TEXT-BOOKS OF SCIENCE
ADAPTED FOR THE USE OF
ARTISANS AND STUDENTS IN PUBLIC AND SCIENCE SCHOOLS.
CHEMICAL PHILOSOPHY.








INTRODUCTION TO TMI  SETJILY
OF
CHEMICAL PHILOSOPHY
TIHE PRINCIPLES
OF
TIEORETICAL AND SYSTEMIA TIC CHEM'ISTRY
BY
WILLIAM  A. TILDEN, D.Sc.LOND., F.C.S.
LECTURER ON CHEMISTRY IN CLIFTON COLLEGE
D. APPLETON  AND  CO.
NEW  YORK
I876








PREFACE.
THIS little volume is primarily intended for the use of
students. It aims at presenting a synopsis, brief indeed, and
probably imperfect, of the leading principles of chemistry
in such a form as to give the subject a more decided
educational direction than has been hitherto customary.
In consideration of its peculiar fitness for developing the
powers of observation, of reasoning, and of memory, no
branch of experimental science deserves more emphatic
recognition at the hands of educators than chemistry. In
order, however, that its advantages may be reaped to the full,
I believe that the methods of teaching very generally prevalent in schools require to be considerably modified. I think
teachers ought to realise the fact that chemistry, as a school
subject, is not taught with a view to its practical applications to medicine, manufactures, or the arts, but because the
study is calculated to quicken the faculties of observation,
to strengthen the memory, and to engender a power and a




vi                     Preface.
habit of continuous thought, as well as to arouse new
interests and open up new fields to the imagination. It is
of little consequence, in this view, whether or not the facts
acquired can be turned to practical account; but it is of
prime importance that the phenomena brought under their
notice, and the manner in which those phenomena are
presented, should be such as will compel the pupils to
think.
Notwithstanding that the book does not profess to be a
complete treatise on the subject, its contents will, I believe,
be found sufficiently comprehensive to afford a tolerably
general view of chemical theory as it exists at the present
time. My desire has been to assist the student in attaining
to broad and philosophic views of chemistry as a whole,
and to accustom him to regard it as one out of many
branches of physical science rather than as a mystery standing apart from other studies.
My little text-book embodies the substance of the lectures I have been giving for some time past to the more
advanced classes in Clifton College. Notwithstanding that
some portions of the book deal with subjects which are
outside the course of ordinary elementary teaching, and
must be admitted to be rather more difficult, I have not
found them beyond the capacity of intelligent boys of fifteen
to eighteen years of age, and I have every reason to be
satisfied with the results hitherto obtained.




Preface.                   vii
The use which I propose to make of the book in my
own.teaching is to get the more advanced classes to read
it by small portions at a time, and to work out all the
exercises, which, it must be understood, are merely suggestive, and will require to be copiously supplemented by any
teacher who adopts the work.  Such a course of study
obviously cannot be undertaken except as the sequel to a
series of experimental lessons, perhaps repeated more than
once, in which the properties of the chief elements and
some of their compounds have been demonstrated. As a
guide to such a course no better book could be desired than
the'Introduction to Inorganic Chemistry,' written by the
late Professor W. A. Miller, and this has been hitherto the
text-book of the junior classes under my charge.
The molecular theory has been adopted in a somewhat
rigid form, not by reason of any special convictions of my
own regarding its permanence as a scientific truth, but
because I am satisfied by long experience that, whatever
form it may ultimately assume, it is even now a most important and almost indispensable aid to teaching chemistry.
I cannot conclude without expressing my thanks to
several scientific friends, who have in the kindest manner
examined my manuscript and have favoured me with
valuable suggestions and encouragement.
I have, of course, not hesitated to avail myself of the




viii                  Preface.
stores of information contained in Watts' most valuable
Dictionary.
The exercises are for the most part culled either from
examination papers given at Oxford, Cambridge, or London,
or are taken from memoirs published in the journals of the
various scientific societies. Many also are original.
W. A. T.
CLIFTON: May I876.




CONTENTS.
rAGH
TABLE OF WEIGHTS, MEASURES, AND TEMPERATURES..   2
SECTION 1.
CHAPTER I.
THE CONSTITUTION OF MATTER.
Molecules-Atoms-Elements and Compounds-Special province
of Chemistry-Solids and Fluids-Special peculiarities of
Gases-Intermediate states of Matter...   3
CHAPTER II.
FUSION AND SOLUTION.
Relation of Melting Point to Molecular Weight-Solution of
Solids-Solution is probably due to Chemical combinationCryohydrates — Crystallisation — Water of CrystallisationEfflorescence and Deliquescence-Solution of Gases-Law of
Henry and Dalton-Absorption of Mixed Gases-Case of
Atmospheric Air.




x                        Contents.
CHAPTER III.
LIQUID DIFFUSION AND DIALYSIS.
PACE
Crystalloids and Colloids; separation of, by diffusion through
Colloid Septa.                                        7
CHAPTER IV.
EVAPORATION AND EBULLITION.
Conditions of Ebullition-Boiling Points, their relation to Molecular Weight-Fractional Distillation-Liquefaction of Vapours
and Gases..21
CHAPTER V.
DIFFUSION AND DIALYSIS OF GASES.
Law of Diffusion-Graham's Experiments-Relation of Diffusion
Phenomena to the Molecular Theory-Passage of Gases
through Colloid Septa-Occlusion of Hydrogen by Palladium, &c.                                             27
CHAPTER VI.
RELATION OF GASES TO TEMPERATURE AND PRESSURE.
Boyle's Law-Law of Gay-Lussac and Charles-Examples of
Calculations-Absolute Temperatures-Law of Avogadro.  32
CHAPTER VII.
SPECTRA.
Continuous Spectra of Solids, Liquids, and Dense VapoursBright Line Spectra of Ignited Gases-Spectral AnalysisAbsorption Spectra-Fraunhofer's Lines-Thermal and Chemical Spectra-Luminosity of Flame-Theories of Davy and
Frankland.                                           39
EXERCISES ON SECTION I.                                  50




Contents.                       xi
SE CTION II.
CHAPTER VIII.
ELEMENTS AND COMPOUNDS.
"AGE
Water a compound, Hydrogen and Oxygen Elements-Allotropic
modification of Oxygen-Number of Elements-Their Distribution in Nature-Symbols and Atomic Weights-Formulae
and Molecular Weights.53
CHAPTER IX.
LAWS OF CHEMICAL COMBINATION.
Definite Proportions-Multiple Proportions-Reciprocal Proportions-Law of Volumes-Percentage Composition-Dalton's
Atomic Theory..59
CHAPTER X.
EQUATIONS. CLASSIFICATION OF REACTIONS.
Calculations of Weight and Volume in Chemical Changes-Classi.
fication of Reactions-Combination of Entire MoleculesDecomposition of Molecules-Metameric Change —Single
Metathesis-Double Metathesis- Substitution..     65
CHAPTER XI.
CHEMICAL COMPOUNDS DISTINGUISHED FROM MIXTURES.
Properties of Compounds always different from those of their
Constituents-Definite Composition of True Compounds



xii                       Contents.
PAGE
Fractional Crystallisation of Solids; Fractional Distillation of
Liquids; Fractional Solution and Diffusion of Gases-Discussion of Special Cases -Iron and Sulphur-Tartaric and
Racemic Acids-Alcohol, Ether, Water-Toluidine-Hydrogen and Chlorine-Ethane and Hydrogen-Atmospheric Air.  72
CHAPTER XII.
NOMENCLATURE.
Names of Elements: of Binary Compounds; of Acids and Salts;
of Carbon Compounds.                                   78
CHAPTER XIII.
CONDITIONS OF CHEMICAL CHANGE. THEORIES REGARDING
THE NATURE OF CHEMICAL ATTRACTION.
In order to produce Chemical Action bodies must be in contact
with each other-One must be Liquid or Gaseous-Assistance
rendered by Elevation of Temperature-Heat generated by
Chemical Combination-Correlation of Electricity and Chemical action —Effect of Physical conditions upon the interaction of
Chemical Elements or Compounds-Effects of Mass-Bunsen's
Experiments-Speculations as to the modus oj5erandi of Chemical Agents.85
CHAPTER XIV.
COMBUSTION.
Modern Explanation of Burning-Theory of Phlogiston-Temperature of Flame-Heat of Combustion-Problems          97
EXERCISES ON SECTION II..                       I04




Contents.                     xiii
SE CTI ON IIL.
CHAPTER XV.'
EQUIVALENTS AND ATOMIC WEIGHTS.
PAGE
Determination of Equivalent precedes the determination of Atomic
Weight-Relation of Atomic Weight to Vapour Density of
Volatile Elements-Atomic Weight the least weight of any
Element found in one Molecule of any of its CompoundsLaw of Dulong and Petit-Exceptions —Isomorphism-Other
Methods.                                             Io9
CHAPTER XVI.
MOLECULAR WEIGHTS AND FORMULIE.
Percentage Composition-Formula deduced from PercentagesApplication of4he Law of Avogadro-Dual Nature of the
Hydrogen Molecule-Relation of Molecular Weight to
Vapour Density-Nascent State-Synthetical production of
Hydrogen and Oxygen-Synthetical production of Compounds
-Acids-Bases-Substitution Compounds.... I20
CHAPTER XVII.
DISSOCIATION.
Ammonium  Salts - Calcium  Carbonate- Efflorescent SaltsWater of Crystallisation-Sulphuric Acid-Physical Explanation of Dissociation-Ammonium Chloride-Phosphoric Chloride-Calomel-Experimental Evidence.... 130




xiv                       Contents.
CHAPTER XVIII.
TYPES. ATOMICITY.
PAGE
Types of Decomposition-Rational Formulse-Case of Sulphuric
Acid —Different Combining Powers of Different AtomsValency and Atomicity-Atomicity of the Chief ElementsCompound Radicles-Saturated Compounds —Formulke Constructed according to the Theory of Atomicity-Graphic
Formulae.1. 35
CHAPTER XIX.
UNSATURATED COMPOUNDS.
Valency generally changes by an even number of UnitsAtomicity therefore uniformly Artiad or Perissad-Even
numbers - Exceptions - Nitric Oxide - Nitric PeroxideUranium Pentachloride-Tungsten and Molybdenum Penta.
chlorides-Unsaturated Compounds acting as Radicles —Uisaturated Bodies are Stores of Energy-Valency of Common
Salt Radicles.. I46
CHAPTER XX.
ISOMERISM.
Physical Isomerides-Polymeric  Bodies-Metamerides-Allotropic Modifications of the Elements-Consumption of
Energy in Production of Isomerides.                  I53
EXERCISES ON SECTION III.                                162




Contezts.                      xv
SECTIONV IV.
CLASSIFICATION OF ELEMENTS.
CHAPTER XXI.
PAGE
Division of Elements into Three Classes-Characters of Nonmetals-The Halogens-Atomicity of the Halogens-Oxygen,
Sulphur, Selenion, Tellurium-Atomicity of Oxygen and
Sulphur-Boron-Carbon and Silicon-Atomicity of Carbon
and Silicon-Nitrogen and Phosphorus....  7I
CHAPTER XXII.
Metalloids-Hydrogen-Tellurium-Tin, Titanium, ZirconiumAtomicity of Tin and Allied Elements-Vanadium, Arsenic,
Antimony, Bismuth - Niobium  and  Tantalum - Molybdenum, Tungsten, Uranium.                            I98
CHAPTER XXIII.
Metals-The Alkali-Metals-Alkaline-earth Metals-Zinc Group
-Silver and Mercury-Indium, Thallium, Lead-Relations
of Indium, Thallium, and Lead-Iron-copper GroupAtomicity of the Iron Group-Copper-Platinum GroupGold-Platinum, Iridium, Osmium-Palladium, Rhodium,
Ruthenium.                                            210
EXERCISES ON SECTION IV........ 237




xvi                       Contelts.
SECTION V.
CLASSIFICATION OF COMPOUNDS.
CHAPTER XXIV.
ACIDS, BASES, SALTS.
PAGI
Classification of Acids-Determination of Basicity-Basic Oxides
and Hydrates-Ammoniacal Bases-Salts.... 239
CHAPTER XXV.
DERIVATIVES OF AMMONIA.
The Ammonium Theory-Amines-Constitution of Ammonium
Compounds-Phosphines, &c.-Amides.         250
CHAPTER XXVI.
CARBON COMPOUNDS.
Hyvdrocarbons and their Haloid Derivatives-Alcohols-Alcohols
Classified-Ethers-Aldehyds  and Ketones-Acids-Acids
containing n-CO -OH are n-Basic-Basic Carbonaceous Derivatives of Ammonia - Compound Ethers - Organo-metallic
Compounds.                                              257
EXERCISES ON SECTION V.                                    273
INDEX......                                            77




CHEMICAL PHILOSOPHY.




METRIC WEIGHTS AND MEASURES.
PREFIXES,-Myria-.............    10,000.
Kilo-..                                               1000.
Hecto-............                           100.
Deka-.............    10.
(.        )............         1.
Deci-.............
CentiMilli-I
Inserting after the hyphen and in the brackets(1.) Metre, gives the measures of length.
(2.)  Litre,,,,,        capacity.
(3.) Gram,,,           weight.
LENGTH.
1 METRE  = 10 decimetres.          (10 dcm.)
= 100 centimetres.      (100 cm.)
= 1000 millimetres.    (1000 mm.)
1000 Metres= I Kilonmetre.
CAPACITY.
1 LITRE   = 1 cubic decimetre.         (1 c.dcm.)
= 1000 cubic centimetres.  (1000 c.c.)
WEIGHT.
1 GRAM or the weight of 1 c.c. of pure water at 4'C.
= 10 decigrams.         (10 dg.)
1= 00 centigrams        (100 cg.)
= 1000 milligrams.      (1000 mg.)
1000 Grams= 1 Kilogram.
ENGLISH EQUIVALENTS.
NEARLY.            ACCURATELY.
1 METRE            = 3 feet 38 inches  = 39'37079 inches.
or 398 inches.
1 KILOMETRE            1100 yards     = 1093'6331 yards.
1 LITRE            =1 Impl. pint           1'760773414 pint.
or 35 ounces.
I GRAM             = 15a grains        = 15'4323s48 grains
1 KILOGRAM         = 2w pounds         = 2'2046213 pounds
MEMORANDA.
Weight of 1 litre of hydrogen (at 0~C. and under 760 mm. bar.
pressure)                           -'0896 gram, or I crith.
Therefore 1 gram of hydrogen measures 11'16 litres.
Specific gravity of Hydrogen (Air = 1)'0693.
Air              (H = 1)   =14'42.
5sC.    -      9'F.. IC.  -    ~'F. and  1IF.  =   ~C.
Cx9
To convert C. into F. temperatures    -     + 32  =   F.
(F - 32)5
To convert F. into C temperatures       9       =   C.




SECTION I.
CHAPTER  I.
THE CONSTITUTION OF MATTER.
IN order to facilitate the explanation of chemical phenomena, modern philosophers have found it convenient to
revive, in a somewhat modified foirm, the ancient hypothesis
that all bodies possessing extension and weight are made
up of stuff, substance, or ntatter, which is not uniform and
continuous throughout, but consists of separate very small
portions. Each of these small masses, which are called
zmoecuzes, is supposed to be to a certain extent independent
of the rest and isolated from them. The hypothesis further
requires us to suppose that the molecules constituting any
given species of matter are all alike, in size, weight, and
properties, and differ in these respects from other molecules.
Thus the molecules contained in one drop of water are
conceived to be precisely like the molecules in any other
drop of the same liquid. Similarly the molecules in a given
globule of the liquid metal mercury, or quicksilver, must be
assumed to be like all other molecules of the same metal; but
water molecules and mercury molecules differ altogether from
each other in weight and chemical properties. It must be
distinctly understood that molecules cannot be seen, and all
the arguments upon which the assumption of their existence
is founded, are derived from the examination of masses
of appreciable magnitude. We know nothing of isolated
individual molecules; the examination of these would be
B 2




4               Chemical Philosophay.
for obvious reasons impossible, and even if such a division
were actually possible, the condition or properties of a single
molecule would be in no way comparable with those of a
mass of matter in which many molecules are imagined to
be naturally aggregated by cohesion or otherwise. But we
may assume that when one kind of matter affects chemically
another kind, the smallest quantity of each which is capable
of entering into the reaction consists of a determinate
number of molecules. For the purposes contemplated in
this book, then, a molecule may be defined as the unit of
chemical action; that is, the smallest quantity which is
able to take part in or result from a chemical change.
The constituent parts of molecules* are called atomns. t
These may be regarded as the primordial masses of which
molecules are supposed to be built up. The atoms constituting a molecule must in some cases be assumed to be
alike. The body made up of such molecules is called an
element. Most commonly, however, there are reasons for
considering that the atoms composing a given molecule are
dissimilar, and then  the body is a compound.  The
number of atoms in a molecule is very variable.  A few
elements have molecules which are assumed to consist of
one atom only, others of two, three, or four; whilst in
compound bodies the number may amount to hundreds.
Chemistry is concerned chiefly with the investigation of
those changes in properties which result from alterations
in the internal constitution of molecules; whilst the study
of those forces which affect entire molecules, and masses
of molecules without regard to their composition, belongs
to the domain of Physics.
The materials which compose the earth's crust with its
ocean and atmosphere and their inhabitants, may be roughly
classified according to their mechanical condition into solids
and fluids.  Fluids are either liquids or gases.
* Molecule, diminutive, from Lat. mzoes, a mass.
t Atom, from &, not, and re/vW', I cut.




