. liquid between tbe plates of a voltaic battery is, there- fore, very raucli greater than that of a comparatively long wire joining them. The larger the plates of a battery, and the nearer they are together, the less is the resistance of the mass of liquid between them. The smaller the resistances of the liquid in a battery, and that of anything placed in the circuit of the conductors outside the battery, the greater is the current which can flow through the circuit. In joining the separate cells of a voltaic battery, so that the current may flow in the same direction in each, if we join the positive elec- trode of one cell to the negative electrode of the next, and so on, we increase the resistance of the liquid conductor, because we increase its length. Such an arrangement is spoken of as a high-resistance battery or connection in series. If, however, we connect all the positive elec- trodes of the different cells by one wire, and all the negative electrodes by another, and then join their separate wires, we decrease the liquid resistance, because we increase its area of cross-section by increasing the size of the plates. Such an arrangement is called a low-resistance bat- tery or connection in multiple arc. 324. Varieties of the Voltaic Cell. — Volta’s orig- inal battery consisted of discs of copper, clotli, and zinc moistened with acid- water, piled on eacli other in the following order, viz. : copper, cloth, ziuc, copper, cloth, zinc, etc. This was called the voltaic pile. It has now been very greatly improved. There are a great many forms of voltaic cells, but they may all be arranged under two classes, viz. : 1st. Those in which but a single liquid is used ; and, 2d. Those in which two different liquids are used. In this case, oue of the metals dips into one of the liquids, and the other metal into the other. Among the most important of the single-fluid bat- teries are Smee's and the Bichromate Battery ; and of the double-fluid batteries, DanielTs. the Gravity. Grove's., and Bunsen's Battery. CURRENT ELECTRICITY. Til 325. Smee’s Battery. — The metals consist of a plate of silver, the surface of which is coated with plati- num in a finely divided state, and a plate of zinc. The metals are dipped into water containing sulphuric acid. 326. The Bichromate Battery. — Zinc forms one of the plates, and a plate of the hard carbon, or graphite, that is formed inside of gas retorts, the other. Some- times a single plate of zinc is placed between two plates of carbon. The plates are immersed in a liquid con- sisting of a substance called potassium bichromate, dis- solved in water containing sulphuric acid. The liquid at first is bright red, but after being used changes to a greenish brown. 327. Daniell’s Battery. — The metals are zinc and copper, and the liquids are water containing sulphuric acid, and a saturated solution in Avater of copper sul- phate. The zinc dips into the acid water placed in a cell of unglazed eartheuAvare, called the porous cell.i placed inside a larger jar, Avhich contains the solution of copper sulphate. The copper is placed around the porous cell in the form of a cylinder. Near the top of the copper cylinder is placed a small cage A\dth a perforated bottom. This cage is kept filled Avith crys- tals of copper sulphate, and is so placed as to be partly covered by the liquid ; by this means the strength of the solution is maintained. 328. The Gravity Battery is a modification of Dan- iell’s, but dispenses AAUth the porous cell. The zinc is hung near the top of a cell containing Avater above and a solution of copper sulphate beloAV. The copper plate is placed in the bottom of the cell, and has crystals of copper sulphate placed on it. The AAure attached to the copper plate is insulated by Avax or India-rubber. After 24 278 NATURAL PHILOSOPHY. the action of the cell has begun, the zinc plate is sur- rounded. bj a solution of zinc sulphate, and the copper bj a solution of copper sulphate : the solutions being of different densities, are thus kept separated. The Daniell’s and the Gravity Batteries give constant currents of electricity; the latter is now almost uni- versally used on telegraph lines. 329. Grove’s and Bunsen’s Batteries. — In Grove’s Battery the metals are zinc and platinum, and the liquids sulphuric acid in water and strong nitric acid. The platinum is dipped into the nitric acid contained in a por- ous cell, and the zinc, in the form of a cvlinder, is dipped in the acid water contained in a larger cell. In Bunsens Battery the metals are zinc and carbon. The carbon dips into nitric acid contained in a porous cell, Pig. 143.— The Nitric Acid Battery, and the ziuc as before, in the form of a cylinder, into water and sulphuric acid in an outer cell. Both of these batteries give very intense currents, which, however, are not constant. They are some- times called nitric acid batteries. 330. Thermo-Electricity. — If two bars of anv un- like metals, as, for example, copper and iron, or anti- mony and bismuth, be soldered together at one end, and the other ends be connected by a wire, or any other conductor, and the soldered end heated, a current of electricity will flow through the circuit so provided, from the bismuth to the antimony, and through the SYLLABUS. 279 wire, or other conductor, back again to the bismuth. If the soldered end be cooled, a current of electricity will also be produced, but in the opposite direction, that is, from the antimony to the bismuth. Such an arrangement is called a thermo-electric couple. Currents of electricity produced in this way by the action of heat, are called thermo-electric currents.^ and will continue to flow as long as there is any difference of temperature between the opposite ends of the bars. Thermo-electric currents are in general of but very feeble intensity. Their intensity varies with the kind of metals used, and within certain limits, with the dif- ference of temperature between the opposite ends of the bars. The intensity or electro-motive force may be considerably increased by the same means as those employed with voltaic batteries, viz. ; by the use of a number of thermo-electric couples suitably connected. In this case a number of bars of two unlike metals, such, for example, as antimony and bismuth, or iron and copper, are soldered together at their alternate ends. Such an arrangement is called a thermo-pile. Syllabus. There are three principal sources of electrical current, viz. : chemical action, heat, and the motion of magnets. Electricity produced by chemical action is called voltaic electricity. Galvani ascribed the convulsive twitchings of the frog’s legs to a vital fluid : Volta ascribed these movements to electricity. A voltaic cell consists of two dissimilar metals, immersed in a liquid capable of acting on one of the metals, and connected outside the liquid by a metallic conductor. The electricity produced in a voltaic cell passes through the liquid from the metal most acted on to the metal least acted on, and out into the liquid from the metal least acted on, back again to that most acted on, thus moving in a circuit called the voltaic circuit. 280 NATURAL PHILOSOPHY. When the conducting-wire connecting the plates outside of the liquid is broken, a charge of opposite electricities collects at the broken ends ; these ends are called electrodes. The electro-motive force is the force which causes the electricity, or that which urges it to flow. Its flow is opposed by the resistances of the materials through which it has to pass. The resistance of the liquid conductor between the plates of a bat- tery can be decreased either by bringing the plates nearer each other, or by increasing the size of the plates immersed. Voltaic batteries can be divided into single-fluid batteries and double- fluid batteries. The principal single-fluid batteries are Smee’s and the Bichromate ; the principal double-fluid batteries are Daniell’s, the Gravity, Grove’s, and Bunsen’s. Thermo-electric currents are those produced by heat. Questions for Review. Name the principal sources of electrical current. Define voltaic couple; voltaic cell. What is the source of voltaic electricity ? Describe briefly the discoveries of Galvani and Volta. Describe a simple voltaic cell. What is a voltaic battery? What is meant by a voltaic circuit? What is meant by making or completing the circuit? What by breaking the circuit? Describe the polarity of the simple voltaic cell. Define electrodes. What is meant by the amalgamation of the zinc? What is electro-motive force? Upon what does it depend? Name the resistances which are present in every voltaic battery. Name all the circumstances which decrease the resistance of any con- ductor. In what two ways may the resistance of the liquid conductor be- tween the plates of a voltaic battery be diminished? Distinguish between a connection of a number of battery cells in series and in multiple arc. Into what two classes may all the different forms of voltaic cell be arranged? Describe each of the following cells, viz.: Smee’s, the Bichromate, Daniell’s, the Gravity, Grove’s, and Bunsen’s. What do you understand by thermo electric currents ? How may these currents be developed? Define thermo-electric couple ; thermo-electric pile. CHAPTER V. PROPERTIES OF AN ELECTRICAL CURRENT. 