Solids and Fluids.                 5
A solid retains its form unless acted upon by pressure,
by division with cutting instruments, by heat, or by solvents.
Many solid bodies under suitable conditions assume definite
geometric figures, which are generally bounded by plane
faces, and thus give rise to crystals. This rigidity, by which
the external form and relative position of parts is maintained, is due to what is called "cohesion."  We should
not, however, be justified in assuming that the constituent
particles of solids are in a condition of absolute repose.
Alteration of temperature and volume produced by the
application of heat are attributable, according to the molecular theory, to the motion of the molecules, this motion
being supposed to consist mainly in their rotation, or
oscillation, or revolution, within a determinate space.
A liquid is recognised by its mobility, and by always
assuming when at rest a horizontal level surface, except just
where it comes into contact with the vessel containing it.
The several parts of a mass of liquid are not held together
with the same amount of force that binds together the parts
of a solid, but that liquids are not altogether destitute of
cohesion is shown by the spheroidal form of the rain drop, or
of water sprinkled upon a greasy surface. The volume of a
liquid is scarcely affected appreciably by pressure, even
when very great, and pressure communicated to any one
part of a mass of liquid is transmitted almost instantly arnd
without loss to every other part.
From the phenomena of liquid diffusion which will be
alluded to further on, it seems probable that the molecules
of liquids move constantly from one part of the mass to
another. These motions, however, are sluggish in comparison with the corresponding intestine movement which
is observed in gases, and this is perhaps to be explained, at
least in part, by the assumption that the molecules of liquids
may be comparatively close together, so that their free
motion is impeded by frequent collision.
Gases differ from solids and liquids in the circumstance




6              Chemical Phzilosophy.
that they seem to be entirely discharged from the influence
of cohesion. A mass of gas exhibits no surface, like that of
a liquid, and no gas can be confined in a vessel which does
not enclose it on every side.
The volume of a gas increases as the pressure upon it
decreases (Law of Boyle), until when the pressure is nothing
the bulk of the gas becomes greater than the capacity of any
conceivable vessel. If, therefore, a small quantity of a gas
be introduced into any part of a vacuous space, it immediately spreads itself out and pervades every portion of that
space equally.
Solids, liquids, and gases alike expand upon the application of heat, but whereas each solid and liquid increases by
a fraction of'its volume which is peculiar to itself, all gases
expand to practically the same extent by equal increment
of temperature. In other words, whilst the co-efficients of
expansion of solids and liquids are all different, those of
true gases are the same in every case.
On the hypothesis that a gas, like a solid or a liquid, is a
congeries of small masses or molecules, there are reasons
for supposing that the molecules of gases are in constant
and very rapid motion from place to place, and that
they move in straight lines. A given molecule, however,
cannot be supposed to pursue an uninterrupted course for
any appreciable distance, but probably takes a new direction
on the approach of another molecule.
These motions of translation which, in accordance with
received views, have been here attributed to the molecules
of gases and liquids, are directly related to the amount of
heat which has been expended in producing the liquid from
a solid, or the gas from a liquid. In other words, when
heat is consumed in melting a solid, or in gasifying a liquid,
it is necessary to admit that the vibratory motion to which
the temperature of the heating agent is supposed to be due
being communicated to the molecules of the object, may
cause them not only to vibrate, but to move from place to




The Molecu/ar Theory.                    7
place; and this altered motion corresponds with change of
state.
For the further discussion of these speculations the reader
must consult works on physics.* He will do well, however,
constantly to set before his mind the fact that we possess
at present no direct or positive proof even that molecules
exist; still less have we any evidence regarding the conditions under which they may subsist in mass. The molecular
hypothesis, however, is no longer in the position which it
formerly held as a relic of the vague speculative philosophy
of the ancients. It has been raised to the rank of a theory
which bids fair to rival in completeness and importance the
Newtonian theory of gravitation itself. In neither case does
the theory admit of direct experimental proof; but both are
accepted because they accord fully with the results of observation. The theory of molecules once admitted, all the
recognised laws of chemical combination by weight and
volume follow  as necessary consequences.  At the same
time the phenomena connected with the physical properties
of gases and liquids, such as the transmission of pressure
and the remarkable laws of diffusion, find a rational and intelligible explanation such as no other hypothesis yet put
forward has been competent to furnish. It is in fact not too
much to assert that the rapid progress of modern chemistry
and the intimate connection which it has been shown to
have with other branches of physical science, as well as the
illustrations it affords of the great doctrine of energy, are
largely attributable to the general acceptance of this hypothesis. The science as it now stands may be regarded as a
practical development of the molecular theory.
nThe mathematical investigations of Krdnig and Clausius, and of
Rankine and Clerk Maxwell, have led to the establishment of the
dynamical theory of gases, and have given a powerful impetus to the
general recognition of the molecular theory by physicists. The student
is recommended to make himself acquainted with the discussion of the
subject from this point of view, which is given in Clerk Maxwell's
" Theory of Heat."




8               Chemical Philosophy.
When a solid is transformed into a liquid, or a liquid into a
gas, an apparently abrupt change of physical properties occurs,
and heat is abundantly absorbed without producing elevation
of temperature. But the transition from one state to another is
by no means so sudden as appears from the consideration of
cases like that of water. Many solids, such as iron, pass through
an intermediate state, in which they are more or less plastic or
viscid before they finally assume the liquid condition; and even
the most perfect liquids with which we are acquainted are far
from being absolutely mobile. Ether and alcohol, for example,
flow more easily than water; but even these liquids exhibit a
certain degree of viscosity.
Experiments commenced in I822 by Caignard de la Tour, and
since continued and extended by Dr. Andrews, have shown that
matter is capable of existing in a somewhat analogous condition
intermediate between the liquid and gaseous states: " By
partially liquefying carbonic acid gas by pressure and then
raising the temperature to 880 F., the surface of demnarcation between the liquid and gas becomes fainter, loses its
curvature, and at last disappears. The space is then occupiec
by a homogeneous fluid, which exhibits, when the pressure is
suddenly diminished or the temperature slightly lowered, a
peculiar appearance of moving or flickering strioe throughout
its entire mass.  At temperatures above 88~ F., no apparent
liquefaction or separation into two distinct forms of matter could
be effected even when a pressure of three or four hundred
atmospheres was applied." (Andrews.) Nitrous oxide and
sulphurous oxide, and other gases, give similar results. The striae
referred to are most probably the result of changes in density,
caused by slight changes of temperature or pressure, as in
ordinary liquids or gases when heated.
It thus appears that the various physical states of matter
merge one into another by imperceptible gradations; and if we
adopt the molecular theory we can see some explanation of this.
The change of a solid into a liquid, and of a liquid into a gas,
is the result of alteration, generally increase, in the distances
between the molecules. It is clear that in passing, for example,
from the relative positions corresponding with the liquid state
into those belonging to the gas the molecules must occupy scccessive intervening spaces. In doing this they must occupy
successively an infinite number of different positions which
may give rise to an infinite number of temporary modifications
in the physical condition of the body.




9
CHAPTER II.
FUSION AND SOLUTION.
Fusion of Solids. —When, by the application of heat, a
dry solid, such as sulphur or lead, is made to assume the
liquid state, it is said to melt or undergo fusion. But when
sugar is placed in water, it disappears, and is said to dissolve,
and the liquid which results from such combination is called
a solution. This distinction in terms is necessary, if only
for practical convenience.
Amongst the elements the temperatures at which fusion
occurs are very diverse, and there are but few cases in
which any relation can be traced between this property and
the chemical or other characteristics of the body. But
among the compounds of carbon, which are very numerous,
the determination of the melting point often serves as a
convenient test whereby to distinguish two similar bodies
from one another or to complete the identification of some
substance under examination. As a general rule, it may,
perhaps, be said that in a series of similar bodies those of
smallest molecular weight melt at the lowest temperatures,
as in the following examples:Melting Point.  Molecular Weight.
Sulphur                  I I5~            n32
Selenion                2170              n79'5
Tellurium         about 5000             nI28
Formic acid                00              46
Acetic acid               17~              6o
Palmitic acid             62~             256
Stearic acid             69'2             284
Cerotic acid             78~              410
Melissic acid            880              452




Io             Chemical Philosophy.
There are, however, numerous exceptions to this rule.
One example will suffice:Melting Point.  Molecular Weight.
Cadmium                 3I5              I 12
Zinc                    4230              65
Magnesium            low red heat         24
In very many cases a mixture of two or more substances
melts at a lower temperature than either of the ingredients.
Mixtures of the fatty and other acids melt at lower temperatures than the pure acids; the carbonates of potassium
and sodium melt more easily when mixed than when alone;
an alloy of potassium and sodium is liquid at the ordinary
temperature; and an alloy of cadmium, tin, lead, and
bismuth melts in hot water.
Solution of Solids.-The extent to which solid bodies are
dissolved by liquids exhibits still greater diversity, and it is
possible to generalise still less with regard to this property
than when referring to the phenomena of fusion.
The most general fact that has been observed is, that
solubility increases with rise of temperature; or, in other
words, a hot liquid dissolves a given solid more freely
than the same liquid when cold. But although this statement is true of a very large number of soluble bodies, it is
subject to some well-marked exceptions. Thus, common
salt dissolves to very nearly the same extent in cold as in
boiling water, and lime, calcium sulphate, barium acetate,
and other salts are very decidedly less soluble in hot water
than in cold.
Liquids, too, are observed to exercise a certain selection
in the solids they take up. Thus, water dissolves a vast
number of salts, which, in the majority of cases, are insoluble,
or nearly so, in alcohol. On the other hand, alcohol dissolves a great many carbonaceous substances, such as
organic bases, resins, and camphor, which are almost unaffected by water. And, without straining the idea too far,
it is not too much to say that there is in many cases a rela



Solution of Solid Bodies.             II
tion observable between the chemical composition of the
solvent and that of the solid it dissolves. Ether and benzene,
both highly carbonaceous liquids, dissolve freely fats and
other substances which are rich in carbon; carbon bisulphide is the best solvent for the common form of sulphur;
phosphorus trichloride dissolves phosphorus. This relation
cannot, however, be said to be general, and at present we
possess no clue to the laws which regulate the solution of
solids in liquids.  It does appear, however, that the union
between a liquid and the substance in solution resembles
more or less closely the act of chemical combination.  One
reason for so regarding it is supplied by the thermal changes
that accompany solution.
In the process of dissolving many solids there is a very
considerable reduction of temperature consequent upon
the change of state which the solid undergoes, its liquefaction necessarily rendering a certain amount of heat
latent. Freezing mixtures are often made on this principle.
But in not a few other cases the temperature rises in a very
marked degree. This is due to the heat generated by the
combination of the solid with the liquid in which it is immersed, and is sufficient not only to satisfy the requirements
of the change of condition which the solid undergoes in
becoming a liquid, but also to raise the temperature of the
resulting liquid. It has also been observed that some solutions, when further diluted with the same liquid, develop successive though smaller quantities of heat. It seems fair to
attribute these continuous manifestations of heat to the
process of aggregation of molecules under the influence of
chemical attraction. This is in accordance with the very
generally observed, though not universal, fact that chemical
combination is attended by liberation of heat.
Sulphuric acid furnishes an instance of the kind referred
to.




I2              CChemical Philosophy.
Relative quantities of heat evolved by mixing sulphuric
acid with successive quantities of water.
One molecule of sulphuric acid
with I mol. of water.               69'7
2 mols.,,..                     33'7
3 mols.,..5'4
4 mols.,,.                   29
5 mols.,,7'3
&c.                             &c.
120 mols.,,.. 
Some interesting facts bearing on this question have
lately been discovered by Professor Guthrie, who finds that
all salts are capable of combining with definite quantities of
water when their solutions are exposed to a sufficiently low
temperature. And from the fact that the bodies thus obtained present a constant melting and solidifying point, a
distinct crystalline form and other well-marked characteristics, it seems not unfair to regard them as true chemical
compounds. Ammonium chloride, for example, is, under
ordinary circumstances, an anhydrous salt; but when the
temperature of its aqueous solution is reduced to - i5~0,
a crystalline body is formed in which one molecule of the
ammonium chloride is united to twelve molecules of water.
In other cases these "cryohydrates," as they have been
called, contain a much larger proportion, amounting to
hundreds of molecules, of water. The process of solution of
a solid body, then, would appear to consist in the combination of the body, in the first instance, with a certain limited,
though relatively large quantity of the solvent forming a
hydrate, which at ordinary temperatures above oo is a liquid,
and this liquid then mixes with the rest of the solvent by
the ordinary process of diffusion.
Naturally connected with the production of solutions is
the process of crystallisation, as it is in the gradual passage
from the liquid to the solid state that we find the most
generally useful method for the formation of crystals.




Water of Crystalliziation.           13
The crystallisation of a crystallisable solid from its solutions occurs when the percentage of solid present in the
liquid exceeds a certain limit dependent on the nature both
of the solid and the liquid in which it is dissolved. This
condition may be brought about either by allowing part of the
solvent to evaporate, or by altering, in general by lowering the
temperature. Crystals are then formed, and these crystals, in
the majority of cases, contain not only the elements of the dissolved substance, but a portion of the solvent united to it in
definite molecular proportions. This is particularly noticeable
in the case of aqueous solutions, and the water thus combined is spoken of as water of crystallisation.  Alcohol and
benzene, and probably other liquids, unite with salts and
other crystallisable bodies in the same manner. It is noteworthy that the proportion of water of crystallisation is
principally dependent upon the temperature at which the
process of crystallisation takes place. Thus, sulphate of
sodium crystallises from water at temperatures above 400 C.
in the anhydrous state. But at the ordinary temperature
of the air, the solution deposits crystals which contain ten
molecules of water with one molecule of the salt, whilst at
- 70, the crystals formed contain I66 molecules of water.
(Guthrie.)
Water of crystallisation is always expelled from a salt by
exposure in a vacuum, or at the temperature of boiling
water. The water which is given off by many bodies at
higher temperatures is supposed not to exist as such in the
compound, but to be produced in consequence of decomposition. Some compounds, however, evolve at ordinary
temperatures, a vapour of appreciable tension, and these
soon lose a part or the whole of their water of crystallisation,
at the same time crumbling away to a shapeless mass. Such
salts are said to be efforescent. On the other hand, certain
dry solids, such as potassium hydrate, and carbonate, calcium chloride, and chromium trioxide, have the power of
condensing and combining with vapour of water so freely,




14              CIhzezmical Philosophy.
that when exposed to the atmosphere, or to any gas containing moisture, they rapidly liquefy in the water which they
thus absorb. Bodies of this kind are said to be deiquzescenzt.
Solution of Gases.-All gases dissolve to a greater or less
extent in water, but, unlike solids, their solubility diminishes
as the temperature rises, so that in most cases* the dissolved
gas may be completely expelled from a liquid by boiling,
whilst the amount taken up may be greatly increased by
cooling the liquid.
Increase of pressure also augments the solubility of gases
in a direct ratio.  It is therefore necessary in making any
statement as to the solubility of a gas to observe the conditions of temperature and pressure under which that solubility
was estimated.  The following examples will serve to show
how greatly gases differ in the extent to which they dissolve
in water.
At oo C, and under a pressure of 7600 mm. barom.
I volume of water will dissolve
Hydrogen....            0193
Nitrogen'020. o2o35
Oxygen....            04 I4
Nitrous Oxide...         1'3052
Carbon Dioxide...'7967
H-lydrogen Sulphide..        4'3706
Sulphur Dioxide.                  79.789
Ammonia....            I488
The numbers given above represent volumes of the
several gases measured at oo and 760 min., and constitute
the co-efficz'ets of absorption of these gases at that particular
temperature and pressure.
The general statement that the weight of a gas dissolved
by a liquid is directly proportionate to the pressure is often
known as the law of Henry and Dalton. It admits of another expression; for since, according to the law of Boyle,
* Exceptions occur in the cases of hydrochloric acid and some other
gases.




Absorption of Gases by Liquids.           I5
the volume of a gas diminishes as the pressure upon it
increases, it is obvious that the volume of gas thus held in
solution must always be the same, whatever the pressure.
These rules no longer hold good when the gas and the
liquid exert a chemical action upon each other, and exceptions must also be recognised in the case of the more
soluble gases, such as hydrochloric acid and ammonia.
The determination of the absorption of gases by liquids
may be applied in certain cases to the elucidation of some
important theoretical and practical questions. Atmospheric
air furnishes an example which will be worth the consideration of the student.
When a mixture of gases is exposed to the action of a
solvent, the quantity of each of the constituents dissolved
by the liquid will depend first upon its co-efficient of
solubility, and secondly upon the proportion in which it
exists in the mixture.  This proportion determines the
pressure which each gas present exerts upon the surface
of the liquid, and consequently regulates the amount of it
which is dissolved. The total pressure produced by the
mixture is therefore the sum of those partial pressures due
separately to the individual constituents.  To make this
more clear:
Suppose a very large vessel containing a very little water,
and filled with oxygen, under a pressure of one atmosphere.
It is plain that if four-fifths of the gas were removed the
pressure would be reduced to one-fifth of an atmosphere,
and the quantity of oxygen dissolved would be only onefifth the quantity taken up under the previous conditions,
provided, of course, that the temperature remain constant.
An exactly similar vessel can be conceived filled with
nitrogen under one atmosphere, and containing a little
water. If one-fifth of the nitrogen were removed, the pressure of the remainder would be only four-fifths of an atmosphere, and the quantity dissolved would be reduced to
four-fifths. Lastly, a similar vessel, filled with atmospheric




i6             Chemical Philosophy.
air, contains a gas in which the conditions of the two previously supposed experiments are combined.
Air is composed very nearly of four volumes of nitrogen
to one volume of oxygen, and by reason of the greater
solubility of oxygen the proportion of the two gases one to
the other is found to be disturbed when air is shaken up
with water, the dissolved gas being richer in oxygen, the
residual air richer in nitrogen, than the original.
No stronger evidence could be adduced in favour of the
view generally held, that in atmospheric air the two main
components are not united chemically, but are in a state of
intimate mechanical mixture.
Note.-The following is an example of the kind of problem
that might occur in connexion with this subject:
Calculate the percentage composition of the gas which would
be dissolved by water exposed in a room full of air containing
79 N, 20'6 0, and'4 of C02 in Ioo volumes (temp. oo and bar.
760 mm.).
Coeff. of sol. for oxygen...'04,,,,      nitrogen..'02
carbondioxide. I79
The pressures are proportional to the volumes of the gases
present. Therefore the relative quantities dissolved would be:
Nitrogen      79    X.02    ='58
Oxygen        20'6  X'04    -'824
Carbondioxide'4  X   I'79    ='716
The total quantity... =      20
The percentage composition of the dissolved gas would
therefore be:
Nitrogen...                  5.6
Oxygen.. 26'4
Carbonic dioxide...             22. 9




I7
CHAPTER III.
LIQUID DIFFUSION AND DIALYSIS.
AN aqueous solution of sugar or of salt is heavier than
water, and may be readily poured through a funnel with a
long stem, into a glass of water in such a way as to form a
separate stratum at the bottom. If the solution is coloured,
it will soon be noticed that the colour gradually extends
upwards through the liquid, until, after a few hours or a few
days, according to circumstances, the whole liquid is uniformly tinged. But it is not necessary that the liquid
should be coloured.  The taste, specific gravity, refractive
power, or the application of chemical tests, will soon give
indications that the solution from below is mixing with the
liquid above. This process of spontaneous intermixture is
called diffusion. It results from the proper motion of the
molecules of the liquid, and cannot be referred to the
disturbing influences of changes of temperature.  The
rapidity with which diffusion of this kind takes place, and
the limit of its action, depend very much upon the nature
of the liquids employed.
The power of interdiffusion is by no means universal
among liquids, some liquids being, like mercury, oil, and
water, quite incapable of mixing together under any circumstances; whilst others, such as water and solution
of hydrochloric acid, mingle spontaneously in consequence
of very rapid diffusion.  We are indebted for nearly all
the information we possess on this subject to the late
Professor Graham. His experiments were conducted very
nearly in the manner already described at the beginning
of the chapter. The glass vessel in which diffusion was
allowed to go on was graduated into equal divisions, from
the bottom upwards; and after the introduction of the
two liquids, the whole was left in a room, the temperature
of which was kept as uniform as possible.
C




I 8              Chemical Philosoply.
After a time, the liquid occupying successive divisions of
the vessel, was removed by a small syphon, or pipette, and
analysed, in order to ascertain the extent to which diffusion
had taken place. In this way a number of conclusions
were arrived at, amongst which the following are the most
important:
i. Bodies are divisible, as regards their diffusive power,
into two classes. Those which diffuse most readily through
a given liquid menstruum are, for the most part, crystallisable
substances, and are termed by Graham crystalloids; whilst
the least diffusible bodies are uncrystallisable with, in most
cases, high molecular weight, and are denominated colloids.,*
from their resemblance to glue, which may be taken as the
type of this class. The following list supplies the times of
equal diffusion by the substances there named; and it will
be seen that albumen and caramel, both of which are
uncrystallisable substances of somewhat indefinite composition, are far behind the rest: —
Hydrochloric acid..            I
Chloride of sodium..         2'33
Sugar......    7
Sulphate of magnesium...    7
Albumen.....   49
Caramel.....   98
Thus hydrochloric acid diffuses more than twice as rapidly
as chloride of sodium, seven times as rapidly as sugar or
magnesium sulphate, forty-nine times as rapidly as albumen,
and nearly one hundred times as rapidly as caramel.
Hydrochloric acid is one of the most diffusive substances
known.
2. Equal rates of diffusion are exhibited in many cases
by the members of isomorphous groups. Thus hydrochloric,
hydrobromic, and hydriodic acids have nearly the same
* K6XXa, glue.




Dialysis.                      9
diffusion rate; so also have the chlorides, bromides, and
iodides of the alkali metals; the nitrates of barium,
strontium, and calcium, and the sulphates of magnesium
and zinc.
3. The rate of diffusion increases with the temperature,
and when the solution is not too concentrated, is proportional to the strength of the solution.
By taking advantage of this difference in diffusibility,
mixed salts may be separated from one another to a certain
extent, and crystalloids may be isolated pretty perfectly
from admixture with colloids. In the practical application
of this process, it has been found convenient to separate
the liquids undergoing diffusion by some membrane or partition composed of colloid material, and this mode of diffusion, through a septum, is called dialysis.  The process is
a very simple one. The liquid holding in solution a mixture of crystalloids and colloids is placed in a sort of tray
or sieve, formed of a sheet of parchment paper stretched over
a hoop. This vessel, which is called the dialyser, is made
to float in a dish of pure water, which, after a time, can be
renewed if necessary.  Under these circumstances, the
crystalloids pass out by diffusion through the membrane
(which must be perfectly free from holes), and by evaporating
the liquid down may be obtained in a condition of tolerable
purity.
The application of this method led to the discovery of
the soluble colloidal forms of ferric hydrate, silicic acid,
alumina, and other bodies which had been previously known
only in the pectous or gelatinoid condition, and the study
of which could not fail to throw considerable light on the
obscure natural processes by which these bodies are
deposited as minerals in the crystalline form, and in a great
state of purity. If, for example, we take a solution of silicate of sodium, and add to it a slight excess of hydrochloric acid, we obtain a perfectly clear liquid, which contains the very substance referred to above as colloid silicic
C 2




20                Chemzical Philosophy.
acid; but in this liquid it is mixed with the acid used
and the common salt formed by the decomposition.  The
process of dialysis furnishes the means of separating
these latter substances without causing the precipitation 6r
other alteration of the silicic -acid, which is left on the
dialyser in the form of a colourless limpid solution. This
solution is, however, very unstable, especially when concentrated, and the addition of even very minute quantities
of various salts causes the whole of the silica to separate
out in the form of a translucent jelly, which cannot be
re-dissolved, except by the addition of a fresh quantity of
alkali.
Liquid stannic, titanic, tungstic, and molybdic acids have
been prepared by a similar process.
The ultimate pectisation of liquid silicic acid and other
colloids is preceded by a gradual thickening of the liquid,
and just before gelatinising silicic acid flows like an oil.
These effects are doubtless the result of the tendency of
the particles of colloids to cohere, aggregate, and contract.
This tendency manifests itself occasionally in the exercise
of very considerable force. Thus the contraction of gelatine drying in a glass dish over sulphuric acid, together
with the adhesion of the gelatine to the glass, is said to be
sufficiently powerful to tear up the surface of the glass.
Glass is itself a colloid, and the permanent adhesion
between the surfaces of polished plates of glass is a wellknown circumstance, which is referred by Graham to this
class of phenomena.




21
CHAPTER IV.
EVAPORATION AND EBULLITION.
A LIQUID boils when, by raising its temperature, the
elasticity of the vapour formed at any point in the liquid
is capable of overcoming the pressure at that point. This
pressure is made up of the pressure of the atmosphere
and that of the superincumbent stratum  of liquid. In a
vessel of inappreciable depth, and when the mercurial
column in the barometer is 760 mm. high, alcohol boils
at 78~'4 and water at 00oo~.
The tension of alcohol vapour is, therefore, at any temperature below its boiling-point, greater than that of water,
and alcohol is said to be more volatile than water. Nearly
all liquids are volatile, but the temperatures at which they
evaporate freely are very diverse. There is, for example,
a wide range between the volatility of ether and of molten
silver, or between that of liquid carbon dioxide and of
metallic mercury.
The observation of the boiling point of liquids is an
operation of daily occurrence in the laboratory, and
although very few general laws connecting the boiling
point with chemical characters have been traced out, a few
general observations have been made, to which the attention of the student must be directed. The conversion of a
liquid into a gas or vapour is attended by the absorption of
heat, and this heat is consumed in giving to the molecules
of the liquid a new and more rapid motion. From this
consideration it would appear that bodies formed of light
molecules would be more easily vaporised than others constituted of complex and consequently heavy molecules.
Such is indeed the case, and, in the broadest sense, such a
statement would be nearly true. But when we examine




22               Chemical Philosophy.
individual cases, we meet with so many exceptions and
anomalies, that it is obvious such a law must be applied
with extreme caution. It is, however, permissible to say
that bodies which are strictly comparable in regard to
chemical and other physical qualities do in nearly all cases
exhibit the relation referred to.  Among the elements the
most noteworthy instances are the following:Molecular Weight.    Boiling Point.
Chlorine...            -50~
Bromine...  6o               63~
Iodine... 254               1750
Oxygen...   32       permanently gaseous
Sulphur....  64?             4400
Selenion     ~   ~ 159?         below a red heat
Tellurium.    258?          white heat
Nitrogen..   28        permanently gaseous
Phosphorus.. 124                 290~
Arsenic... 300           volatilises at red
heat without fusion
Antimony.. 244?          white heat
Bismuth... 420?            white heat
Groups of similarly constituted compounds show the
same relations.
Molecular Weight.    Boiling Point.
Sulphur Dioxide.  64                -I0o
Sulphur Trioxide.  80                460
But the most important cases are observable among the
carbon compounds, which form homologous series, to which
reference will be made hereafter.  One example will suffice
in this place. The following compounds all contain carbon
and hydrogen in proportions which may be represented by
the general formula CH2n +2:- 




Boiling Points.                   23
Molecular Weight.    Boiling Point.
Methane (marsh gas)    i6              gaseous
Ethane...   30              gaseous
Propane...   44              gaseous
Tetrane...   58               ~1
Pentane...   72                38~
Hexane...   86                 700
Heptane...   00oo              990
Octane... II4                1240
&c.             &c.
In this series, for every increase of I4 on the molecular
weight, the boiling point rises by about 300 to 35~.
In the determination of boiling points it is usual to
observe, by an accurate thermometer, the temperature of the
vapour evolved by the boiling liquid, and not that of the liquid
itself. This precaution is necessary when glass vessels are,
as usual, employed, in consequence of a peculiar adhesive
attraction which glass exercises, and which causes the boiling point to be slightly raised above the true temperature
of ebullition. This adhesion sometimes gives rise to the
phenomenon of irregular ebullition or "bumping."  The
entire thread of mercury in the thermometer should be immersed in the vapour, so as to be heated by it, and the
height of the barometer should be noted at the time of the
experiment.
This last is a precaution which is very generally
neglected, and its neglect is probably the cause of some
of the' discrepancies noticed between calculated and
observed boiling points and between the results of different
experiments.
The determination of the boiling point is often useful in
deciding as to whether a given liquid is a mixture or a
homogeneous body.  When heat is applied to a mixture of
volatile liquids the mixture begins to boil at a temperature
very near to the boiling point of its most volatile constituent, and if the temperature is not allowed to rise above




24               Chemnical Philosophy.
this point, ebullition in most cases soon comes to an end.
But if the application of heat is continued whilst the thermometer is kept in the vapour, the temperature may be observed
to rise continuously till the whole of the liquid has boiled
away. If this operation is conducted in a flask connected
with a condensing apparatus and a receiver, and if the
receiver is changed at intervals, so that the several portions
which pass over between certain limits of temperature are
received in separate vessels, a more or less complete separation of the constituents of the liquid may be effected.
Such a process is called fractional distillation.
The compression of a vapour tends to produce the same
change of state as lowering its temperature. In either case
the approximation of the molecules is attended sooner or
later by the liquefaction of a part of the vapour. With these
facts in view, and considering the generally close resemblance
between vapours and those bodies which are commonly
called true gases, Faraday came to the conclusion that the
latter are not essentially different from the former, but are
in truth the vapours of volatile liquids far removed at
ordinary temperatures from their boiling points. This conclusion he verified experimentally by enclosing in strong
A-shaped glass tubes materials capable of evolving the gases
he wished to examine. On the application of a gentle heat
to these materials gas was generated, and by its accumulation in the confined space, sufficient pressure was exerted
to cause its partial liquefaction. In this way ammonia,
chlorine, and other gases were reduced to the condition of
limpid liquids, and by the combined use of pressure and
low temperature, produced by powerful freezing mixtures, a
great many other bodies, which till then had been known
only in the gaseous form, were also liquefied.  Larger
apparatus constructed on the same principle as Faraday's
glass tubes, were subsequently employed by different experimenters, and at the present time several gases, such as




Liquefaction of Gases.               25
carbonic anhydride and nitrous oxide, are liquefied on a
large scale by compressing them by powerful force-pumps
into iron cylindrical bottles fitted with stopcocks. By such
methods all known gases have been, with six exceptions,
reduced to the liquid state.
Those which refuse to liquefy, viz., oxygen, hydrogen,
nitrogen, carbonic oxide, nitric oxide, and methane, are all
gases of comparatively small density, though not less dense
than some, e.g. steam and ammonia, which assume the liquid
condition very readily.
There can be very little doubt, therefore, that these six
gases would yield like the rest if a sufficiently great pressure,
assisted by very low temperature, could be applied.
The evaporation of liquefied gases is attended by the
absorption of much heat, and in some cases the reduction
of temperature is such as to cause the solidification of part
of the liquid.  Carbonic anhydride can be obtained in the
form of a white snow-like solid by allowing a fine stream of
the liquefied gas to escape into the air. Part of it evaporates very rapidly, and so much heat is thus rendered latent
that the remainder freezes.
The following table shows the amount of pressure in
atmospheres necessary at the temperature of o~ to liquefy
some of the more important of the liquefiable gases:PRESSURE IN ATMOSPHERES-TEMP. 00 C.
Sulphur dioxide..'53
Cyanogen..3 7
Hydriodic acid...       3'97
Ammonia.....   4'4
Chlorine.      about  5
Hydrogen sulphide...
Nitrous oxide....  32
Carbon dioxide....  38'5
Hydrochloric acid.. about 42




2G               CChemical Philosopphy.
Some of these may be liquefied by cold alone under the
ordinary atmospheric pressure. ThusSulphur dioxide condenses at.. — Io0
Cyanogen...,,.. -22~
Ammonia...,,.   -36
Chlorine...,,. -500
Carbon dioxide..,,. -87~
On the other hand, Dr. Andrews has found that if the
temperature be raised to a certain point, a gas which is
otherwise liquefiable can no longer be liquefied by pressure,
even when it amounts to several hundred atmospheres. This
point, which varies with the nature of the gas operated upon,
is called the crilicalp/oint. In the case of carbon dioxide it
is 3I~ C.
It has been already stated earlier in the chapter that if
we compare together liquids which consist of the same
elements, and which present the same general properties,
those which have the simplest constitution are the most
readily converted into vapour.  The converse is equally
true. Comparing together similarly constituted gases and
vapours, we find that those which are composed of simple
molecules are more difficult to liquefy by cold or compression than others of more complex constitution. These
facts are sometimes serviceable in helping to decide questions as to the relative complexity of two nearly allied
compounds. For example, there are two oxides of carbon,
both gaseous at ordinary temperatures, but one of them
capable of liquefaction under pressure. It follows that the
liquefiable oxide is in all probability made up of heavier
molecules than the other, and this view is supported by a
comparison of the densities of the two gases. Carbonic
anhydride gas, which is liquefiable, is bulk for bulk I'57
times heavier than carbonic oxide.
Nitrous and nitric oxide, ethylene and marsh gas, furnish
examples of the same kind.