331. Effects Produced by an Electrical Current.— The passage of an electrical current through a wire or other conductor, produces in the wire or other con- ductor a number of effects, the principal of which are as follows, viz. : 1st. Thermal effects. — The wire becomes heated. 2d. Luminous effects. — If the wire be broken at any point, a brilliant flash of light appears. 3d. Physiological effects . — An electrical current sent through the body of an animal produces involuntary movements of the muscles. 4th. Chemical effects. — An electrical current sent through a compound liquid conductor causes a de- composition and recombination of its constituent ele- ments. 5th. Magnetic effects. — All conductors conveying electrical currents are thereby rendered magnetic, that is, acquire the property of attracting or repelling bod- ies called magnets. 332. Thermal Effects. — Whenever a definite vol- taic current flows through a conductor, it heats the con- ductor. The elevation of temperature, in the case of a wire, is almost inappreciable if the wire be stout and I 24* 281 282 NATURAL PHILOSOPHY. of good conducting material, unless tlie current be very great. If, however, the wire be fine, so as to offer a great resistance., it may become intensely heated or even melted by the current, if the latter is sufficiently great. 333. Luminous Effects. The Voltaic or Elec- tric Arc. — When a conductor conveying the current from a powerful battery is broken at any point, a brill- iant flash of light is seen. If the ends of the wire are connected with two pencils of hard carbon, or some other material, and brought together, and then slowly separated, a brilliant arc or light, called the voltaic arc, will continue to pass between the electrodes, unless they be too widely separated. The light of the voltaic arc is of dazzling brightness, and the arc itself one of the most intense sources of heat that can be produced artificially. AVhen the carbon electrodes are separated from each other, portions of the positive electrode are volatilized by the current of electricity and carried through the air to the negative electrode, thus forming a bridge of vapor over which the electricity passes. The positive carbon decreases in size and the negative carbon increases. Besides this, the carbons being in- tensely heated, are gradually consumed by ordinary combustion. 334. Illumination by the Electric Light. — The intense brilliancy of the electric light renders it ad- mirably adapted for the illumination of light-houses, or large buildings, or the streets of cities. The consumption of the carbon electrodes by com- bustion, and the growth of the negative carbon at the expense of the positive, render it necessary to adopt PROPERTIES OF AN ELECTRIC CURRENT. 283 Fig, 144. The Jab-' locbkoff Candle. some means bj 'vvbicb the carbons may he Tcept a con- stant distance apart; for, if they sliould get too far apart, the current at once ceases, and tlie light goes out, in which case the carbons must be brought to- gether again, and slowly separated before the light reappears. The carbons are kept at the same distance apart by various forms of regu- lators. One of the simplest of these regulators is a late contrivance, called the Jahlochhoff candle. It consists of two carbon pencils, A and i?. Fig. 141, placed parallel to each other, and separated by some non-conducting mate- rial, such as- pure clay or alumina, or plaster of Paris. As the ends of the carbons are con- sumed, the material separating the pencils is fused and volatilized, thus expos- ing fresh carbons for con- sumption. The intense brilliancy of the electric light renders it impossible to directly ex- amine the carbon electrodes between which the arc is passing. Colored glasses, which cut off most of the light, may be employed for this purpose. A more sat- isfactory way is to form a magnified image of the car- bon electrodes, by means of a suitable lens placed in front of them. The image so formed is received by a distant screen. Fig. 145 represents an image so obtained. If a small piece of any metal, such, for example,, as copper or silver, is Fig. 145. An Image of the Carbon Electrodes. 284 NATURAL PHILOSOPHY. placed ou the positive electrode, it is at once volatilized and carried over in the form of vapor. 335. Physiological Effects. — An electrical current passed through the nerves of a recently killed animal, causes convulsive movements in the muscles that are connected with these nerves. Passed through the nerves of the living animal, it produces vmrious physiological actions, many of which are favorable to the cure of cer- tain diseases. Electricity, however, as a curative agent, may do more harm than good, and should never he em- gjloyed except by a skilful and intelligent physician. 336. The Chemical Effects. Electrolysis. — -TYhen a voltaic current is passed through any compound sub- stance in the liquid condition, it decomposes the sub- stance — one of its elements, or sets of elements, appear- ing at one of the electrodes, and the other at the other electrode. This decomposition is called electrolysis. W hen two elements combine chemically with each other, one is considered to be electro-positive and the other elec- tro-negative. When a substance undergoes electrolysis, the electro-positive element appears at the negative electrode, and the electro-negative element at the positive electrode. In salts of the metals, the metal is electro-positive, and the element or elements Avith Avhich it is combined are electro-negative. 337. Electrolysis of Water. Electro-Metallurgy. — If two platinum strips be made the electrodes of a voltaic battery, and plunged into water Avhich has been rendered slightly acid for the purpose of increasing its conducting poAver, the current in passing through the Avater Avill decompose it, and hydrogen Avill be giA'en off at the negative electrode and oxygen at the posi- tive. If the electrodes be dipped into a solution of any PROPERTIES OF AN ELECTRIC CURRENT. 285 salt of a metal, as copper sulphate, the passage of the ! current will decompose the salt, and metallic coi^per will appear at the negative electrode, and sulphuric V' acid and oxygen will he set free at the positive electrode. \ If the positive electrode he of copper, instead of sulphuric acid and oxygen being set free, sulphate of copper will I , he formed, and thus keep up the strength of the solution. In this way we can deposit strong adherent films of metal on the surface of any conductor ; for if the article to be coated be attached to the negative electrode of a ■ battery, and dipped into a solution of the metal with ; which we desire to coat the article, say copper, and the i positive electrode be attached to a plate of copper, and also dipped into the liquid, when the current passes, the salt will be decomposed, and the metal deposited in a uniform layer over the article at the negative electrode. This process is called electro-metallurgy , and by it arti- cles may be coated with gold, silver, copper, iron, and other metals. The cuts in this book are prepared as follows ; They are first cut in. wood by a skilful artist; the wood-cuts, however, are not used directly for printing, as they are expensive and would soon wear out. The wooden cuts are pressed into a mixture of soft wax and black-lead, and a perfect impression is thus obtained. This impression is covered on the back with some non-conductor of electricity and attached to the negative electrode of a battery, and immersed in a solution of copper sulphate. By the passage of the current, the wax mould is thus covered with a thin sheet of copper, which is now the exact coun- terpart of the figure on the wooden Mock. This film is removed from the wax mould and stiffened by being filled with stereotype metal, and the form thus obtained is used for printing. 