27
CHAPTER V.
DIFFUSION AND DIALYSIS OF GASES.
A VERY remarkable property of gases and vapours is their
power of mixing with one another, even in opposition to
gravity.
" If a bottle of any odorous gas is opened in any part of a
room of constant temperature and free from draughts, the
smell of the gas soon becomes perceptible in every part
of the room, and, after the lapse of a short time, equally
in every part. Notwithstanding that, as in the case of
sulphuretted hydrogen, a heavy gas may be selected for the
experiment, it would be easy to prove by anaylsis of the air
that every part of it is equally impregnated with the foreign
matter. Other experimental illustrations of the same law
may easily be devised. A bottle of hydrogen held mouth
downwards in the air for a short time soon becomes explosible.  A jar of air inverted over another filled with
carbonic acid gas, soon acquires the power of giving, like
carbonic acid, a precipitate with lime water. And this
process of intermixture proceeds almost equally well if the
gases are separated from each other by a partition formed
of some porous material.
A thin plate of unglazed earthenware, a slice of artificially
compressed graphite, or a cake of dry plaster of Paris, may
be employed for the purpose. A very effective form of
apparatus consists of a clay battery cell closed by a cork,
through which a yard or so of glass tubing open at both
ends is made to pass. By means of this simple apparatus
it may be shown that different gases penetrate the porous
clay with different degrees of rapidity, and that light gases
effect a passage more quickly than heavier ones. The con



28               Chemical PhilosofiAy.
sequence of this difference of diffusion-rate is that a
difference of pressure is established inside the cell, and if
the open end of the tube is dipped into water a certain
quantity of gas is expelled in bubbles from below, or the
liquid is forced by the atmospheric pressure up the tube.
If, for example, the clay vessel previously full of air is
surrounded by hydrogen gas, intermixture of the air within
and the hydrogen without takes place through the clay; but
since the hydrogen diffuses more rapidly than the air, the
quantity of gas within is rapidly increased, and some of it
visibly finds its escape from the open end of the tube
through the water.
Early observations of and experiments upon gaseous
diffusion were made by Priestley in the last century, and
by Dobereiner in 1825; but Graham gave the explanation
of the phenomena, and by precise and long-continued
experiments established the law:
The velocities of dzJflzsion of different gases are inversely
proportional to the square roots of their densities.
Graham's experiments were for the most part conducted
with a very simple apparatus, consisting of a straight wide
glass tube closed at its upper extremity by a disc of porous
stucco or graphite. This tube was filled with hydrogen or
other gas over the mercurial trough, the graphite plate being
covered during this operation with a sheet of gutta percha.
The mercury within the tube was kept at the same level as
the mercury in the trough, in order that there might be no
alteration of pressure whilst the diffusion was proceeding,
and at the same time the temperature and barometric
pressure were recorded.  After the lapse of a certain
interval, measured by a chronometer, the volume and coimposition of the residual gas could be determined.
The  following table  embodies some  of the results
obtained in this way, the pressure and temperature being
supposed to be the same in all cases:



Law  of Gaseous Diffision.                   29
DIFFUSION OF GASES.
~~Name of Gas.  D  Square Root           Velocity of
NameofGas. Density.   of Density.   Density    iffusion.
Air.                                 I
Hydrogen..  o693'2632     3'7794    3'83
Marsh gas..'554'774      1'3375    I'344
Carbonic oxide.   *9678'9837     I0'o65    1'oI49
Nitrogen..'9713       *9856    I'oI47   I'0143
Oxygen.       I'Io56     I'055'95IO'9487
Carbonic dioxide  I'529      I'2365'8087'812
It will be noticed that the observed rate of diffusion
agrees very nearly with the rate calculated from the density
of the gas, but in no case is there absolute concordance.
This is, probably, in part due to the errors inevitable in
any experimental investigation, especially where gases are
concerned, but is also in some degree attributable to the
fact that the diaphragm employed possesses an appreciable
thickness, so that in passing through the pores the gas
encounters considerable resistance.
By taking advantage of the unequal diffusibility of gases
of different density, a partial separation of mixed gases may
in some cases be effected.
The gases constituting atmospheric air, for example,
may be, to some extent, separated from each other by
causing a slow current of air to flow through a clay tube
passing through a glass tube which has been exhausted as
completely as possible by the air-pump.  The nitrogen
being lighter, and consequently more diffusible than the
oxygen, passes more abundantly into the vacuous space,
leaving the residual air richer in oxygen than it was originally.
The physical explanation of the phenomena of diffusion
depends directly upon the mechanical theory of gases,
which has been (Chap. I.) already discussed. The molecules of a gas are supposed to move constantly in straight




30               Chemical Philosophy.
lines till they come nearly into contact with other moving
molecules, or with the walls of the containing vessel. If
these walls are perforated at intervals with apertures large
enough to permit the passage of a molecule, we may conceive that although many molecules continue to rebound as
though the surface were impervious, yet that a great many
others may find their way into and through these short passages, and so into the atmosphere beyond. Molecules from
the external atmosphere may be assumed to pass inwards in
precisely the same manner, and if the densities of the
gases on the two sides of the partition are the same, the
number of molecules passing inwards in a given time is
exactly equal to the number passing outwards, and no
change of volume or of pressure can result. But if the
gases are of different densities, molecules of the lighter gas
pass through more rapidly than those of the heavier, and a
change is produced in the tension or elastic force of the gas
enclosed in the porous vessel.
This pressure, exerted by a gas in opposition to that
which it has to bear when enclosed in a vessel, is represented according to the dynamical theory as the result
of the continuous showering down  of its moleeules
upon the surfaces with which it is in contact. The molecules of different gases being of different weights, they
must move with different degrees of velocity, the light
molecules more rapidly than heavier ones, in order to produce the same amount of pressure. Hence, when the gas
in contact with the porous surface is a light gas, its molecules must be supposed to fall upon a given area more frequently than when the gas employed is heavier, and consequently the opportunities for the escape of molecules
through the pores are more frequent. Hence it is that
light gases diffuse more rapidly than heavy gases.
Gases have not only the power of passing by diffusion
through porous substances, but under certain circumstances
penetrate membranes, and even sheets of metal which are




Passage of Gases througlh Membranes and Metals. 3 I
absolutely destitute of pores. This phenomenon differs
entirely from diffusion, for it is not found that the lightest
gases traverse such substances most rapidly; indeed, the
contrary is more generally the case. Moreover, the metals
which are so remarkable f6r their power of transmitting
some gases are absolutely impermeable by others.
A few simple experiments will give the student an idea
of the general character of the phenomena we are discussing. If a thin india-rubber balloon (such as are sold at
the toy-shops), inflated with air, is immersed for a few
minutes in a vessel full of, carbonic dioxide gas, the balloon
becomes largely distended, and if a band of tape is fastened
round it before the experiment, it generally bursts after
immersion in the gas for a short time. A similar balloon
filled with hydrogen or carbonic dioxide gas quickly collapses when exposed to the air.  Such a film of rubber
appears to have no porosity, but rather to resemble a film
of liquid in its relations to gases.  The penetration of the
rubber and similar colloids by a gas appears to be due to
the absorption of the gas by one surface of the colloid and
its transmission to the other surface by the agency of liquid
and not gaseous diffusion. The liquefied gas then volatilises into the vacuum or atmosphere on the other side.
The passage of gases through metallic plates at a red heat
is referred by Graham to a somewhat similar cause. Thus,
at a red heat, both platinum and palladium, and even iron,
are permeable by hydrogen gas; and this is evidently connected with the fact that the same metals are capable
of absorbing and retaining considerable quantities of
hydrogen when that element is presented to them under
suitable conditions. Thus a sheet of palladium  connected with the negative pole of a battery, and immersed
in acidulated water, becomes charged with upwards of 200
times its volume of hydrogen gas. The same metal in a
spongy state absorbs 686 times its volume of hydrogen
when heated in the gas to 200~.  This "occlusion" of




32               Clzenical'Phzilsoph y.
hydrogen is not attended by any alteration in the appearance of the metal, although its volume is increased, and
consequently its density diminished.
Other gases are occluded in a similar manner by other
metals; but in each case a certain selective power is manifested on the part of the metal.  Thus, platinum and palladium take up hydrogen freely, but no other gas to an
appreciable amount; iron takes up hydrogen and carbonic
oxide, and melted silver absorbs' oxygen.
CHAPTER VI.
RELATION OF GASES TO TEMPERATURE AND PRESSURE.
Law  of Boyle.-" The volume of a given mass of any
gas varies inversely as the pressure."
Thus, if V is the volume when the pressure is P
The volume of the gas becomes
2 V when the pressure is..   2 P
3 V,,   3 P
4V,,,,,,,,            4 P
- V,,,,,,,)           ~    n P
Also
2V,,,,,,,,   P
3 V                                P
nV                                P
Hence the pressure which is produced by the elastic force
or tension of a gas is proportional to its density.
So that if pressure increases,
density increases,
and volume diminishes.
Also if volume increases,.fressure and density diminish.




Law of Boyle.                  33
Boyle's law is not absolutely obeyed by any known gas;
but hydrogen and the other incondensable gases conform
to it very nearly, and thus present the nearest approach to
the condition of a perfect gas with which we are acquainted.
With change of pressure the liquefiable gases and vapours
increase or decrease in volume to a greater extent than
permanent gases.
The pressure and density of atmospheric air, and of
gases which are in communication with it, are estimated by
the aid of the barometer. This instrument, in its simplest
form, consists of a straight glass tube, somewhat less than
a metre long, and closed at one end. The tube is filled
with pure mercury, free from air, and then inverted with
the open end beneath the surface of pure mercury. The
liquid metal then falls from the closed extremity, leaving a
space which is generally referred to as the " Torricellian
vacuum."  It contains nothing but mercurial vapour. It
is usual to consider that the atmosphere possesses its
average and normal density when, at the sea level and at the
temperature of o~C, the column of mercury sustained by
the atmospheric pressure is 760 millimetres (or 29'92
inches) high, measuring from the surface of the mercury
in the reservoir to the surface of the mercury within the tube.
This amount of pressure is often spoken of as one athmospere.
In accordance with the law of Boyle, the volume of a
gas under altered barometric pressure, can be calculated by
the formula:
V PI
V- P
In which V is the given volume under pressure P, and V1 is
the new volume when the pressure is altered to P1. So that
Vi =    x P.
Examlpe.-Ioo volumes of air are measured off when
D




34               Chemrnical Philosophy.
the barometric pressure is 740 mm.; what will be the
volume of the same air when the barometer stands at
76o mm.?
Here V = ioo, P = 740, P1 = 760.
Then V, =00 X 740 = 97'3 vols.
760                  Ansrcer.
LAW OF GAY-LUTSSAC AND CHARLES.
Air expands by.   of its volume at o~ for every increase
273
in temperature of I~ C.
Thus 273 volumes of air at..   00
Become 274,,,,.         I~
275,,,,20
276,,,.    3~
2713+t,,,,,..       t~
Also 273,,, 
Become 272,,,,.. - I
271,, I,,. -  2~
270,,,,    ~  - 3
273-t,,,,.         t~
And generally 273 + t at t~ become 273 + T at T~.
This fraction  - or'003665, is called the co-efficient of
273
expansion,, and represents almost exactly the increment or
decrement which occurs in a measured volume of air or
other permanent gas for every change of temperature of
one degree centigrade, provided the pressure remains
unchanged.  The co-efficient of all gases is very nearly
coincident with that of air, and for chemical purposes may,
without inconvenience, be assumed to be the same. Strictly
speaking, however, every gas has a co-efficient of its own,
which, in the case of the liquefiable gases, is perceptibly




Expansion of Gases by Heat.             35
greater than the number given above, as may be seen by
the following table:Co-efficients of expansion for I~ C.
Unliquefiable.
Air....'003665
Nitrogen....  oo003668
Hydrogen....  003667
Carbonic oxide...'003667
Liquefiable.
Carbonic anhydride.       003. oo3688
Nitrous oxide...  003676
Cyanogen....'003829
Sulphurous anhydride..'003845
It seems not unreasonable to suppose that such differences are due in part to the fact that in vapours and in the
liquefiable gases the influence of cohesion is not altogether
annulled.  It is conceivable that a vapour may consist of
molecules which, unlike the independent and mutually
repulsive molecules of perfect gases, may be connected
together into companies, which move about much in the
same way as individual molecules, but less rapidly.
Examples.-A certain mass of air measures ioo cubic
centimetres at o~; to find its volume at 1O~.
273 vols. of a gas at o~ become 273 + t vols. when the
temp. is t~ C.
In this case t = I o,
Then 273 c.c. at o0 become 283 c.c. at o"~.
I C.C.,,,,becomes  3 c.c. at I0O.
273
And Ioo c.c.,,,,,,          283 X 100. at.
273
Ans.  Io3'66 c.c.
Or employing the decimal equivalent to -2,, let V1 be the
required volume and V the vol. given.
D 2




36                Chemical Philosophy.
Then V, = V (I +'00366 t)
Ioo (i +'oo366 x io)
10o3 66 Ans.
300 c.c. of air at 20~; find the volume at o~.
273 + 20 at 20~ become 273 at o~
I,, becomes -7_   at o~
293
And 300,,,,    273 x 300
at o~
293
279'5 Ans.
Or, using the decimal co-efficient, we say Vo, a certain
volume of air at o0, becomes 300 c.c. at 20~, or
Vo (I +'00366 x 20) = 300
Whence V. =        3- -        =  279'5 c. c. Ans.
I + -'oo366 x 20
500 c.c. of air at oo~; find the volume at -  o0~.
In this and all similar problems it is to be remembered
that the co-efficient of expansion is a fraction of the volume
which the gas occupies at o~, not at any other temperature.
273 + Io c.c. of air at Io~
measure
273 - io c.c. at - Io0
Therefore 500 c.c. of air at Io~
measure 263 x 5~~ or 464'6 c.c. at - io"
283
Or let V1, VO, and V T- o, be the volumes at the temperatures I10~, o, and --  o~ respectively; then
V  10 -- o  Vo(I- oo366 x Io)    3      x (I- oo366x Io)
Ii+'.00366 x io
= 464'6 c.c. Ans.
Air or any other permanent gas diminishes by 2-73 of
its volume for every degree of temperature travelling down
the scale.  If the same relations of volume to temperature




A bsoliute Temperatures.           37
were maintained, it is obvious that at- 2730 the volume
would be nil, and the gas cease to exist. Such a temperature has, however, never been attained; and if ever such a
degree of cold were reached, there can be no doubt that a
gas exposed to it would liquefy, or that some change would
occur whereby the gas would be released from obedience to
the ordinary law.
Notwithstanding, however, that such a condition of things
is practically beyond the reach of experiment, this consideration is important as furnishing the basis of an absolute scale of temperature. Calling - 2730 C the zero point,
we represent absolute temperatures by adding 273 to the
number of degrees upon the ordinary Centigrade scale.
From what has already been stated regarding the expansion
of gases, it follows that pressure being constant, the volume
of a mass of gas varies directly as the absolute temperature.
This statement is sometimes referred to as the law of
Charles, to whom we owe the discovery* of the equal
expansibility of the principal gases by heat.
LAW OF AVOGADRO.
It has been shown in the foregoing paragraphs that all
gases, when under conditions sufficiently remote from those
which induce their liquefaction, are affected in the same
manner and to the same extent by changes of pressure and
of temperature.  Differences of density, of chemical composition, or of chemical properties, do not affect the
generality of this statement. The volumes of heavy oxygen
and light hydrogen, of simple nitrogen and compound
marsh gas increase and decrease according to the same law.
It is impossible to avoid the inference from these facts
that these gases, so different chemically, must be physically
constituted alike. If now we admit the hypothesis that
gases, like other bodies, are made up of small independent
* Towards the end of the last century.