338. Magnetic Effects. — All conductors, no matter what the nature of their substance, become magnetic during the passage of an electrical current through them, and thereby acquire all the properties of mag- nets. Since, however, magnetic properties are capable 286 NATURAL PHILOSOPHY. of existing in certain substances, under circumstances in wbicli the presence of electrical currents through the magnets are not evident, we will first discuss the properties of such bodies. Bodies that are capable of acquiring magnetic prop- erties under circumstances in which the presence of electrical currents are not evident, are called perma- nent-magnets ; those in Avhich magnetic properties are only produced during the passage of an electrical cur- rent are called electro-magnets. Hard iron and steel are the principal substances that can be rendered per- manently magnetic. Any conductor of electricity can become an electro-magnet. Syllabus. The effects produced by the flow of an electrical current are, 1st. Thermal; 2d. Luminous; 3d. Physiological; 4th. Chemical; and, 5th. Magnetic. An electrical current traversing a wire raises the temperature of the wire. If the wire be thin and the current powerful, the wire will become luminous, and may be fused. If a wire conveying a powerful current be broken at any point and separated a slight distance, a brilliant flash of light is seen. If two pieces of hard carbon be connected to the wires and then separated, a brilliant arc, called the voltaic or electric arc, will continue to pass between them. The light of the electric arc is very intense, and well adapted for the lighting of large areas. During the passage of the current the positive carbon decreases in size and the negative increases. To maintain a constant distance be- tween the electrodes, various devices, called regulators of the electric light, are used. Electrical currents passed through the bodies of dead animals cause muscular movements. Electricity should never be employed as a curative agent except by a skilled and intelligent physician. QUESTIONS FOR REVIEW. 287 The chemical effects of an electric current are seen in the combina- tions and decompositions produced by the passage of a current through a compound substance. Electro- metallurgy is the deposit of one metal on another by the action of an electrical current. Conductors conveying electrical currents become magnets. Questions for Review. Enumerate the different effects produced by the passage of an elec- trical current. How may the heating effects of an electrical current be shown ? What is meant by the voltaic or electric arc? How may it be ob- tained? What changes are produced in the positive and negative carbons during the passage of the current through the arc? Why must the carbon electrodes used in the employment of the elec- tric light for purposes of illumination be kept a constant distance apart? Describe the Jablochkoff candle. How are the carbon electrodes maintained at a constant distance in this candle? Describe the appearance of the image of carbon electrodes used for producing an electric arc. Describe any of the physiological effects of an electrical current. What is meant by electrolysis? Describe the process of electro- metallurgy. Describe the magnetic effects produced by the passage of an elec- trical current through a conductor. Distinguish between permanent- and electro-magnets. CHAPTER VI. MAGNETISM. 339. Natural Magnets. — There exists in nature an ore of iron called magnetic oxide, specimens of which are sometimes found, which, possess the properties of magnets. Such magnets are called loadstones, or natural magnets, to distinguish them from those which may be obtained artificially. 340. Permanent Magnets. — When a piece of hard- ened steel is rubbed by a magnet, the steel itself be- comes magnetic. Steel can be magnetized in a variety of ways, and as the magnets so obtained are stronger than those found in the earth, they are preferred to them. Various forms are given to these artificial mag- nets, but they are generally made either in the form of a straight or curved bar. 341. Distribution of the Magnetic Force. — If some iron filings be scattered over the surface of a bar- mag- net, they will be found to collect mainly at the ends of the bar, while the middle will be quite free. The ' ends of the bar, or the points where the force is mani- fested with greatest power, are called the poles of the magnet. 288 MAGNETISM. 289 A magnetic needle consists of a magnetized bar of steel, made in the form shown in Fig. 146, and supported at its centre of gravity on a point, around which it is free to move. When sucb a needle comes to rest, it will, if no other magnet is near it, point nearlv due north and south. 146. — A Magnetic Needle. That end which points to the north pole of the earth is called, in this country, the north pole.^ and the other end the south pole. There are always two opposite poles in every mag- net. Even if the magnet were suddenly broken in the middle, the broken ends would be found to have polar- ity opposite to that of the other ends. 342. Attractions and Repulsions of Magnets. — If the north pole of a magnet be brought near the south pole of a magnetic needle, they attract each other ; but if the north pole be brought near the north pole of the needle, they repel each other. We generally express these facts as follows, viz. : Like magnetic poles attract.^ and unlike poles repel; that is, north attracts south, or south north ; but north repels north and south repels south. When magnets are at a considerable distance from one another, the force with which they attract or repel decreases with the square of the distance between them. 343. Magnetic Field. — Since magnets attract or repel all other magnets brought near them, they are 25 T 290 NATURAL PHILOSOPHY. supposed to be surrounded by an atmosphere of mag- netic influence called the macjnetic field. The direction of the lines in which the magnetic force acts in this field is beautifully shown by the following Experiment. — Place a magnet on a table, and lay over it a piece of smooth window-glass, or a sheet of stiff paper stretched in a frame. Sprinkle some fine iron filings on the glass or paper, and then tap the edge gently with the finger, and the filings will be arranged in curved lines, which are the lines in which the magnetic force acts in the mag- netic field. Experiment. — Magnetize a large needle by rubbing its ends against the opposite poles of a magnet. Stick it through a small piece of cork large enough to float it on water, and place the cork so that the needle may be parallel to the water. If properly magnetized, the needle will j)oint north and south. Now approach to it the poles of a magnet, and prove that like poles repel and unlike poles attract. 344. Magnetism by Contact. — When a bar of steel, or other body capable of being magnetized, is rubbed against a magnet, it becomes itself magnetic. Iron, steel, nickel, cobalt, manganese, and a few other sub- stances, can be magnetized in this way. Pure, soft iron is very easily magnetized, but only retains its magnet- ism while in contact with the magnet. Hardened steel is not so readily magnetized, but retains its magnetism after being removed from the magnet. 345. Magnetism by Induction. — When any body capable of being mag- ' netized is brought within the magnetic .field, it becomes a Jfiff. 147. — Magnetic Induction. , magnet without hav- ing actually touched the magnetizing body. Magnetism produced in this waj'^ is said to be produced by induc- tion. The polarity of the magnetism produced in any body hy contact and by induction is the same in either MAGNETISM. 291 cose, and is always of the opposite polarity to that of the magnet to which the body is touched.^ or near which it is brought. Thus, suppose a bar of steel be touched to the north pole of a magnet ; the point touched will have south goolarity ; or suppose the end of a bar, /S", to be brought near the north pjole, N, of the magnet. Fig. 147, without touching it; then the nearer end will, as before, have south polarity. Experiment. — Touch one end of a steel pen to the north pole of a magnet. Throw the pen in some iron filings, and it will be found to have a pole both at the end touched and at the opposite end. By means of a magnetic needle, test the polarity of the end touched, and it will be found to be south, and that of the opposite end north. Experiment. — Move one end of a steel pen several times very near the north pole of a magnet, without actually touching it. Iron filings will show poles at each end, and the little floating magnetic needle will show that end which was brought near the north pole of the magnet to be of south polarity, and the opposite end north. 346. Magnetization of Soft Iron and Hardened Steel. — Soft iron and hard steel differ greatly in the ease with which they become magnetized. If a piece of soft iron be touched to the pole of a strong magnet, it will at once become magnetized throughout. As soon, however, as it is removed from the magnet, it at once loses its magnetism. This is not the case with a piece of hardened steel. When once magnetized by touch- ing a magnet, it will retain its magnetism after separa- tion therefrom. 347. Electro-Magnets . — When a current of electricity is caused to flow through any conductor.! it gives that con- ductor the properties of a magnet. If, for example, the current flows through a copper wire, the wire will acquire the property of attracting iron filings, which it will instantly lose when the circuit is broken. 292 NA TURAL PHIL 0 SO PHY. If the wire conveying the current be held near a magnetic needle, it will deflect the needle. If the wire be held above and parallel to the needle, it will deflect the north pole in one direction ; but if it be held below the needle, it will deflect the north pole in the opposite direction. In order to obtain strong magnets by electrical currents, a considerable length of copper wire, covered with cotton, silk, or some other insulating substance, is wrapped in the shape of a coil around a bar of soft iron, as seen in Fig. 1-18. When an electric current is sent through such a coil, and the coil thereby becomes magnetic, it strongly magnetizes, by induction, the soft- iron core within it. The strength of the magnetism so produced is very much greater than that of the coil without any soft-iron core. Magnets so obtained differ in no respect from per- manent magnets, except that they retain their proper- ties only during the passage of the current. 348. Methods of Magnetization. — Bodies capable of receiving magnetism, may be magnetized hy touch, by induction, and by electrical currents. Merely touching the end of a needle or penknife to the pole of a powerful magnet will render it magnetic throughout; but the needle or knife can be more pow- erfully magnetized by drawing it a number of times from its centre to the end over one of the poles of a magnet, being careful to return the bar each time through the air, and to make the stroke always in the same direction. Then put the middle of the needle or knife over the other pole of the magnet, and rub towards the opposite end in the same manner. Sometimes two magnets are placed with their oppo- MAGNETISM. 293 site poles in the middle of the bar to be magnetized, and moved to the ends in opposite directions, and then returned through the air and stroked as before. By far the most powerful magnets, however, are pro- duced by means of electrical currents. The electrical cur- rent is caused to flow through a hollow coil of wire in which is placed the bar to be mag- netized ; or, as is most fre- quently the case, the current is used to excite magnetism in an electro - magnet, and the bar to be magnetized is rubbed against the poles of the electro-magnet so pro- vided, as shown in Fig. 148. 349. Cause of the Needle Pointing to the North. — The magnetic needle points to the north pole of the earth for the same reason that the opposite poles of magnets point to each other, if they are sufficiently near and free to move. The earth acts as a huge mag- net.^ with its magnetic poles in the neighborhood of the poles of the earth and the magnetic needle points towards these poles on account of their attraction. Since it is the opposite poles of magnets that attract each other, the end of the needle that points towards the north pole of the earth must be of the opposite polarity to the earth's polarity at the north. The French, for this reason, call the end of the needle which points to- wards the north pole of the earth, the austral or south pole. 350. Origin of the Earth’s Magnetism. — The origin of the earth’s magnetism is not exactly known, though there is no doubt that it is in some manner con- nected with the sun’s action. It is quite probable that the principal causes are induction from the sun, and 25 * Fig. 148. Magnetism hy Electro-Magnets. 294 NATURAL PHILOSOPHY. electrical currents developed in tlie earth in a variety of ways, though perhaps chiefly by heat and chemical action. 351. The Declination or Variation of the Needle. — It is a very common, though mistaken notion, to sup- pose that the magnetic needle invariably points to the true geographical north. The fact is that except in a few localities, it actually points to the east or west of the true north. This deviation of the needle from the true north, is called the declination or variation., and in some localities amounts to a considerable deviation. 352. The Inclination or Dip of the Needle. — When a magnetic needle is free to move in a vertical as well as a horizontal direction, it remains in but few parts of the earth in a true horizontal position. In most places one of the poles is inclined or dipped to- wards the earth. This is called the dip or inclination of the needle. In the northern hemisphere it is the north pole, and in the southern hemisphere the south pole that is inclined. The cause of the dip is the greater pull or attraction of one of the poles of the earth on the needle than the other, so that the needle is pulled down to the earth as well as directed to the north. Thus, in the northern hemisphere, the north pole of the needle being nearer the earth’s magnetic pole than the south pole of the needle, is pulled down or dipped, the dip being greater the nearer the needle is to the earth’s magnetic pole. When, however, the needle is midway between the poles it remains horizontal, because the attraction of the opposite poles of the earth is equal. SYLLABUS. 295 Syllabus. A magnet is a body that possesses tlie power of attracting and re- pelling other magnets. Magnets are either natural or artificial. If iron filings be sprinkled on a bar-magnet, they will all collect at the ends of the bar, leaving the middle free from attracted particles. Those ends where the greatest amount of filings collect, or where the magnetic force is greatest, are called poles. There are always at least two opposite poles in every magnet. A magnetic needle consists of a small magnetized bar, supported at its centre of gravity so as to be free to move. When such a magnet comes to rest it points nearly due north and south ; the end which points towards the north pole of the earth is called the north pole of the needle, the other end is called the south pole. Like poles of magnets repel one another, unlike poles attract. The magnetic field is the atmosphere of magnetic influence surround- ing the poles of a maguet. When a body receives magnetism from another by being rubbed or touched with it, it is said to be magnetized by contact ; when it re- ceives magnetism from another body by being brought into its mag- netic field, it is said to be magnetized by induction. klagnetism by contact, or by induction, is always of the opposite polarity to that of the body giving the magnetism. Bodies capable of being magnetized, may receive their magnetism by touch, by induction, and by electrical currents. The latter produce the most powerful magnetism. The magnetic needle points to the north pole of the earth because the earth acts like a huge magnet, with its poles near the north and south geographical poles ; the magnetic poles of the earth attract the poles of the needle, and cause them to point towards them. The earth’s magnetism is most probably caused mainly by inductiop from the sun, and electrical currents in the earth. The needle does not, in the majority of places on the earth, point to the true north, but to the east or the west of it. This deviation is called the deelination or variation. When the ends of a magnetic needle are free to move in all direc- tions, in most places one of them dips or inclines to the earth. This is called the dip or inclination of the needle. 296 NATURAL PHILOSOPHY. Questions for Review. What are loadstones? Distinguish between natural and artificial magnets. How is the magnetic force distributed in a magnetic bar ? Define magnetic poles. How many poles must there be in every magnet? What names are given to these poles? Describe the magnetic needle. State the law of magnetic attrac- tions and repulsions. How may these laws be experimentally verified? How may a very simple magnetic needle be constructed ? Define magnetic field. How may the directions in which the mag- netic force acts in a magnetic field be ascertained ? Distinguish be- tween the manner of producing magnetism by contact and by induc- tion. If a bar of steel be touched by the north pole of a magnet, what will be the polarity produced in the steel at the point touched? If the end of a bar of steel be brought near the north pole of a magnet, what polarity will be produced at that end? Describe the methods of producing magnetism by contact. Describe the method usually adopted for producing magnetism by electrical currents. What is an electro-magnet? Why does the magnetic needle point nearly to the geographical north? What is the probable cause of the earth's magnetism? Define declination or variation of the magnetic needle. Define inclination or dip. By what is the inclination or dip of the needle caused ? 0 CHAPTER VII. MAGNETO-ELECTRIC CURRENTS.— APPARA- TUS DEPENDENT ON ELECTRO-MAGNETS. 353. The Galvanometer. — In order to ascertain whether an electrical current is flowing through anj conductor, it is only necessary to bring near it a mag- netic needle, and observe whether or not the needle is deflected. Unless, however, the current is powerful, the needle, even if delicate, is not visibly deflected. In order to magnify the effect of the electrical current, it is caused to pass through an instrument called a galvanometer. The galvanometer is an instrument used to detect the presence of elec- trical currents, and to measure their intensity. It consists of a length of copper wire insulated by being wrapped with silk or cotton, and wound in the form of a flat ring or helix, a. Fig. 149. A magnetic needle is suspended inside the coil by a fibre, 5, of silk. The current is sent through the coil by making it enter at one of the binding-posts, x or ?