38               Chemical Philosophy.
masses called molecules, and that heat causes these molecules to separate from one another, whilst cold or pressure
causes them to approach, we are led to the assumption
that in equal volumes of different gases* there must exist
the same number of molecules.
This statement, originally enunciated by an Italian
physicist, Avogadro, in I81I, may now be regarded as a
well-established truth.
But, like every other part of the molecular theory, this
law owes its recognition by physicists and chemists not to
any direct proof that can be adduced from experimental
sources in support of such hypotheses, but to the fact that
nearly all observed chemical phenomena do not only
harmonise with such views, but find in them complete and
satisfactory explanation.
Admit the law of Avogadro, and we see at once why
gases are equally expanded by heat, why they are equally
contracted by cold and pressure, and why they combine
together, according to the discovery of Gay-Lussac, in
simple proportions by volume.
In a later chapter will be shown some of the consequences
which follow upon an application of this law, and the important progress of chemical theory which has resulted
from its adoption.
* Under the same circumstances of temperature and pressure.




39
CHAPTER VIT.
SPECTRA. EMISSION SPECTRA.
All bodies when heated to a sufficiently high temperature emit light; those which are densest being, as a rule,
the most intensely luminous.
When this light is examined by a prism the image formed
by the refracted and dispersed rays appears in the form of
a coloured band, which is called the spectrum.  The apparatus employed for the purpose of observing the spectra of
different kinds of light is called a spectroscope.  The details
of its construction are described in nearly all works on
physics.  Suffice it, therefore, to say that the light under
examination is allowed to pass first through a fine slit in a
metallic plate so arranged that the slit is parallel to the
edges of the prism. The rays are then rendered parallel,
by means of a pair of lenses placed in a tube which is fixed
at the angle of minimum deviation with the first face of the
prism. After passing through the prism the light is viewed
through a telescope, which gives a magnified image of the
spectrum.
The spectrum of the light emitted by solids, liquids, and
very dense gases, is found to be a continuous one, that is
to say, the simple colours, red, orange, yellow, green, blue,
indigo and violet, which are its components, merge one
into the other gradually, and are not separated by dark
intervals.   The spectra furnished by ordinary gases or
vapours, when ignited, consist, on the contrary, of bright
lines, which are so many images of the slit of the spectroscope refracted in different degrees so as to be separated
from  one another.  In the spaces between these bright
bands there is in general no light. In some cases, however,
a continuous spectrum is more or less distinctly visible.
These differences are shown as far as possible without the
aid of colour in the following diag a ms. The first represents




40               Chemica/l Philosoaphy.
the appearance which is presented by the spectrum of an
ignited solid, such as lime, or of a flame such as that of a
candle, which is supposed to contain either solid matters
or very dense vapours. The second is the spectrum of heated
hydrogen gas.
CONTINUOUS SPECTRUM.
BRIGHT LINE SPECTRUM.
Salts of the alkalis, of the alkaline earths, of copper and
many other metals, have long been known to give light
having peculiar and characteristic colours when heated in
the blow-pipe flame or in the non-luminous flame of the
Bunsen burner.   Thus, sodium  salts give yellow light,
potassium violet, barium green, calcium orange-red, and
lithium and strontium crimson flames.
When flames coloured by the vapours of these salts are
viewed with the spectroscope, they are found to exhibit a
character similar to that of the gas hydrogen. That is to
say, these spectra are made up of bright lines which are
separated from each other by dark intervals. The appearance shown in each case is indicated by the diagrams
on next page.
When the same prism is employed these lines always
occupy the same relative positions, and the lines produced
by any one substance are not changed in position, or
breadth, or intensity in the presence of another substance
which, when ignited, gives out a different kind of light.
Thus, if a flame coloured by sodium or one of its salts is
viewed through the spectroscope, the bright yellow double
band characteristic of sodium is alone visible.




Spctra of Coloured Flames.           41
Another flame coloured by some compound of potassium
gives only the red and deep blue lines peculiar to the
ignited vapour of that metal. A third flame, into which is
RED.     YELLOW.            BLUE.         VIOLET.
0
introduced a mixture of potassium and sodium salts, appears
U) ~       C   y
z ~ ~ ~  




42               Chemical Philosophy.
yellow to the unaided eye, if the proportion of sodium
present is more than infinitesimal. But after passing through
the prism of the spectroscope this yellow light is resolved
into, red and indigo potassium bands and the yellow sodium
lines occupying exactly the same position as when observed
separately.  These facts constitute the basis of the method
of spectral analysis.
It is only necessary to bring before the slit of thespectroscope the incandescent vapour of a metal or other
substance which it is desired to examine. The position
and number of the bright lines visible through the telescope
are at once an indication of the nature of the substance. The
volatilisation of solid bodies and the necessary ignition of
the vapour is effected most generally by the aid of the
Bunsen gas-flame; but when this is incompetent to produce
a temperature sufficiently high, the' oxy-hydrogen blow-pipe
or the electric arc may be employed.  The transmission of
sparks from an induction coil between terminals to which
metals can be attached is a convenient method for obtaining
the spectra of such bodies as well as of gases such as
nitrogen, hydrogen, and carbon dioxide, through which the
sparks can be passed.
The delicacy of the method of spectral observation is
very great, far surpassing that of the most exact and sensitive of chemical tests, and by its aid the presence of exceedingly minute quantities of various elements can be
detected with certainty and ease. Sodium and lithium, for
example, are bodies which, even in excessively small
quantity, are capable of giving very easily recognisable
spectra, and, accordingly, their presence has been discovered
in many substances in which it was formerly unsuspected.
Sodium salts are, indeed, almost universally diffused in water,
in the mineral constituents of the soil, in the tissues of
plants and animals, and even in the dust suspended in the
atmosphere.
Spectroscopic analysis has also led to the discovery of




Spectroscopic Analysis.            43
several elements previously unknown, and existing as unrecognised impurities in various substances. These newlydiscovered elements are all metals. Their names are given
below:
NAME.      ORIGINAL SOURCE.   DISCOVERER.   DATE.
Rubidium      Diirckheim         Bunsen     1859
Caesium      Mineral Water
Thallium   Seleniferous pyrites   Crookes   I857
Indium      Freiberg blende Reich & Richter I863
Gallium       Zinc blende       Lecoq de   I875
from Pyrenese     Boisbaudran
It will be seen by reference to the diagrams that the
spectra of the metals of the alkalis and of the alkaline
earths are comparatively simple, consisting, in most cases,
of a small number of lines, which are often widely separated
and easily recognised.  Several other metallic elements,
under the same conditions, yield equally simple spectra::that
of thallium, for example, consisting of a single green line,
whilst the spectrum of indium exhibits one line in the blue
and another in the indigo. The spectra of the heavy and
less volatile metals are, however, in general, much more
complex; the spectrum of iron, for instance, showing
upwards of four hundred and fifty lines, many of them
crowded together in the green.
When a compound, such as the chloride of a metal like
sodium, is heated in the Bunsen flame, it gives out light
which is very generally identical with the light obtained
from the incandescent vapour of the metal itself. Sometimes, however, this is not found to.be the case, and the
spectra of certain compounds is different from that of either
of the constituents, taken separately, and is, in general,
more complex.
The accompanying diagram exhibits a comparison of the
spectra obtained by the introduction of solid calcium
chloride into the non-luminous gas flame, and when the
electric spark is passed over the same compound.




44               (kChemical Philosophy.
The higher temperature in the second case causes a
decomposition of the compound, and the observed lines,
which are changed both in number and in refrangibility, are
attributed to the glowing vapour of the metal calcium itself.
ID
The position of such lines is not altered at still higher
temperatures, though not unfrequently new lines make their
appearance.
ABSORPTION SPECTRA.
When a transparent coloured medium, such as a piece
of glass or a coloured liquid, is brought into the path of a
ray of light before it enters the spectroscope, certain portions of the spectrum disappear. The bands of darkness
which are thus produced are due to the interception of
certain portions of the light by the coloured glass or solution, or vapour, as the case may be. The resulting spectrum is called an absorption spectrum. It presents characteristics which in many cases are as decided as those of
the emission spectrum, and the observation of absorption
spectra may occasionally be turned to practical account in
the recognition of the colouring matter of blood, of wines,
and many other substances, as well as of certain coloured
gases and vapours.
When the absorbing medium is the vapour of an element




A bsorption Spectra.            45
or of a compound which is volatile without decomposition,
the dark lines of absorption occupy the same position as
the bright lines in the spectrum of the light produced by
the ignition of the same vapour. Thus, if a beam of light
from a lamp is allowed to traverse a sufficiently thick stratum
of sodium vapour, two dark lines close together make their
appearance in the yellow; and if such a spectrum is viewed
side by side with the ordinary spectrum of the monochromatic
sodium light, the position of the dark absorption band is
seen to coincide with that of the bright yellow lines. Such
phenomena are explained by the law that gases and vapours
are capable of absorbing and stopping the same rays of
light which they emit at higher temperatures, when in the
state of ignition or incandescence.
This law has been experimentally verified by the examination not only of the absorption spectrum of sodium, but of
other elements which give more complex spectra.  The
facts thus established have been employed in the solution
of the very interesting problem presented by the light
which reaches us from the san and other heavenly bodies.
Sunlight examined by a spectroscope exhibits a spectrum
from which none of the primary colours are absent, but
which is traversed by a very large number of fine black
lines, many of which were discovered so long ago as I814
by a German optician, Fraunhofer. They are usually known
as Fraunhofer's lines.
In the figure, the position of a few only of the most
prominent is indicated, and they are distinguished by the
letters used originally by Fraunhofer.
A  ]3C   D.     o   F.                   H
These dark spaces are now known to be absorption
bands. It has been found that their positions coincide with




46               Chemical Philosophy.
the lines of the spectra of many terrestrial elements; and
the coincidences which have been observed are so absolute
and so numerous as to lead to the inevitable conclusion that
they are produced by the vapours of such bodies existing in
the gaseous envelope of the sun.
It is beyond the purpose of this book to give further
details with regard to this most interesting question, which
is, moreover, fully treated of in several well-known works
specially devoted to the subject.
THERMAL AND CHEMICAL SPECTRA.
The eye enables us to recognise only a part of the spectrum of a luminous object.  In addition to the rays which
give the impression of light, there are others which produce
heating effects, and others, again, the special function of
which is to promote chemical combinations and decompositions.
A sensitive thermometer exposed successively to different
portions of the visible spectrum will indicate that the heatl
ing effect is produced only towards the red end, and even
that the maximum is attained amongst rays which are less
refrangible than the extreme end of the visible red, and the
spectrum of which lies beyond in darkness. The yellow
and red luminous rays and the dark heating rays possess,
however, little or no chemical power. This accords with
the experience of photographers, who are accustomed to
manipulate their sensitive plates in rooms illuminated by
yellow light.  The chemically active rays are confined
almost entirely to the violet end of the spectrum, the exact
position of the most active being found, however, to depend
to a certain extent upon the nature of the substance submitted to their influence. Thus, silver salts are blackened,
and hydrogen and chlorine are caused to combine most
rapidly when exposed to that part of the violet end situated
between the lines G and H of the solar spectrum, though
more or less chemical activity is manifested some distance




Thzermal and Chemical Spectra.        47
beyond the visible violet on the one side and as far as the
middle of the green on the other; all perceptible action
ceasing in the yellow or most luminousposition of the visible
spectrum.
These relations are exhibited by the three curves in the
following diagram:A3 3B      C         D    E!    c 
The summit of each curve indicates the position of the
maxima of heating, luminous, and chemical effects in the
spectrum of sunlight, the vertical lines representing the
chief dark lines in the solar spectrum.
These curves are represented as overlapping one another,
but it must not be supposed from this that there are three
distinct sets of rays in the spectrum. In all probability the
rays are of the same kind from end to end, but differ in
wave-length and rapidity of vibration. The least refrangible
rays are capable of producing the effects of heat, but are
not capable of exciting the sensations of vision, whilst the
vibrations which communicate the sensation of violet to the
eye are also capable of effecting chemical change.
LUMINOSITY OF FLAME.
The cause of the luminosity of common candle and gas
flames has long been a subject of interest, and two chief
hypotheses have been framed with the object of explaining




48               Chemical Philosophy.
it. Though neither of these hypotheses alone is capable of
furnishing a complete explanation of every case, yet, taken
together, they serve to account satisfactorily for the phenomena which are usually observed.
According to the earlier hypothesis, proposed in I817 by
Sir H. Davy, the luminosity of flame is attributed to the
existence in the flame of particles of solid matter, which,
being heated to a high temperature by the burning gases,
emit light. These solid matters, supposed to exist in the
flame, are deposited in the form of soot when a cold body
is plunged into the flame. Such facts as the following were
adduced by Davy in support of this hypothesis.
The flame of hydrogen or of alcohol, burning in the
usual way, emits only a very feeble light; but the introduction of solid matter, such as powdered charcoal, oxide
of zinc, or dust of any kind, whether combustible or not,
serves to render such a flame luminous.
A bright flame is also produced by the combustion, in air
or oxygen, of substances which, like metallic zinc, form
solid products of combustion. On the other hand, sulphur,
hydrogen, and carbonic oxide, which, in burning, yield
entirely gaseous products, give, out light very sparingly.
The spectroscope has also shown us that ordinary
luminous flames yield a continuous spectrum of the same
character as that usually produced by incandescent solids.
The production of solid particles in a candle flame, or in
the flame of coal-gas, or other hydrocarbon, is supposed to
be due to the selective power of the oxygen of the air,
in virtue of which it unites preferably with the hydrogen,
leaving a part of the carbon in a solid form to be consumed
as it reaches the higher parts of the flame.
A candle flame burning steadily exhibits the form of a
long cone with the apex pointing upwards.  Other flames
taper upwards in the same way. This form is, of course,
due to the strong upward current produced in the air which
immediately surrounds the flame, in consequence of the




Luminzosity of.Fame.               49
heat. A close examination of the flame of a candle will
show that it consists of several parts.  At the base is a
faintly luminous stratum of a blue colour.  Higher up the
luminosity increases, the most intense light proceeding from
those parts which are near the middle; whilst the whole
structure is surrounded by a transparent and almost invisible,
but very hot, envelope of ignited gas.  It is easy to show
by many simple experiments that such a flame is only a
shell of ignited gas, the process of combustion occurring
only on the outside, the interior being filled with comparatively cool gas or vapour. Thus, if a sheet of paper is held
for a moment horizontally across the flame it will receive a
deposit of soot in the form of a ring.  A slip of wood or a
wire passed through the flame becomes ignited only at the
two points where it cuts the outer portions of the flame, the
part in contact with the interior remaining dark.
The luminosity of such a flame is diminished if a sufficient
quantity of air or oxygen is thrown into it, and at the same
time it loses the power of depositing soot upon any cold
object held in it.  The flames of the blowpipe and the
Bunsen burner are produced in this way, and present this
character.
Some experiments made a few years ago by Dr. Frankland,
indicate that the ignition of solid particles of sooty matters
in hydrocarbon flames may occasionally be the cause of the
light emitted by such flames, but that in some cases at least
the effect is not wholly attributable to this circumstance. Dr.
Frankland has observed that when hydrogen is burnt in
oxygen under great pressure the light of the flame, usually
so pale, is increased to such an extent as to be capable of
illuminating a page of print so that it can be read at some
distance.
The spectrum  of hydrogen  burning  under pressure
exhibits the three bands characteristic of hydrogen, but
much broader and more or less nebulous at the edges, so
that an approach to a continuous spectrum similar to that
E




50                Chemical Philosophy.
obtained from solid bodies is the result. A similar effect is
produced by burning carbonic oxide. So that the argumer,
founded upon the nature of the spectrum of an ordinary
luminous flame has less weight than might otherwise be
supposed.
Moreover, in burning a series of substances, all of which
yield volatile.products of combustion, it is found that many
of them are capable of emitting a very vivid light, quite
equal in brilliancy to the light produced by the ignition
of many substances which are solid and not capable of
vaporisation. Thus, the combustion of arsenic in oxygen is
attended by the emission of a very brilliant white light,
although all the substances present-the arsenic, the oxygen,
and the arsenious oxide which is formed-are, at the
temperature of ignition, entirely in a state of vapour.
In such cases, therefore, the light cannot be attributed to
glowing solid  matter.  It has been  observed  that the
brilliancy of the light emitted by the combustion of such
substances is nearly proportional to the density of the ignited
vapours existing in the flame, provided that in each case the
temperature is sufficiently high.
In all probability, then, the luminosity of burning gas or
tallow is due to the ignition of vaporous, but very dense,
hydrocarbons, and is not to be ascribed to the presence of
solid particles of carbon. The fact that ordinary flames are
transparent is also difficult to reconcile with the latter
hypothesis.
EXERCISES ON SECTION I,
I. Water is shaken up with a large volume of oxygen gas under a
constant pressure of 765 mm. What volume of the gas will be contained in Io c. c. of the solution?
2. Water is exposed to an atmosphere consisting of 2I vols. of oxygen,
with 79 volumes of nitrogen.
Temp. o~; pressure, 760 mm.
Coeffs. of sol., N -'02, O ='04.