/, and pass out at the other. Since each turn of the wire becomes magnetic during the passage of the current, it is evi- 297 298 NAT UR A L PHIL OSOPIIY. dent that an increase in the uumher of turns will cause an increase in the attraction or repulsion which the coil has for the magnetic needle. Before using the galvanometer, the coil is placed so that the direction of the wire is parallel with the needle, that is, the coil is placed with the wire extending in a north and south direction. On the passage of the cur- rent, the needle is deflected so as to tend to he placed at right angles to the direction in which the current is flowing;. The streng:th of the current is then deter- mined from the deflection of the needle. 354. Induction by Current Electricity. — We have seen that permanent magnets are capable of producing magnetic properties in suitable substances placed near them, that is, they are capable of producing magnetisjn by induction. AYe have also seen that electro-magnets possess all the properties of permanent magnets. AY e would sup- pose, therefore, that electro-magnets should also possess the property of causing magnetism by induction, and this, as Ave have also seen, is the case. But electrical currents passing through conductors render them magnetic ; therefore we might suppose that magnets should be capable of producing elec- trical currents. That electrical currents can be so produced Avas first proved by the illustrious Faraday. 355. Production of Electrical Currents by the In- duction of other Electrical Currents. — AYheuever an electrical current begins to flow^ or ceases /to floA\', through a conductor, it causes, by induction, electrical currents in neighboring conductors. The currents so produced are said to be due to the induction of the current floAving through the first conductor. MAGNETO-ELECTRIC CURRENTS. 299 The induced currents continue but for short inter- vals, that is, they only flow at the moment of making or of breaking a circuit. The presence of induced currents produced in this manner can be shown by means of the apparatus seen in Fig. 150. A hoUow coil, A, of moderately stout, in- Fig. 150. — Indnctioa by Cnrrent Electricity. sulated Avire, called the primary coil, is connected by wires, a, fc, c, cZ, Avith a battery -cell, C. Another holloAV coil, B, called the secondary coil, formed of a consider- able length of insulated wire, surrounds the primary coil. The ends of this coil are connected by means of the Avires e and / to a galvanometer, G. If, noAV, one of the wires conveying the battery- current, as d, be raised from the mercury in the cup, B, so as to break the circuit, that is, to cause the electricity to stop floAving through the primary coil, a momentary induced current Avill at once be pro- duced in the secondary coil, as will be shown by the ijiovement of the needle of the galvanometer in a cer- tain direction. After a moment the needle Avill come to rest, thus showing that the current in the secondary coil has ceased to floAv. If, noAV, the Avire d be again placed in the mercury cup, so that a current from the battery can begin to fioAV through the primary 300 NATURAL PHILOSOPHY. coil, the galvanometer needle will again be deflected, but in the opposite direction to that produced by the breaking of the circuit of the primary coil, thus show- ing that the induced current produced in the secondary coil, by making contact with the battery-circuit, is in the opposite direction to that produced by breaking the contact. It can be shown that, at the moment of breaking the circuit of the primary coil, the current is flowing through the secondary coil in the same direction as the current in the primary coil, and that at the moment of mahing the contact it is flowing in the opposite direction. The former is sometimes called a direct, and the latter an inverse current. The current producing the induction is some- times called primary current, and the current which is induced, the secondary current. It is only at the moment of making or of breaking the primary current that the secondary current is in- duced ; tluit is, the induced current is only produced while the intensity of the primary current is either in- creasing or decreasing . 356. Induction Currents Produced by the Move- ment of the Primary Current. — If, while the current is still flowing through the primary coil, the latter be drawn out or away from the secondary coil, the intensity of its influence on the secondary coil will gradually decrease, and a direct current of short duration will be induced in the secondary coil. If the primary be pushed within or towards the secondary, the intensity of its influence on the secondary will gradually increase, and an inverse current of short duration will be induced in the secondary. 357. Currents Produced by Magnets — Magneto- Electricity. — If the ends of the secondary coil be con- ELECTRO- MAGNETIC CURRENTS. 301 nected with the galvanometer, and a permanent bar- magnet be thrust into the coil, a momentary current is produced, as is shown by the deflection of the needle. If the magnet is now drawn out of the coil, a momentary current is also produced, but in the opposite direction to the first, as is seen by the movement of the galvan- ometer needle. Currents produced by the motion of magnets are called currents of magneto-electricity . They will also be produced if the magnet remains stationary and the coil is moved. 358. Production of Electricity from Power — Dynamo-Electric Machines, — Very powerful elec- Fig. 161. — The Gramme Dynamo-Electrio Machine. trical currents, similar to those obtained from large voltaic batteries, can be obtained directly from me- 26 302 NATURAL PHILOSOPHY. clianical power by means of dynamo-electric machines. In these machines, by means of a steam-engine or other source of power, a number of coils of unre, called the armature.^ are set into rapid revolution between the poles of powerful electro-magnets. The currents so produced in the armature are caused to take the same direction by means of a contrivance called the commutator. A variety of dynamo-electric machines are now con- structed. Fig. 151 shows a very successful form known as the Gramme Machine. The field-magnets are shown at M M and M' M\ and the poles of the magnets at A and A, which are north and south poles respectively. The armature is seen at A, and the commutator at (7. The coils on the armature and those on the field-magnets are con- nected in the same circuit, so that on first starting, the current pro- duced in the armature coils, flowing through the coils of the field-mag- nets, makes them stronger. The stronger magnetic field so produced reacts on the armature coils, and thereby increases the intensity of the currents produced therein. Dynamo-electric machines are now very successfully employed for the production of electricity for illumi- nation and for electro-plating. 359. Applications of Electro-Magnets. — The power possessed by electro-magnets of retaining their magnet- ism only during the passage of the electrical current, enables them to be employed in a great variety of elec- trical apparatus. 360. The Electro-Magnetic Telegraph. — Electro- magnetic telegraphy depends for its operation on the fact just mentioned, that a bar of soft iron can be in- stantly magnetized and de-magnetized by making and breaking the circuit of a helix of wire wrapped around the iron. ELECTRO- MAGNETIC CURRENTS. 303 There are many diftereut systems of telegraphy. That most generally used in this country is called the Morse system, after the name of its inventor. In the Morse system a battery., a hey^ and a Morse in- strument are placed in an electrical circuit, at a distant portion of which is another key and Morse instrument. The key consists of an arrangement by means of which the circuit may be easily made or broken. The Morse instrument consists of an electro-magnet, i/, Fig. 152, with a piece of soft iron, J., called an armature or keeper., attached to one end of a bar, B, pivoted at C. The other end of this bar is furnished with • , ,7 Ti Fig. 152, — A Morse Eeceiving Instnunent. a point or stylus, B, ^ ^ which presses against a long strip of paper, S, moved by clock-work under the point. If the operator at the distant station should depress his key when the circuit is broken, he thereby closes it, and M, becoming mag- netic, pulls down the armature. A, and causes the stylus, P, to indent the paper. If he keeps the key down for some little while, the paper will be drawn over the stylus, and a dash or long mark made on it ; but if the key is only kept down for a moment, then but a dot will be given to the paper. When the operator raises the key, the circuit is broken, M ceases to be a magnet, and the point is pulled away from the paper by the spring, Q. By adopting a system of dots and dashes to represent the letters of the alphabet, commu- nications can be carried on very rapidly between dis- tant places. 304 NATURAL PHILOSOPHY. Skilled operators soon learn to readtke dispatcli from tlie sound. In most offices the paper instrument just de- scribed is replaced by an instrument called the sounder., similar in construction to the other, but dispensing with the paper and clock-work. 361. The Morse Alphabet. — In the following table are given the combinations of dots and dashes employed in the Morse system to represent the letters of the al- phabet and the numerals. a -— i s --- b k t - c -- - 1 u d m V e - n — - w f o - - X g — p y — h q z --- - i -- r - -- & - --- 1 6 2 7 3 8 4 9 6 0 To avoid the running of one letter into another, such as t - - and e - , which might be mistaken for - - - s, or - - - c, a space is left between successive letters longer than that between any of the sepa- rate dots of any single letter, and a still longer space is left between words. It will be noticed that two. dots, an interval, and one dot stand for c, while three dots stand for s; f is represented by two dots, while o is represented by a dot, an interval, and a dot. Similar differences are noticed in the signs for h and y, c and r, 2 and d-. These differences are more marked when only the sounds are regarded, and, indeed. ELECTRO-MAGNETIC CURRENTS. 305 most telegraphy by the Morse system is effected by means of the sounds, as already explained. 362. The Magneto-Electric Telephone is an in- strument b}'" means of which the sounds of the human voice, as in articulate speech uttered in any place, can be audibly reproduced at places hundreds of miles dis- tant. This wonderful instrument is the invention of an American citizen by the name of Bell. The telephone consists of a permanent magnet, d/. Fig. 153, with a e a l it' a B H The Magneto-Electric Telephone Circnit. coil, (7, of insulat- ed wire wrapped around it near one of its ends. One end of this coil is connected to a wire, E X, which passes to a dis- tant station, where it is connected to one end of the coil. O', wrapped around the permanent magnet, M'. The other ends of the coils C and C are either con- nected by means of a wire between A B H., or, Avhat is the same thing, are connected to metallic plates buried in the earth at A and H. The circuit so pro- vided is called the telephone circuit. The apparent break in the wire, E X, at a, is intended to represent a great length of wire. The method of operation of the instrument is as fol- lows : A circular diaphragm, X, of thin sheet-iron is fast- ened at its edges to a mouth-piece, P, of wood. When a person speaks into this mouth-piece, the diaphragm is moved in and out by the sound-waves striking it ; but the soft-iron diaphragm, X, being near the magnet, JX, becomes a magnet by induction ; and as it is moved towards and from the magnet, iX, by the sound-waves, it produces induced electrical currents in the coil of 26* U 306 NATURAL PHILOSOPHY. wire on if, and these currents traversing the circuit, flow through the coil on if', at the distant station, and, by changing the strength of the magnetism of if', cause the diaphragm, f)', to pass through movements precisely similar to those produced by the sound-waves in D. An ear, therefore, placed at P' will distinctly hear all that is said at P, even though P' be hundreds of miles distant from P. One can even recognize the peculiarities of the distant speaker’s voice. The manner in which the currents flowing through the coil on if' cause movements in the diaphragm, P', can be readily understood by considering a single mo- tion of the diaphragm, P, towards and from if. Sup- pose that, by moving towards if, it causes an electrical current which, traversing the telephone circuit and flow- ing through the coil, C", increases the magnetism of if '; the diaphragm, P', is at once drawn nearer if'. When, now, P moves away from if, it produces a current of electricity in the opposite direction to that produced by its first movement, which current flowing through C', decreases the magnetism of if', when the elasticity of the diaphragm, P', causes it at once to move away from if'. Since these movements of the diaphragm correspond precisely to the movements of the sound-waves, it will be seen that the diaphragm, P', is moved in preciselv the same manner as the diaphragm of a person’s ear would be if he were near the speaker. A person list- ening at P' should, therefore, be able to hear all that is spoken at P. It will be seen that the magneto-electric telephone is in reality a magneto-electric machine operated by the voice. The sound-waves striking the diaphragm produce electrical currents ichich, flowing through the ELECTRO- MAGNETIC CURRENTS. 307 telephone circuit.^ reproduce in a distant diaphragm the exact movements of the first. Electrical currents then^ and not sound-waves, pass through the wire connecting the telephones. The construction of the magneto- electric telephone will be better under- stood by an inspection of Fig. 154, which represents a section of the instrument. The magnet, H F, has a coil, C, at one of its ends, the ends of the coil being connected to binding screws at x and y. The diaphragm, D, is placed near that end of the magnet which is surrounded by the coil. The centre of the dia- phragm, D, is directly opposite the opening in the mouth-piece, P. The Bell Telephone. 363. The Microphone is a variety of telephone by means of which faint sounds can be heard at very great distances. The microphone was invented by Professor Hughes. In this instrument the sound-waves are made to vary the electrical resistance of a circuit, and thereby pro- duce variations in the quantity of electricity which flows through the circuit. These variations, by means of a telephone, are made to reproduce the sounds caus- ing them. The microphone is placed near the place where the sounds are originated, and connected by means of a conducting wire with a battery and a tele- phone. Whatever sounds are made near the micro- phone, such as talking or even breathing, can be heard by any one listening at the telephone, even though it be hundreds of miles distant. Fig. 155 shows the construction of the microphone. 308 NATURAL PHILOSOPUY. A small rod, A 7i, of hard carbon, sharpened at both ends, is loosely placed in small cavities in two other carbons, C and supported, as shown, by a piece of thin Avood, A, at- tached to the base, S. The Avires, a and ^ 7, connected to the W ends of C and A, are placed in the circuit of a voltaic battery. Fig. 165. — The Microphone. in Avhich is also placed a magneto-electric telephone. The amount of electricity from the battery that can floAV through the microphone will be dependent on the position of the upright carbon, A B. If it rest against the horizontal carbons, C and 1), so that a comparatively large extent of the surfaces of its sharpened ends are in contact thercAvith, more electricity Avill floAv through the microphone than if a smaller extent of its surfaces Avere in contact Avdth C and D. Any Amriations in the strength of the current flowing through the micro- phone, by producing Amriations in the strength of the telephone magnet, Avill cause movements in the dia- phragm of the telephone. If, uoav, a person talks in a faint tone near the microphone, the sound-Avaves strik- ing different parts of the instrument — mainly the board, E — Avill cause movements of the carbons, C and A, Avhereby the portions of A A that are in contact with C and D are A’aried in exact accordance with the movements of the sound-waA'es ; so that a person listening at the telephone Avill be able to hear dis- tinctly all that is said. ELECTRO-MAGNETIC CURRENTS. 309 So -wonderfully sensitive is this instrument that the ticking of a watch held near it can be heard by a person at the telephone. Even a fly walking over the board, E. can be heard at the distant telephone. The sound of the voice as in speaking can be most distinctly heard if the board, E, is inclined at an angle of about 50° to a horizontal. The joint at/ permits of such inclination. 364. The Phonograph. — The discovery of the tele- phone directed the attention of many experimenters to the mechanical work which the voice is able to per- form, and soon residted in the invention of an appa- ratus for recording the sounds of the voice, and repro- ducing them at any future time. This instrument is called the phono gi'cq:ih^ and was invented by Mr. Edi- son. Pig. 156. — The Speaking Phonograph, The construction of the phonograph is shown in Fig. 156. A cylin- der, C, is supported at X and E on an axis, A, so as to be readily turned by means of the handle, TT. At one of the supports, as at A, a screw-thread is cut on the axis. This thread fits into a correspond- ing thread in the support at X, so that one complete turn of the handle, ir, causes the C3dinder to move forward the distance between any two consecutive threads. A screw-thread of the same pitch is cut on the surface of the cylinder, C. A mouth-piece, I/, attached to a cir- cular diaphragm, E, of elastic metal, similar to the mouth-piece of the magneto-electric telephone, is connected to the upright support, S. A piece of watch-spring, Q, fastened at its lower end, is placed, as shown in the figure, near the diaphragm, D, but prevented from touching it by means of a small piece of rubber tube, R. A needle-point, P, is soldered to the rod, Q, near its upper end. 310 NATURAL