Exercises on Section I.                    5 1
Calculate (a) the total volume of gas dissolved by 52'5 c. c. of water,
and (b) the percentage composition of the gas.
3. Soda-water is charged under a pressure of 2'3 atmospheres. Calculate the volume of carbonic anhydride contained in 300 cubic-centimetres of such water.
An atmosphere = 760 mm. barom.
Coeff. of sol. for carbonic anhydride I'7967.
4. Sulphur dioxide is passed into water as long as it is absorbed. If
the barometer stands at 745 mm., calculate the volume of gas contained
in half a litre of the solution.
Coeff. for sulphur dioxide, 79'789.
5. Water is shaken up with its own bulk of a mixture ot I volume of
oxygen with 3 vols. of nitrogen. Supposing the temp. and pressure to
remain normal and constant throughout the experiment, calculate the
composition of the residual air.
Coeff. of oxygen,'0411I4
Coeff. of nitrogen,'02035
Diflsion of Gases.
6. The specific gravity of chlorine is 35'5 (H = I).  Compare its
velocity of diffusion with that of hydrogen.
7. Specific gravity of ozone, 24; of carbonic anhydride, 22. Compare their velocities of diffusion with each other and with that of HI
(sp. gr. I).
8. The rate of diffusion of a gas is observed to be'8 when that of
air is I. Find its density.
9. Oxygen and hydrogen are separated by a porous plate, and 3'83
cubic-centimetres of hydrogen pass through the plate in a second, what
volume of oxygen passes during the same time in the opposite direction?
io. In the last question, suppose the original volume of the oxygen
to have been 20 c. c., what will be the composition of the mixture formed
in its place after three seconds, assuming the apparatus so arranged that
no change of pressure occurs?
Corrections of Gas-volumes for Chanzges of Pressure and Tenmperature.
I.  00oo c. c. of air when bar. = 7.50 mm. Find the volume when
bar. = 790 mm.
I2. 250 c. c. of air when bar. = 765 mm. Find the volume when
bar. = 745 mm.
x3. What pressure in atmospheres would be required to make the
density of hydrogen (sp. gr. o'0693) equal to that of air?
E 2




52                  Chemsical Philosophy.
I4. Calculate the atmospheric pressure per square centimetre when
the barometer stands at 760 mm.
Weight of I c. c. of mercury I3-596 grams.
15. Find the atmospheric pressure per square decimetre when the
barometer stands at 750 min.
I6. What change of atmospheric pressure will be denoted by a change
of I2 mm. in the barometric column.
I 7. The weight of one litre of hydrogen at o~ and 760 mm. is'0896
gram or I crith. Calculate the weight of I litre of hydrogen measured
off under a pressure of I400 mm.
I8. Find the weight of I litre of nitrogen (sp. gr. 14); of lo
litres of carbonic anhydride (sp. gr. 22); of 250 c. C. of oxygen (sp.
gr. i6).
i9. A mass of air at o~ measures Ioo0 c. c. What volume will it occupy
at 20; at I5~'5; at I000?
20. A certain quantity of air is measured at 75~.  What volume will
it have at o~?
21. I000 c. c. of a gas at I2~'5.  What volume at 75~?
22. 500 c. c. of a gas at Io0. What volume at 400~?
23. 300 c. c. of a gas at 25~. What will its volume be at - Io~?
24. 75 c. c. of nitrogen measured at 50o~. What volume at - 350?
25. 1500 c. C. of hydrogen measured at 20~.  At what temperature
will it measure Iooo c. c.?
26. Five degrees centigrade correspond with nine degrees on the
Fahrenheit scale.  Find the co-efficient of expansion of gases for
0~ F.
27. I50 c. c. of nitrogen are measured at Io~, and under a pressure of
500mm. of mercury.  What will the volume become at I 6~4 when the
pressure is 540 mm.?
28. A quantity of nitrogen confined in a tube standing over mercury
in a mercurial trough measures 75'5 c. c.; temp. 15~; bar. 742 mm.;
surface of mercury inside the tube above surface of mercury in the
trough 122 mm. Find the volume which the gas would occupy at
normal temperature and pressure.
29. I000 cubic feet of gas are put into a balloon of 1250 cubic feet
capacity; temp. I8~; bar. 765 mm.  After ascending a certain height
it is found to be fully distended. What is the atmospheric pressure, temperature being 80?
30. A certain balloon is just capable of holding IO grams of hydrogen
under standard conditions: what is its capacity?  How much larger
must it be made if it is required to sustain a diminished atmospheric
pressure equal to 650 mm. bar.?




53
SECTION II.
CHAPTER VIII.
ELEMENTS AND COMPOUNDS.
WHEN water is exposed to a very high temperature or
made the vehicle of an electric current, it disappears and is
replaced by an equal weight of a mixture of two gases,
hydrogen and oxygen. These two gases will, under certain
conditions, again give rise to water and to exactly the same
amount of water as at first.
Water then is said to be composed  of oxygen  and
hydrogen. It is worth noting, however, that strictly speaking this can only mean that in proportion as the water is
destroyed or ceases to exist, the gases make their appearance,
and vice versa, for in water we have no resemblance to hydrogen or oxygen, neither can we detect either of those bodies
in water except by this process of so-called decomnposition.'
Now, if the hydrogen thus obtained from  water is submitted to a repetition of the same kind of treatment or to any
other that may suggest itself, it refuses utterly to yield up
anything that is not hydrogen. In other words, it cannot
* " Cavendish and Watt both discovered the composition of water.
Cavendish established the facts; Watt, the idea.  Cavendish says,'From inflammable air and dephlogisticated air water is produced.'
Watt says,' Water consists or is composed of inflammable air and
dephlogisticated air.' Between these forms of expression there is a
wide distinction."-Liebig's Letlers on Chemistry, p. 58.




54              Chemical PhilosopIhy.
be decomposed.  We find then that certain bodies,* such
as water, may be resolved into two or more different kinds
of matter, and these are called compounds; whilst others like
hydrogen cannot be split up in this manner by any means
with which we are at present acquainted, and are regarded
as elemrens.
Instances, however, have not been wanting of bodies for
a long time regarded as chemical elements ultimately
showing their true character as compounds under the
influence of some neWv agent or some improved mode of
operating. For example, the alkalies potash and soda were
regarded as elementary till Sir H. Davy showed them to be
compounds of the metals potassium and sodium with oxygen
and hydrogen, and the body represented as metallic vanadium
by Berzelius turned out to be an oxide of vanadium when
examined long afterwards by Roscoe. This term element is,
therefore, used in no absolute sense, but is merely intended
to imply that in the present state -of knowledge the bodies
Lnus designated mus oDe regarded as simple substances.
In the case of oxygen it is found that, by the action of
electricity and otherwise, it may be converted into another
gas, ozone, possessing remarkable characters quite distinct
from those of oxygen. Nevertheless, oxygen ranks as an
element because it yields in this way only one new body at
a time, which by mere application of heat recovers its original
properties, and that without loss or gain in weight. It must,
therefore, be assumed that the altered properties exhibited
under these circumstances are owing to a temporary rearrangement of its constituent particles. When, as in this
case, elementary matter, stuff, or substance is capable of
making its appearance in the form of two or more bodies
having different properties, these are said to be alltrolric
modifications of the element, and the phenomenon is spoken
of as allotropy. (See Isomerism, Chap. XX.)
* It must be understood that bodies of definite characters, and not
mere mixtures, are here referred to. (See Chap. XI.)




Distrizbution of the Chzief Elemients.    55
About sixty-three elements are known at the present
time, but it is not improbable that a few new substances
may be hereafter added to this number. It is not, however, very likely that any hitherto unknown elements will
be found to occur in any considerable quantity among the
constituents of the earth's crust, for every substance within
reach of man has already been subjected to a very close
scrutiny by chemists.
It will be seen by reference to the following tabie that
the materials which compose the solid earth, so far as we
know it-the ocean, the atmosphere, and the bodies of the
living beings which inhabit it-are made up of a few of these
elements, the rest occurring in much smaller quantity, in
some cases discoverable only by specially delicate methods:
Water consists of                 i Hydrogen.. Oxygen.
Air consists chiefly of..     f Oxygen.
t Nitrogen.
/Silica..     (Silicon.' Oxygen.
|  Calcium.
Limestone.     -.- i Magnesium.
Carbon.
Solid Earth                           Oxygen.
consists chiefly of                    ( Silicon.
Various   silicates | Oxygen.
forming crystallineJ Aluminium.
rocks or  beds of Iron.
clay.  Calcium.
Potassium.
( Carbon.
Plants consist chiefly of.           Hydrogen.
Oxygen.
Carbon.
Animals consist chiefly of.       Hydrogen.
Oxygen.
l Nitrogen.




56              Chemical Philosopay.
Chemists have found it convenient to adopt a system of
symbols and formulae, whereby to represent the elements and
those compounds the composition of which is known.
Thus, to each of the elements is assigned a symbol
formed generally of the initial letter of the Latin name of
the element. For example, sulphur is represented by the
symbol S; selenion, Se; silicon, Si; strontium, Sr; but
silver (argentunz) is Ag, and sodium  (natron) has the
symbol, Na.
But chemical symbols are not merely abbreviations
contrived, like shorthand characters, for the purpose of
saving trouble in writing the names. Each symbol represents one atom of the element for which it stands, and hence
expresses a definite weight and volume of the element,
which are identical with the proportions by weight and
volume in which it enters into chemical combination.
The weight of an atom of hydrogen is less than the
weight of an atom of any other element. It is, therefore,
convenient to consider the value of the symbol of hydrogen as cunity and the values of all other symbols greater
than unity. But since nothing is known as to the absolute
weight of an atom of hydrogen or of any other element, it
should'be borne in mind that the atomic weights are in
reality ratios or fractions, whose denominator is I, although
they are always written in the form of whole numbers.
In tthe following list are given the names of all the known
elements, together with their symbols and atomic weights,
and the student is recommended at once to commit to
memory those which are printed in capital letters, leaving
the rest to be learnt gradually as occasion may require:



Elemeznts.                          57
Name of Element.           Symbol.        Weight.omic
I  ALUMINIUM.  Al                          27'3
2  Antimony..  Sb (Stibium).   I22
3  Arsenic...  As...    75
4  Barium..  Ba..      I 3 7
5  Bismuth...  Bi.             2Io0
6  Boron..        B...     11 I
7  BROMINE.            Br...    80
8  Cadmium..  Cd...  112
9  Ccesium...  Cs...         I33
io  CALCIUM..  Ca..          40
I   CARBON..  C...    I2
12  Cerium..           Ce...    92
I3  CHLORINE..  Cl...    35'5
I4  Chromium..  Cr...    52
I5  Cobalt... C.          58'7
I6  COPPER.           Cu: (Cuprum).    63.5'7  Didymium..  D...    95
I8  Erbium..              I 13
I9  FLUORINE..  F...    I9
20  Glucinum.  G1                          9'3
21  Gold                  Au (Aurum)..   I97
22  HYDROGEN              H.           I
23  Indium..  In..  I3'4
24  IODINE.     I..   I27
25  Iridium...  Ir'.   I97
26  IRON.              Fe.               56
27  Lanthanum             La...    93
28  LEAD                  Pb (Plumbum). 207
29  Lithium.              Li...
30  MAGNESIUM             Mg..        24
3I  Manganese             Mn.                55
32  MERCURY              Hg (Hydrargyrum)   200
33  Molybdenum            Mo...    96
34  Nickel..  Ni.58 7
35  Niobium.  Nb...    94
36  NITROGEN..  N...    14
37  Osmium...  Os.             1.   I99
38  OXYGEN..  0...    I6
39  Palladium.       Pd.                   6..    o65
40  PHOSPHORUS.  P...    31
41  Platinum..  Pt... I     97'5
42  POTASSIUMI            K (Kalium).           39-I
43  Rhodium..  Ro..  I.4'5
44  Rubidium. Rb...         85'4
45  Ruthenium.  Ru..  I04 5
46  Selenion.    Se.                79 5




58               C/zeemical Philzosop/zy.
Name of Element.       Symbol.     Wtigc t
Weight.
47 SILVER.. Ag (Argentum).   o8
48 SILICON            Si                 28
49 SODIUM. I Na (Natrium)      23
50 Strontium..  Sr...       875
51 SULPHUR.. S..   32
52 Tantalum..    Ta...   82
53 Tellurium.. Te...I28
54 Thallium.  I T1.  203'5
55 Thorium.. Th.             Ii6
56 Tin... Sn (Stannum)        I 18
57 Titanium.      Ti                 50
58 Tungsten.      W (Wolfram).  184
59 Uranium.     U.. U   240
6o Vanadium. V.     ~  5I
6I Yttrium.            Y.     6I'7
62 ZINC.    I Zn..         65
63 Zirconium.. Zr...        89 6
In order to indicate combination between two elements
their symbols are placed side by side, thus, HC1. When a
molecule of a compound contains more than one atom of
each or either of its constituents, the number of atoms is
indicated by a small figure placed below the line.
Thus the formula for water OH2 represents one atom of
oxygen united with two atoms of hydrogen, and H3PO4
means that three atoms of hydrogen, one atom of phosphorus, and four atoms of oxygen are bound together in
phosphoric acid.
The formula, taken as a whole, is invariably assumed to
represent a molecule of the compound, and the relative weight
of this molecule is easily found by adding together the weights
represented by the several symbols of which it is made up.
This weight is called the molecular weight. In order to
express two or more molecules a figure is placed at the beginning of the formula, and must be understood to multiply
every symbol that follows it. For example, 20H2 represents
two molecules of water, each consisting of two atoms of
bydrogen with one atom of oxygen; in all, four atoms




Laws of Ch/emical Combination.         59
of hydrogen and two atoms of oxygen.  Occasionally
brackets have to be introduced when some group of symbols occurs more than once.  Aluminium  sulphate, for
example, has the formula Al, (SO4)3, which is thus written
for the sake of assimilating the appearance of the formula to
that of other sulphates, such as H2 SO4, BaSO4, FeSO4 &c.
Resemblance between them would be less apparent if it
were expressed as Al, S3 0,,.
Care and a little practice are all that is necessary to avoid
confusion in the use of formulae, and the student is therefore
recommended to work out conscientiously all the examples
given at the end of the section.
CHAPTER IX.
LAWS OF CHEMICAL COMBINATION.
The extraction of the metals from their ores, the manufacture of alkalies, soap, glass, dyes, and a variety of' other
useful applications of practical chemistry, were known to
man in a more or less practical form from very early times.
The alchemists extended the art of chemistry by the discovery of the processes for producing many acids and salts,
and by the invention of much useful apparatus. The discovery of oxygen by Priestley, of chlorine by Scheele, the
proof of the composition of water supplied by Cavendish,
and the overthrow of the phlogistic theory by Lavoisier's
explanation of combustion, were all great strides in advance
which followed one another in rapid succession.  But the
foundation of chemistry as an exact science was only laid
towards the end of the last Century, when the balance
began to be used systematically in all chemical investigations. Exact determinations of the relative weights of bodies
engaged in various chemical reactions were necessary for
the establishment of those laws upon which chemical ideas




6o              Chemical PhilosopAy.
of the present day are founded. If we -start by admitting
the molecular constitution of matter, it becomes unnecessary to make any formal statement of the laws which have
been found to regulate the distribution of weight or quantity of matter involved in chemical actions. If we assume,
for example, that the element oxygen is made up of molecules, all having the same properties and each composed of
two atoms whose weight is sixteen times that of the hydrogen atom, it is obvious that only a definite number of those
atoms can take part in forming a chemical compound. And
if, as we believe, these atoms are indestructible by the chemical force, no fractional part of an atom can enter into
such process; and if the number of atoms thus employed
be represented as n, the weight of the substance will be
I6n.
But since the molecular theory was not always employed
by chemists, and even at the present day is not adopted
universally, it is desirable still to indicate the facts which
have been observed in connection with this question. The
three general statements or laws may be expressed as
follows:LAW OF DEFINITE PROPORTIONS.
The proportions in which bodies unite together chemically are definite and constant. In other words, a given
chemical compound always consists of the same elements
united in the same proportions.
In order to form water, for example, union between hydrogen and oxygen occurs exactly in the proportion of two
measures of the former to one measure of the latter. This
corresponds with two parts by weight of hydrogen to sixteen
parts by weight of oxygen, since oxygen is sixteen times
heavier than hydrogen.  The employment of any larger
quantity of either element would only result in the excess
being left uncombined.




MJilpti.le and Reciprocal Proportio zs.    6 
LAW OF MULTIPLE PROPORTIONS.
When one body unites with another in several proportions, these quantities have a simple relation to one another.
Oxygen combines with carbon, forming two oxides of
carbon. In the one, three parts by weight of carbon are
united to four parts of oxygen, whilst in the other three
parts of carbon combine with twice this quantity or eight
parts of oxygen. A great number of equally simple cases
might be cited, and, doubtless, it was the study of such
cases which in the first instance led to the enunciation of
the law. In perhaps a still greater number of instances,
however, it is by no means easy to trace its application.
Among the numerous compounds of carbon with hydrogen,
for example, are found such relations as the following,
~which represent the composition of the series of paraffins:
Carbon.     Hydrogen.
Methane...   3 parts with I part
Ethane      ~    ~   ~   4,,    I 
Propane.    ~   ~    9,,    2,,
Butane       ~   ~   ~   24,,    5,,
Pentane     ~   ~    ~    5,,    I
Hexane      ~   ~    ~  36,,    7
Heptane...   21,,    4,,
Octane..    6,,    3,,
Nonane...  27,,    5
Decane...  60,,   I 
&c.                    &c.           &c.
LAW OF RECIPROCAL PROPORTIONS.
The weights of two different elements, A and B, which
combine with a third, C, represent the proportions in which
they will themselves unite together if union between them
is possible, or they bear some simple relation to those proportions. Thus, 351 parts of chlorine and 8o parts of
bromine combine with 23 parts of sodium. Then, according to the law, when chlorine combines with bromine,




62              Chemical Philosophy.
351 parts of the former are required for every 80 parts of
the latter. This may be rendered graphically somewhat in
this manner:Sodiunl.
m 23,
/
n So                 ---                 p 35'5
Bromine,             with               Chlorine.
In other words, the combining weight of a body may always
be represented as a multiple of the same number whatever
state of combination it enters into, or m n is the combining weight, where m is a number peculiar to the body
and n is some integer,
LAW OF VOLUMES.
When gases combine together they do so in equal
volumes, or in volumes which have some simple relation one
to another, as i to 2, I to 3, 2 to 3, and so on.  For
example, hydrochloric acid is formed by the union of
I volume of hydrogen with
i volume of chlorine;
water -s formed by the union of
2 volumes of hydrogen, with
i volume of oxygen;
ammonia by the combination of
3 volumes of hydrogen with
i volume of nitrogen;
nitrous anhydride by the combination of
2 volumes of nitrogen with
3 volumes of oxygen, &c.




Gay-Lussac's Law of Volumes.           63
These facts are recorded in the formulae
HC1       H2O       H3N        N203
by which these compounds are commonly represented.
It is important also to remember that whatever the
volume of the vaporous elements before combination, the
bulk of the resulting compound, measured in the gaseous
state under the same conditions of temperature and pressure, is always two volumes.
This will be again referred to in connection with the
methods employed for the determination of molecular
weights, and until those subjects have been discussed the
student must be content to take upon trust all statements
concerning molecular volumes, weights, and formulae.
This law was first enunciated by Gay-Lussac very soon
after the laws of reciprocal and multiple proportions by
weight had been established by Dalton. We now see the
connection between them and the dependence of the one
upon the other, though this relation, from want of accurate
knowledge, was not at first admitted by Dalton and other
chemists. Originally a fact, depending upon experimental
evidence only, this statement of Gay-Lussac's now stands in
the more important position of a logical deduction from
another law, the law of Avogadro (Chap. VI. p. 37).
The results of analysis before Dalton's time were represented by numbers which expressed the proportion of each
constituent in TOO parts of the compound. For example,
in Ioo parts of red oxide of copper there areCopper....   88.8
Oxygen....   I2
100'0
In black oxide of copper:Copper....   7987
Oxygen..            20o'I 3
10000




64               Chemical Philosophy.
In carbonic oxide: —
Carbon....    42'85
Oxygen....    57'14
I00'00
In carbonic anhydride:Carbon...          27'27
Oxygen.                    72'72
I00'00
Dalton discovered reciprocal and multiple proportions
by stripping from them the disguise in which this mode of
representing composition enveloped them. For by taking
the proportion of some one of the elements in a series of
similar compounds as unity, and ascertaining by calculation the proportions which the others bear to it, it is easy to
show that these proportions are simple multiples one of
another.
In the case of the two oxides of carbon, the composition
of which has just been stated, if we calculate the ratio of the
oxygen to the carbon in both of them, we find the proportion ill the second double that in the first. Thus, taking
the carbon as unityCarbon.      Oxygen.       Carbon.     Oxygen.
42'85         57'I4:      I   I'33
and 27'27         72'72::    I           2 66
Then plainly
I'33          2'66::    I:   2
In other words, the proportion of oxygen by weight in
carbonic anhydride gas is double that contained in carbonic
oxide.  Obviously the same fact may be expressed in
another way by taking the oxygen as unity instead of the
carbon:Oxygen.       Carbon.      Oxygen.     Carbon.
57'I4         42'85::    I          750
72 72         27'27:      I'375




Daltont's Atomic Theory.            65
Then-'750'375           2         I
Or, we may say that carbonic anhydride contains half as
much carbon as carbonic oxide. Similarly it will be found
that the ratios of the oxygen in the several oxides of
nitrogen, of sulphur, and of many of the metals are as I, 2,
3, &c., to I, 2, 3, &c., of the other constituent.
Looking about for an explanation of these observations of
his, it occurred to Dalton to resuscitate the ancient theory
of the limited divisibility of matter.  According to this
view, matter is made up of definite small parts or particles
called " atoms," because it was assumed that they could not
be in any way cut or divided. This assumption of finite
particles as an explanation of the phenomena of chemical
combination thus became the basis of the modern theory of
molecules. But Dalton's term "atom" was applied indiscriminately to the ultimate particles of compound as well as
elementary bodies. The relative weights also assigned to
these atoms were inferred from the results of the analysis
of only a few compounds. Consequently the word " atom "
in the' Daltonian system may be interpreted sometimes
"'molecule," sometimes "atom,"  and in other cases it
corresponds to the term "equivalent," as those words are
now understood by chemists.
CHAPTER X.
EQUATIONS.
CLASSIFICATIONS OF REACTIONS.
Equations.- Chemical changes involve neither the destruction nor the creation of matter, but simply a redistribution of the materials of which the acting masses are
composed,
F




66              Chemical Philosopbly.
In order, therefore, to represent symbolically the results
of any given action it is only necessary to write down the
formulae of the bodies engaged, and then to transpose their
symbols in such a manner as to build up the formulae of the
bodies which are produced. We thus arrive at equations in
which the signs + - and = are employed, so far as the
weights of matter are concerned, in the same sense as in
algebra.
The following examples will serve to show the manner
in which chemical equations are to be read, as well as the
mode of investigating the relative weight of the bodies
which are formed or decomposed. The student should
practise reading equations aloud, or writing them out at full
length, according to these instructions:Equation given,
2HgO      =     2Hg    +       02.
This means that two molecules, or 432 parts, by weight,
of mercuric oxide yield two molecules, or 400 parts, by
weight, of metallic mercury, and one molecule, or 32 parts,
by weight, of oxygen.
Young students, however, would do well to ensure
precision by filling up such a scheme as the following:2HgO         =      2 Hg   +  02
No. of molecules   2         give      2    and  I
Name.. Mercuric oxide         Mercury   Oxygen
Wt. of I mol.. 200 + I6 = 216        200       32
Whole weight.   used 432         obtained 400  32
When gases or vaporisable bodies occur in such an
equation we can express the volume of each by recollecting
that, according to the law of Avogadro, the volumes of
gaseous molecules under like conditions are equal. We may
also connect together the weight and volume of a gas by
applying a rule which will be more fully explained in the
next section, namely, that the specific gravity of a body in




Equations.                      67
the gaseous state* is the half of its molecular weight. The
weight of a molecule of carbonic anhydride, for example,
is 44: its specific gravity is, therefore, 22, and the weight
of a litre of the gas is 22 times as great as the weight
of a litre of hydrogen measured at the same temperature
and pressure. Now, the weight of a litre of hydrogen at
o~C, and under a pressure equal to 760 mm. of the barometric column is'o896 gram or I crith. Hence, we may
say that a litre of carbonic anhydride weighs 22 criths, or,
if the weight is to be expressed in grams, it will be
22 X'o896 grams.
Example.
CI2 +    2- =  2HC1
A molecule of chlorine and a molecule of hydrogen produce two molecules of hydrochloric acid.
C12 +     H2      -       2HC1
No. of Molecules    I       I                 2
Name.. Chlorine  Hydrogen        Hydrochloric acid or
hydrogen chloride.
Vt. of I molecule   7I     2                  36'5
Whole weight. used 7I      2          obtained 73
Volume...    I        I          obtained 2
or  2   or 2                or 4
If, now, we choose to attach a concrete value to the
symbols, we express it thus:
C12    +      H2    =      2HC1.
Weight 71 criths.      2 criths.      73 criths.
or
7I x'o896 grams, 2 X'o896 grams, 73 x'o896 grams.
Vol.  2 litres.      2 litres.       4 litres.
The student having thoroughly mastered the foregoing
examples, and worked some of the exercises given at the
end of the section, is now in a position to solve such pro* The specific gravity of hydrogen being taken as I.
F 2




68               Chemical Philosophy.
blems as the following, which may be taken as representative:I. How many pounds of zinc are required to make 500
pounds of the crystallised sulphate?
Formula of one molecule of crystallised zinc sulphate,
Zn SO4. 7H20.
Molecular weight 287.
The symbol of zinc, Zn = 65, occurs only once in the
formula of the sulphate. Hence,
In 2871bs. of zinc sulphate there are    651bs. of zinc.
ilb.,,,,,,65 lbs. of zinc.
287
500 lbs.,,,,,, would require   65 x 2 oolbs..287     6 287.      8ow many litr   I3 3   of Zilstandard temperature
2. How many litres of oxygen (at standard temperature
and pressure) are obtained by heating io grams of potassic
chlorate?
2KCI03 = 2 KC1 + 302.
or more simply  KC10,  =    KC1 +  I 02.
I26'6      74'6   48.
Taking the potassium chlorate in criths, we should obtain
3 litres of oxygen gas. ThenI22'6 criths
or       I22'6 x'o896 grams give 3 litres of oxygen.
I gram would give        3         litres.
I22'6 X'o896
Ans.       30       = 2'731 litres, or 273I c.c.
122'6 x 0o896
NOTE. -Taking'o896 gram as the weight of I litre of hydrogen, I gram
is the weight of II I6 litres. This number is sometimes useful. Thus




Classzfication of Reactions.                   69
CLASSIFICATION  OF REACTIONS.
Chemical action may take place in a great many different
ways, but every known chemical change to which bodies
are subject may be referred to one or other of the following
five typical modes of action:I. —Combination of entire molecules.
ExamplesHg        +         CI2      =      Hg C12.
Mercury.             Chlorine.     Mercuric Chloride.
CO    +            C12   =           COC12
Carbonic Oxide.       Chlorine.       Carbonyl Chloride.
NH3   +    HC1  =    NH4C1
Ammonia.         Hydrochloric Acid.  Ammonium Chloride.
CaO    +    OH2  =    Ca(HO)2
Calcium Oxide.        Water.          Calcium Hydrate.
PbO2    +             SO2  =             PbSO4
Lead Peroxide.    Sulphurous Anhydride    Lead Sulphate.
II. —Splitting zp of a comnpound molecule into its elements,
or into simpler molecules.
ExamplesCaCO,3                CaO    +    CO2
Calcium               Calcium           Carbonic
Carbonate.             Oxide.            Anhydride.
H2SO4          =       H           — 0  +    SO3
Sulphuric Acid.           Water.          Sulphuric Anhydride.
MnC14    =    MnCl2   +                   C12
Manganic            Manganous           Chlorine.
tetrachloride.        Chloride.
in the example worked out in the text, the calculation would be a trifle
shorter, thus:
KC103          give  03
I22'6 grams       I6 x 3 grams.
or I I I6 x 3 litres.
Then Io grams of chlorate would give
33'48 x Io
48 X IO  --  2'73 litres.
I22 *6




70              Chemical Phziosophy.
HgI2    -=       Hg        +      HgI2
Mercurous Iodide.   Mercury.        Mercuric Iodide.
C7H605     =    C6H603   +         CO2
Gallic Acid.     Pyrogallol.  Carbonic Anhydride.
Many familiar decompositions, which at first sight appear
to belong to this class, are in reality double decompositions, as an examination of one or two cases will show.
The decomposition of mercuric oxide by heat, for example,
seems to consist in a simple resolution of the body in its
elements,
HgO =  Hg +  0.
But when we write the equation molecularly,
HgO  + HgO =  Hg +  Hg +  0,
we may see that the decomposition of one molecule of the
oxide necessitates the splitting up of another. In order to
form one molecule of oxygen-and we believe one molecule
to be the smallest quantity of the element capable of independent existence-we must take two atoms of it from two
separate molecules of the oxide.
The same remarks apply to other cases, such as the decomposition of potassium chlorate by heat.
III.-Rearrangement of the atoms constittizng a molecule
so as to give rise to a newr body.
Two examples of this mode of transformation may be
given here.
(NH4)CNO converted by heat into CHN2,O
Ammonium cyanate.                   Urea.
C6H5CH3HN converted by heat into C6H4CH3H2N
Methyl-Aniline,                    Toluidine.
The further explanation of this kind of change is postponed to the chapter on " Isomerism." (Sec. III.).
IV. Single Metalhesis.-In this kind of change an atom,
or group of atoms, contained in a molecule is displaced by
another atom or group.




Dozuble and Single Decovmpositions.            7I
Examples.
Zn     +     2HC1       =       ZnC12       -      H2
Zinc.    Hydrochloric acid.  Zinc chloride.     Hydrogen.
MgBr2    +       C12   =       MgC12    +    Br2
Magnesium bromide.    Chlorine.    Magnesium chloride,   Bromine.
2C5,HONO        +    SO2  =   (C5H,10)2 S02 +  2NO
Amyl nitrite.   Sulphur dioxide.    Amyl sulphate.   Nitric oxide.
Many cases of precisely the same character are not easy
to find. The fact is, the great majority of reactions belong
to the next class of double decompositions. Even some
which seem to be single decompositions must in strictness
be so considered; thus, the decomposition of hydrochloric
acid by sodium must be represented in this mannerNa Na  +   HC1 +   HC1
or     Na5        +      2HC1        2NaCl +  H2,
not   Na          +      HC1  =       NaCi +  H.
V. DoubleklDeconrgosition, or Mfetathesis.-This is by far
the most general mode of reaction. Two or more molecules
coming together exchange some of their constituents so as
to give rise to the same number, or to a greater number of
molecules. We may, for the sake of completeness, classify
double decompositions under three divisions:I. Those in which one of the reacting bodies is an
element; e.g.,
Na2    +      20H12    =     2NaOH        +     H2
Sodium.        Water.        Sodium hydrate.  Hydrogen.
2. Those in which both are compounds; e.g.,
Na,CO3   +      CaCL2    -      2NaCl   +    CaC03
Sodium carbonate.  Calcium chloride.  Sodium chloride.  Calcium carbonate.
3. Those which result in the formation of " substitution"
compounds.   This  kind  of reaction is not essentially
different in its nature from cases i and 2, but substitution products among carbon compounds constitute




72               Chemical Philosophy.
a class of bodies so remarkable in their characters as to
deserve special notice. Some examples of their formation
are given in this place, more particular mention being reserved for a later chapter.
Examples.
CH4    +      C12   =   CH3C1        +   HC1
Methane.     Chlorine.    Chloromethane.   Hydrochloric acid.
C6H{   +   NO2HO  =   C6HNO2  +   H20
Benzene.     Nitric acid.  Nitrobenzene.  Water.
Co0H6  +   NOC1   =   C,,H,,NO   +   HC1
Turpentine.  Nitrosyl chloride.    Nitrosoterpene.  Hydrochloric acid.
CHAPTER XI.
CHEMICAL COMPOUNDS DISTINGUISHED FROM MIXTURES.
WHEN two bodies unite together to form a chemical compound, they merge so completely one into the other as to
be no longer recognisable by any physical character.  The
properties of chemical compounds are always quite distinct
from those of their constituents, and in general have not
even a remote resemblance to them. In the glistening red
crystalline powder called by chemists mercuric oxide and
vulgarly " red precipitate," no trace can be detected by the
eye, or by any other sense, of the liquid, volatile, silvery
metal mercury, and the colourless, gaseous oxygen into
which it is resolved by the action of heat. In water, again,
whether examined in the condition of solid ice, liquid
water, or vaporous steam, we should look in vain for any resemblance to the gaseous, unliquefiable elements, hydrogen
and oxygen, of which it consists. Neither can we detect in
the properties of water any that can be regarded as intermediate between those of the two constituents, or such as
we should expect to find exhibited by a mixture in which
each element retained its independence.