PHILOSOPHY. In using the instrument, the surface of the cylinder, C, is covered with a smooth piece of tin-foil, and the mouth piece is placed so that the needle-point, P, just touches the tin-foil surface directly over one of the grooves of the thread. If, now, a person talk in a loud voice into the mouth-piece, M, the sound-waves, striking the diaphragm, move it in and out ; and this motion being imparted to the needle- point, P, causes it to indent the tin-foil on the surface of the cylinder. Were the cylinder at rest, these indentations would all be received on the same point ; but if the cylinder be kept constantly turning while the person is speaking, the indentations are made on different portions of the surface. During this time the cylinder has been moved so that the mouth- piece is now in a different position from that it occupied when the person first began to talk. To make the phonograph talk back, move the support, S, so that the mouth-piece assumes the position shown in the figure at S'M' by the dotted lines, and reverse the motion of the handle, W, until the cylinder is again in the position it occupied when the person began to talk. Now move the mouth-piece back, so that the point, P, again touches the foil directly over the groove, and again turn the handle in the same direction in wliich it was turned when the person began to talk. If the ear be now pilaced near the mouth-piece, or, still better, if a large paper cone be inserted in the opening, M, all that was spoken to the instrument will be distinctly repeated by it. The cause of the sound is as follows: The movements of the point, P, produced by words spoken into M, caused the surface of the tin- foil on C to be marked with minute elevations and depressions ; and when the point is again caused to pass over these irregularities, it re- produces in the diaphragm, D, all the movements by which the irregu- larities were originally caused ; for when the point is forced to climb any of the elevations on the foil, it moves the diaphragm outwards; and when it is forced to enter the depressions, the diaphragm, D, moves inwards. A person near M will therefore hear all that was origi- nally spoken into the phonograph. Syllabus. In order to ascertain whether an electrical current is flowing through any conductor, it is only necessary to bring the conductor near a mag- netic needle, and observe whether the needle is deflected or not. Galvanometers are used to detect the presence of a current of elec- tricity, or to measure its intensity. They consist essentially of a long. QUESTIONS FOR REVIEW. 311 insulated wire, wrapped in the form of a hollow helix, with a magnetic needle inside the helix. A conductor conveying a current of electricity will induce a mo- mentary current of electricity in a neighboring conductor, whenever the intensity of the current is increasing or diminishing. If a magnet be moved towards or from a coil of wire, it will pro- duce a current of electricity in the wire. In dynamo-electric machines, powerful electric currents can be pro- duced by the movements of electro-magnets between the poles of other electro-magnets. The electro-magnetic telegraph depends for its operation on the fact that a core of soft iron, surrounded by a coil of insulated wire, can be instantly magnetized and de-magnetized by completing or breaking an electrical current, in the circuit of which the coil is placed. In the magneto-electric telephone the sound-waves, striking a me- tallic diaphragm placed near a permanent magnet surrounded by a coil of insulated wire, produce electrical currents in the coil, which, flowing through a telephone circuit, reproduce in a distant diaphragm the exact movements of the first diaphragm. The microphone is a variety of telephone, by means of which faint sounds can be heard at very great distances. The phonograph is an instrument by means of which the sounds of the voice, as in speaking, may be recorded and reproduced at any future time. Questions for Review. How can we ascertain whether an electrical current is flowing through a conductor ? Describe the construction and use of the galvanometer. What is meant by induced electrical currents ? How may such cur- rents be developed ? Are such currents continuous or momentary ? What are inverse currents? What are direct currents? How may induced currents be produced by permanent magnets ? What are dynamo-electric machines ? Describe their general con- struction. Describe the general construction and method of operation of the Morse Telegraphic Instrument. Explain in full the construction and method of operation of the magneto-electric telephone. What is the use of the microphone ? Describe its construction and method of operation. Describe the construction of the phonograph. INDEX. A FACE Absorption of heat 187 of light 216 selective 188 Adhesion 84 between liquids 85 between liquids and gases 90 between solids 85 between solids and gases 89 between solids and liquids 86 Adhesion, influence of on boiling- point 201 varieties of. 85 Affinitt/, or chemical attraction... 84 Air, buoyancy of 134 Air-Pump, construction of. 131 Alcoholometers Ill Alphnbet, Morse’s telegraphic 304 Amnlgumntion of zinc of batter- ies 274 Annenlini/ 91 Aqueous humor of eye 23G Archimedes’ principle, experi- mental proof of. 105, 106 principle of 105 Artesiun well 104 Athermnnous bodies 189 Atmosphere 127 composition of 127 diffusion of gaseous ingredients of 128 greater mass of, near earth’s sur- face 13.5 height of 1‘27 pressure of 128 AtmosjAieric electricity 264 P.tGE Atmospheric pressure, amount per square inch 129 pressure, illustrations of 132 pressure, influence of on liquid state 32 pressure, simple experiments in 132, 133 Atoms 19 Attraction of gravitation, causeof unknown 65 of gravitation, effect of di.stance on 67 of gravitation, effect of mass on.. 66 Attractions and repulsions, elec- trical 2.52 and repulsions of excited bodies. cause of 2.57 and repulsions of magnets 289 B Balance, use of in determining specific gravity 108 Balloons 134 Barometer 129 construction of 130 tests of accuracy of. 130 use in measuring height of moun- tains 1-30 use of as a weather-glass 130 Batteries, double-fluid 276 single-fluid 276 Battery, bichromate 277 Bunsen’s 278 Daniell’s ffii Grove’s 278 312 INDEX. 313 PAGE Battery, high resistance 276 low resistance 276 Smee’s 277 the Gravity 277 A’oltaic 27-t Beaut of light 212 BoUiuy of liquids, laws of 199 point, circumstances influencing 199 point, influence of adhesion on.. 201 Boiler, steam-engine 201 JJoff/e, specific gravity 110 BriHlettesx 91 Banseti Burner, how to make 181 photometer 221 Buoyancy, centre of 106 of air 134 of liquids 105 c Calorimeter 203 Camera, the photographing 238 obscura 239 Candle, Jablochkoff 283 Capillarity 87 familiar instances of 88 phenomena, cau.se of 88 phenomena of 88 Carbon electrodes, image of 283 Cau.xe and effect 12 of color 244 of musical sounds 155 of winds 177 Centiyrade thermometer scale.... 173 Centre of buoyancy 106 of curvature of mirror 226 of gravity 68 of gravity, bodies supported at, at rest 70 of gravity, method of determin- ing 69 Centrifugal force, so called 48 Centripetal force 48 Change of latent into sensible heat 195 Charge, positive and negative 254 Chemical or atomic attraction 84 Chemistry 11 Chimneys, cause of draught in... 177 Circnit, electrical 273 Cohesion 82 of leaden bullets 83 of liquids 83 Cofd produced by evaporation 201 27 PAGE Color, cause of 244 disc 243 Comniunication of heat 180 Commutator of dynamo-electric machines 301 Compotients and resultants 43 Compound microscope 239 Com pressibility of liquids 98 of matter 21 Concave mirrors, formation of images by 226 Condensation of vapors and gases 34 Condenser of TEpinus 261 of steam-engine 206 Conduction of heat 180 of heat by fluids 182 Conductivity of bodies for heat, illustrations of 181 of solids, applications of 182 Conductors of electricity, list of.. 251 Connecting-Bod of Steam-engine 205 Connection of voltaic cells in multiple arc 276 of voltaic cells in series 276 Contraction of matter 21 Convection of heat 183 Cornea 236 Couple, thermo-electric 279 Cranh of steam-engine 205 Cry.stalline form 94 lens of eye 236 Crystallization, force of 95 paradox 200 Current, electrical, definition of. 275 electricity 250 ' electricity, induction of. 298 electricity, sources of 271 D Bead points of steam-engine 206 Beclination of magnetic needle. 294 Biatherma nous bodies 187 Biffusion of gases 127 of light 214 of liquids 86 Birective tendency of magnetic needle, cause of. 293 Bispersion of light 242 Bistillation 201 Bistinct vision, conditions requi- site for 236 vision, limit of. 234 314 INDEX. PAGE Divisibility of matter 19 Double fluid electrical hypothesis 255 Drnuyht in chimneys, cause of... 177 Ductility 90 Dynamo-Electric machines 301 E Ear 157 iimits of audition 1.58 Eccentric shaft 206 Echoes 148 multiple 149 Elasticity 93 how developed 93 limits of 94 measure of 94 Electric discharge, effects of. 263 light, illumination by 282 Electrical attractions and repul- sions, law of. 253 charge 249 charge, distribution of 258 charge, effects produced by 250 circuit 273 current, chemical effects 284 current, effects of 281 current, luminous effects 282 current, magnetic effects 285 current, physiological effects 284 current, thermal eflects 281 discharge, chemical effects 264 discharge, luminous effects 263 discharge, mechanical effects 264 discharge, physiological effects.. 263 discharge, thermal effects 263 energy, varieties of. 249 hypotheses 255 machine, plate 260 tension 258 Electricity, atmospheric 264 condensation of 262 conductors of 2.51 current 2.50 induction of 2.56 nature of 249 thermo 278 Electrodes 274 Electrolysis 284 Electro-Maynctic telephone 307 magnets, definition of. 286 Electro-Metallurgy 284 PAGE Electro-Motive force 274 Electrophorus 2.59 simple, how to make 260 Electroscope 253 Elements 10 Energy and force 13 not gained by machines 54 Engine, steam 204 units of measure 17 Equilibrium, neutral, of bodies supported on axis 71 neutral, of floating bodies 107 of bodies resting on a flat sur- face 72 of bodies supported on an axis.. 71 of liquids in communicating vessels 104 stable, of bodies supported on an axis 71 stable, of floatiug bodies 107 unstable, of bodies supported on axis 71 unstable, of floating bodies 107 Eqttivulent, .loule’s 204 Ether, the luminiferous 170 Evaporation, circumstances in- fluencing 198 production of cold by 201 Expansibility of gases 126 of matter 21 Expansion of gases 176 of liquids 175 of solids 174 of solids, examples of 174 Experiments in high-tension electricitj' 266, 267, 268 Extension or magnitude 16 Eye, the human 235 F Fahrenheit's thermometer 173 bodies, laws of 74 Field, magnetic 289 Floating bodies, equilibrium of... 106 Flow of liquids 116 amount of, rule for calculating... 117 of liquids through horizontal pipes 118 velocity of, rule for calculating.. 116 Fluids, conduction of heat by 182 varieties of. 30 INDEX. 315 PAGE Foci, conjugate, of concave mir- rors 226 Focus, principal, of concave mir- rors 226 virtual, of concave mirrors 226 Force and energy 13 centrifugal, examples of 49 centrifugal, so called 48 centripetal 48 definition of 37 direction of action of. 38 indestructibility of. 13 intensity of. 39 magnetic distribution of 288 of crystallization 95 point of application of 39 representation of. 37 unaffected by condition of rest or motion .^ 42 varieties of 37 Forces, direction of. 44 molecular 29 parallel 46, 47 parallelogram of. 44, 45, 46 Form, crystalline 94 French units of measure 17 Ft •eezitnj a gradual process 196 effect of, on temperature of air... 196 mixtures 196 Friction, cause of. 25 definition of 25 Fusion, laws of. 193 of solids, cause of 193 o PAGE Gravity, force of. 65 intensity of, determined by the pendulum 78 point of application of 68 specific 108 H Hardening 91 Hardness 91 Head of liquid 116 Heat, absorption of 187 cause of. 170 communication of. 180 conduction of 180 convection of 182, 183 emission or radiation of. 187 general effect of. 171 how caused by chemical combi- nation 207 influence of, on condition of matter 33 latent 193 luminous 186 mechanical equivalent of. 203 obscure 186 opacity 187 radiated in all directions 184 radiation of 184 ray 185 reflection of 186 sensible 193 shadows 185 specific 202 transparency 187 High-Tension electricity 249 Galleries, whispering 150 Galvani and Volta, experiments of. 272 Galvanometer, construction of... 297 use of 297 Gas, tension of 126 Gases, condensation of. 34 expansibility of 126 expansion of. 177 incoercible. so called 34 nature of 32 Gramme electrical machine 301 Gravitation, law of 66 Gravity, centre of 68 direction of. 68 effect pf. 65 Tinman eye 235 eye, defects of. 237 Hydranlics 115 Hydrodynamics 9S Hydrometer Ill Hydrostatic pre.ss 100 I Hhiminated bodies 211 Illumination by electric light 282 Images, apparent size of. 225 formation of by lenses 234 formed by small apertures 221 virtual 223 Impenetrability of matter 18 316 IND EX. PACE Inclination or dip of magnetic needle 294 Inclined plane 59 efficiency of as a mechanical power 60 Inde.<le of A'elocities 52 Properties common to gases and liquids 126 of matter 16 Pulley, the fixed 59 the mOA'able 59 the air 131 the force, for water 137 the suction, for water 136 Q Quality of sounds, cause of differ- ences of 160 Quantity oi motion 40 R Padiant heat and light 245 heat, intensity of. 186 Padlation of heat 184 of heat, how effected 184 Painbotv 244 Jtaij of heat 185 of light 212 Pcaction of escaping jet 121 PAGE lieaction, vase 122 Reeds 166 Reflecting toloscoyo 241 Reflection of heat 186 of light 214 of light, amount of. 215 of light, larvs of ; 214. 215 of sound 147 Refracting toloioopo 240 Refraction, effect of on apparent direction 231 of light 217 of light, effects caused by 219 of light, laws of 218 of sound 151 Relation betAveen absorbers and reflectors of heat 188 Resistaiice, electrical 274 Resistances, Qmd 25 to motion 25 Resonance 149. 162 experiments in 163, 164 Resonators 163 Resulta nts and components. 43 Retina 236 Rivers, velocity of Avater in 118 s 5cre?c, the. as a mechanical power 60 Selective absorption 188 absorption, cause of 189 Senses, the 10 Shadows, cause of 212 of heat 185 Simple microscope 237 voltaic cell 272 Single-Fluid electrical hypothe- sis 255 Siphon, the 136 .Si'rcH, the 159 Sive, limits of, of structures 93 S/irfe valve 205 Small apertures, images formed by 221 Solid condition of matter, cause of. 30 Sot idiflcation — 33 increase of volume during 197 laAVS of 194 of fused solids, cause of 193 Solids, expansion of. 174 Solution of liquids 86 INDEX. 319 PAGE Solution of solids by liquids 86 Sonorfrun medium 145 Sonnet, cause of. 143 effect of distance on 151 intensity of. 156 mu.sical and noisy 155 not transmitted through vacuum 145 pitch of 157 quality of. 160 reflection of 147 reflection of by mirrors 150 refraction of. 151 the characteristics of 155 transmission of. 145 transmitted by elastic media 146 use of word 140 velocity of 147 Sound-Waves, amplitude of, ef- fect of 156 how conveyed to ear 143 interference of 165 nature of 143 Sources of light 211 Speaking phonograph 309 trumpet 156 tubes 151 Specific gravity 108 gravity, bottle 110 gravity of liquids, how to deter- mine 110 gravity of solids lighter than water, how to determine 108 gravity, rule for determining 108 gravity, table of. 112 heat 202 heat of water 203 Spectrum 242 colors of 242 colors of, refrangibility of. 242 Specula and mirrors 222 Steam-Chest 204 Steam-Engine 204 high pressure 205 String telephone 146 Structures, limits of the size of... 93 Sublimation 197 wees, compound 10 elementary 10 Surface action of bodies on heat. 186 Sympathetic vibrations 160 vibrations, examples of 161 Synthesis of light 243 T FACE Telegraph, electro-magnetic 302 Telegraphic key 303 Telephone, electro-magnetic 307 the string 146 Telescope, penetrating power of... 241 the refracting 240 Temper, drawing the 91 Temperature, definition of 171 how measured 172 Tempering 91 Tenacity 92 influence of sectional area on 92 Tension, electrical 258 of gas 126 of vapors, maximum 198 Thermo-Electricity 278 Thermometer, construction of..... 172 graduation of. 173 scales. Centigrade and Fahren- heit 173 uses of. 173 Thermo-File 279 Thunder, cause of 265 Torricelli’s experiment to sh\)w atmospheric pressure 129 rule 117 Trnnslticent bodies 211 Transmission of sound 145 of sound by elastic media 146 Transparency of metals for light 211 Ti-ansparent bodies 211 Trtimpet, ear 157 speaking 156 Tube, capillary 87 speaking 151 Tuning-Fork 143 u JTmbra, or complete shadow 212 Undershot water-wheel 119 Undulation, definition of. 142 Units of measure, English 17 of measure, French 17 V Uapoi'ization 197 Vapors, condensation of 34 formation of, in vacuum 198 latent heat of. 201 maximum tension of. 198 Variation of magnetic needle 294 Velocities, principle of 52 320 INDEX. PAGE Velocities, table of. 40 Velocity of falling bodies, effect of mass on 74 of falling bodies, effect of shape of body on 74 of light 213 of sound 147 of sound, effect of temperature of air on 147 of water in rivers 118 Vlhrotion, definition of 142 Vibrations, sympathetic 160 angle 224 Vitreous humor of eye 230 Voltaic couple 272 currents 271 or electric arc 282 pile 276 Vol atne, increase of during solidi- fication 197 of gas, effect produced by press- ure on 135 W Water, compressibility of 98 electrolysis of. 284 specific heat of. 203 temperature of maximum den- sity of 176 wheels 119 PAGE Water-Pump, the force 137 the suction 136 Wave, amplitude of 142 definition of. 142 length of. 142 motion, nature of 140, 141 period or time of vibration of,,,,. 142 Wave.s of condensation and rare- faction 144 Wedge, examples of 60 the, as a mechanical power 60 Weight, French and English sys- tems of. 65 French system, values of 66 Well, artesian 104 Wheel and axle .58 Whtfcls, breast 121 overshot 120 turbine 122. 123 undershot 119 water 119 Whispering galleries l.'O Winds, cause of. 177 Work done by machine, how to determine 54 X Xylophone 167 z Zinc, amalgamation of 274 The End. 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