Mixtitures and Chzemical Compounds.       73
The law of definite proportions furnishes another criterion
by which the character of a body under examination may
be judged as to its title to rank as a definite chemical
species. In order to decide whether a given substance is a
true chemical compound or a mere mechanical mixture,
various considerations are employed by chemists, the nature
of which depends very much upon the circumstances of
each particular case. If a solid body is the subject of
investigation, it is examined under a microscope, in order to
see if its appearance is uniform throughout; or, if crystallisable, it is recrystallised, and the crystals compared with
those of the original substance. If soluble in any liquid, it
may be treated with a quantity of the solvent insufficient to
take up the whole. The part dissolved, after getting rid
of the solvent by evaporation or otherwise, ought to agree in
every respect with the undissolved portion, if the original
body is one compound and not a mixture.
In the case of those liquids which are volatile, and which
bear the application of heat without decomposition, the boiling point should remain constantly at the same temperature
during the distillation of the whole, and portions taken from
the retort and from the distillate in the receiver ought, in
the case of definite compounds, to correspond in specific
gravity and all other physical and chemical characters.
When the body to be examined is a gas, the action of
solvents is tried; and if, after such treatment, the relative
proportions of the ingredients are undisturbed, the body
may be regarded as probably consisting of one compound.
This may be confirmed by observing whether these proportions agree with the combining weights of the elements
present. The phenomena of gaseous diffusion are not unfrequently useful in helping to decide whether the elements in
a given gas are chemically combined or mechanically mixed.
Other means of a mechanical nature may be resorted to in
special cases, and occasionally considerable ingenuity is
called for in devising methods suited to the occasion.




74              Chemical Philosoplay
One or two examples will render these matters more
intelligible to the student. We will select cases in which
the law of definite proportions would afford no assistance in
the solution of the problem.
Fifty-six parts of iron would unite with thirty-two parts
by weight of sulphur; but the two elements may be mixed
together in the state of fine powder, without exerting upon
each other any chemical action whatever, the compound,
ferrous sulphide, which would be formed by their union in
these proportions being produced  only when they are
strongly heated together. The mixture of these two bodies,
though it might be indistinguishable from the compound by
appealing to the proportions of the two ingredients, would
yet be easily recognised by such properties as the following:-Under a microscope particles of iron and particles of
sulphur would be visible; a magnet would withdraw the iron
from the powder, and leave the sulphur; carbon bisulphide
would dissolve the sulphur, but would not affect the iron; a
separation could be effected by merely stirring up in water,
when the iron, by reason of its greater specific gravity, would
sink quickly to the bottom, leaving the sulphur suspended;
diluted sulphuric acid poured upon the mixture would
evolve hydrogen gas. The chemical compound has a uniform
appearance under the microscope; if reduced to powder, it
could not be divided into two different portions by the use
of a magnet, by any solvent, or by elutriation with water;
and, lastly, the action of diluted sulphuric acid would result
in the evolution of hydrogen sulphide gas, easily distinguishable from hydrogen by its odour, by its solubility in water,
and by many other properties.
The domain of organic chemistry supplies numerous
problems of the kind we are considering.
In certain kinds of tartar there exist the potassium salts
of two acids, which have the same composition, but somewhat different properties. These acids are called respectively tartaric and racemic acids. The former rotates the




Mixed L iquids.                 75
plane of polarisation of a ray of polarised light to the right,
whilst the latter is optically inactive. Crystals of tartaric
acid are permanent in the air, whilst those of racemic acid
contain a molecule of water, which, escaping at ordinary
temperatures, renders the crystals efflorescent. Racemic
acid and calcium racemate are decidedly, though not very
greatly, less soluble than the corresponding tartaric acid
and calcium tartrate.  Hence it will be perceived that
whilst it is perfectly easy to distinguish the pure acids from
each other, the mere estimation of the amount of carbon
and hydrogen, or even an examination of a great many of
the salts, would not suffice to decide between the one or
the other of them and a mixture of the two.
A mixture of alcohol, water, and ether might be made
in such proportions (46: I8: 74) that it would possess
exactly the same composition as alcohol. But such a mixture would be at once distinguished from alcohol by its
peculiar odour, and by separating into two layers on addition of water. When distilled, it would be found to boil
at a much lower temperature than the boiling-point of
alcohol; and after about three-fourths had passed over into
the receiver, the liquid left behind in the retort would no
longer smell of ether, and would be easily recognised as
weak alcohol.
Again, the commercial liquid alkaloid toluidine, is an
oil which, when distilled, boils steadily at about 200~, and
no difference of composition can be detected between the
first portions of the distilled liquid and the last. And yet
this substance is a mixture of two alkaloids of the same
composition, one liquid, the other solid, the boiling points
of which (200~ and i98~) differ so slightly that they cannot
be separated by any kind of fractional distillation. The
only process by which they can be completely separated depends upon the fact that the oxalate of solid toluidine is
much less soluble in ether than the oxalate of the liquid
base. By patient repetition of this treatment with ether two




76              Chemical Philosopa/y.
salts are obtained, which, when recrystallised any number
of times, undergo no further change in crystalline form and
solubility. In this condition they are believed to be pure
and homogeneous.
Examples of mixtures of gases, presenting the same
composition as true chemical compounds, might be easily
multiplied. Thus, equal volumes of hydrogen and chlorine
constitute a gaseous mixture which exhibits the colour and
bleaching action of chlorine; and after shaking up with
solution of soda, just one half its bulk of colourless inflammable hydrogen remains.
Hydrochloric acid gas, which contains the same elements
combined in the same proportions, is, on the contrary, a
colourless gas which no longer possesses the bleaching
power of chlorine, and is readily and completely soluble
in water or in solution of soda.
Ethane and hydrogen gases in equal volumes furnish a
mixture which would be indistinguishable from marsh-gas
by ordinary quantitative analysis. But recollecting that
ethane is more than four times more soluble in water than
hydrogen, whilst the rate of diffusion of hydrogen is nearly
four times that of ethane-gas, it would not be difficult to
distinguish the mixture from  the compound.  Further
assistance might be derived from a study of the action of
chemical agents upon the two gases.
The. case of atmospheric air is one of so great irmportance, that its consideration demands some attention in this
place.
Neglecting accidental constituents, as well as the water
vapour, and carbonic dioxide, which are always present, the
analysis of air from various localities has led to the conclusion that it consists almost uniformly of 20o9 volumes of
oxygen, with 79-I volumes of nitrogen.  The question
whether these two elements are united together chemically
has been decided in the negative, in accordance with such
considerations as the following:



A tmospheric Air a Mixture.           77
I. The most accurate analyses seem to indicate that the
proportion of oxygen to nitrogen in the atmosphere is not
absolutely uniform, as would be the case if it were a
compound.
2. The quantities of oxygen and nitrogen present do not
bear any simple relation to the combining weights of those
elements.
3. When oxygen and nitrogen are mixed together they
show no signs of chemical action by evolution of heat or
contraction of volume; and such a mixture, when due proportions are employed, resembles atmospheric air in every
respect.
4. Water dissolves the constituents of the air in unequal
proportions, so that by reason of the greater solubility of
oxygen, the air which may be expelled from common water
by boiling contains a larger proportion of that element
than is present in atmospheric air.* A chemical compound
would dissolve, as a whole, without change of composition.
5. When the rays emitted from a given source of heat are
transmitted through different gases, it is found that compounds absorb a much larger amount than elementary
gases or mixtures of elementary gases. Thus the amount
of radiant heat absorbed by nitrous oxide, a colourless and
transparent gas, is more than 350 times as great as the
amount absorbed by a column of equal length of oxygen,
nitrogen, or atmospheric air.  Between the absorbent
powers of the last three gases no difference can be detected, and the natural inference, therefore, is that they are
similarly constituted.
6. The elements may be separated to a certain extent by
the mechanical process of diffusion through a porous plate,
called, in this case, atmolysis.
It has also been discovered by Graham (see Chap. V.,
p. 3') that gases have the power of penetrating thin sheets of
india-rubber, and that the rate at which oxygen passes
* See also Chapter II., p. I5, Solubility of Gases.




78              Chemical Philosophy.
through this material is more than two and a-half times
that of nitrogen. Upon this observation he has based a
very instructive experiment, which proves conclusively the
fact that the oxygen of atmospheric air is not combined
with the nitrogen.  An airtight india-rubber bag is exhausted as completely as possible by the Sprengel air-pump.
When the exhaustion is nearly perfect, it is found that gas
can still be slowly extracted from the bag by continuing the
operation; and this gas is found by analysis to consist of
a mixture of nitrogen and oxygen, containing upwards of
forty per cent. of the latter ingredient. The gases thus
withdrawn from the bag result from the passage of the
gases of the atmosphere through the india-rubber partition;
the oxygen, however, more rapidly than the nitrogen. The
explanation of this dialytic passage of gases through the
apparently impermeable caoutchouc, appears to be that the
gases are absorbed by the external film of that material,
that they penetrate in this condition to the other side of
the sheet, where evaporation occurs in consequence of
exposure to an atmosphere of very feeble density.
CHAPTER XII.
NOMENCLATURE.
A NAME may be used either for the purpose of indicating
some particular person or object, or it may serve to point
out relationships and to define the position which a thing
holds in some system of classification.
In the early days of chemistry the number of different
bodies known was comparatively small, and mere indicative
names fulfilled ali the requirements of the time. But when
chemical research began to be regularly followed and
crowds of new compounds were constantly presenting
themselves, it became necessary to devise names which
would serve, not merely to distinguish one compound from




Cheimical Names.                 79
another, but to indicate, at least in some degree, the
relationship subsisting between allied bodies.  The first
attempts of this kind were naturally imperfect, the devices
employed being wholly inadequate to the requirements of
the case. For example, it soon became evident that the
mere employment of adjectives, as in the names blue, green,
and white vitriol, and the like, could have only a very
limited application.
It was only after oxygen had been discovered, and its
compounds were called oxides by Lavoisier, that chemical
names began to assume some appearance of precision. The
nomenclature adopted by the leading chemists of that
period has met with very general acceptance; and, although modified in detail, the system of the present day is
based essentially upon the same principle. The names now
employed by chemists for scientific purposes sometimes
assume rather formidable dimensions, but unlike the fanciful
names in use in connection with some branches of natural
history, every syllable has a significance of its own. And,
in spite of their length and frequent uncouthness, it may
fairly be claimed for these names that they are in few cases
inconvenient practically, whilst they do very fairly realise
the idea originated by Lavoisier, namely, that of representing, as by a formula, the composition of the bodies for which
they stand.
Nevertheless, many of the old names dating their origin
from the times of the alchemists have become, by long familiarity, so incorporated into the language of medicine, of
commerce, and the arts, that it is neither possible nor advisable for the chemist to reject their use. In the majority of
cases, indeed, they are very serviceable, and no chemical
student should disdain to make himself acquainted with
such terms as caustic ypotash, alum, borax, or oil of vitriol,
wood-spirit or marsh-gas, and to use them when they express all that is required, alternatively with the more formal
and systematic names, the employment of which, for certain




80                 Chemical PhilosopAy.
purposes, has been rendered necessary by the advance of
knowledge.
Lames of Elements.-Several of the metals —iron, copper,
lead-were known in very early times.  Those elements
which have been discovered by more modern chemical research have received names which in some cases recall their
origin, e.g., silicon (silex, flint); whilst others were suggested by some prominent characteristic, e.g., chlorine
(XXwpoc, green), or by their chemical relations, real or supposed, eg., oxygen ('vbc, acid; yjEvvai, I generate).
Metals which have been discovered in modern times
have all been designated by Latinised names, with the termination " ium " * —potassium, sodium, lithium, thallium, &c.;
whilst the names of the non-metallic elements are characterised by no syllable which is common to them all.
lVames of Binary  Comjounds.-When two elements are
combined together, the last syllable or two of the name of
one of them is changed into the suffix-ide. Thus, we
have CaO, calcium oxide; HC1, hydrogen chloride; PbS,
lead sulphide. In nearly all cases it is the name of the
more negative or chlorous element which is thus modified,
the name of the metal or corresponding element remaining
unaltered.
Prefixes from  the Greek-mono-, di-, tri-, &c.-serve to
indicate the number of atoms of the chlorous element
present in a molecule of the compound. ThusN0,O   is  nitrogen monoxide.
N202,,  nitrogen dioxide.
N203,,  nitrogen trioxide.
N204,,  nitrogen tetroxide.
N,O5,,  nitrogen pentoxide.
* Unfortunately this termination has been erroneously applied to the
name of the non-metal selenium from a belief entertained at the time of
its discovery that it was a metal. I have ventured in these pages to
change the final syllable into on, selenion. The alteration of the name
silicium into silicon within the last few years will, I hope, be considered
sufficient precedent.




Names of Binary Comnpounds.              8I
Not unfrequently, however, when the actual number of
atoms is doubtful, or when it is desired simply to indicate a
relation between two compounds, that which contains the
larger proportion of the chlorine or oxygen or similar
substance is distinguished by the suffix ic, whilst the other
ends in ous.  Thus, in the foregoing series, we may distinguish:
N20 as nitrous oxide.
N0,O or NO as nitric oxide.
SimilarlyN203 is nitrous anhydride.
N205 is nitric anhydride.
A few other marks serve in special cases, thus, Fe2O3,
Cr2Oa, Mn2O3, A1203, are often called sesquioxides.* When
there is a series of oxides, chlorides, or sulphides of
the same elements, the syllables protot and per+ are sometimes prefixed to the name to indicate respectively the
poorest and the richest in oxygen, chlorine, or sulphur, as
the case may be. Thus, the two oxides of iron, which are
usually represented by the formulae FeO and Fe2O3, may
be distinguished by one or other of the following pairs of
names, according as it is desired simply to indicate that the
ratio of oxygen to iron is greater in the one compound than
in the other; or to imply, what is a matter of far less certainty, that so many atoms of the two elements form one
molecule of the compound.
FeO, Ferrous oxide:  Iron protoxide; Iron monoxide;
or             or
Protoxide of iron. Monoxide of iron.
Fe203, Ferric oxide:  Iron peroxide;  Iron sesquioxide;
or              or
Peroxide of iron. Sesquioxide of iron.
* Latin, sesqui, one and a half.
+ Greek, rpiroS, the first.
+ Greek,'re0p above, over, exceeding.
G




82              Chemical Philosophy.
Names of Acids and Sals. —The same principles guide
the construction of the names of these compounds, and a
single example will go far towards explaining their application. It happens, in the case of chlorine, that an unbroken
succession of compounds is formed by the union of this
element with hydrogen and oxygen. These are their names
and formule:
HC1... Hydrochloric acid.
HC10... Hypochlorous acid.
HC102... Chlorous acid.
HC103... Chloric acid.
HC104... Perchloric acid.
Here, again, the terminations ous and ic serve to indicate
different grades of oxidation, whilst the prefixes hypo and
per respectively announce a smaller and a larger amount of
oxygen than is contained in the chlorous and chloric acids.
In the name hydrochloric acid, it is evident that the termination has nothing to do with the presence or absence of
oxygen, and this use of the termination forms an exception
to the general rule. If we consent to regard acids as salts
of hydrogen, we may write names for this series of compounds which are constructed in all respects in the same
manner as those which are applied to salts in general.
Instead of                  We may write
Hydrochloric acid.. Hydrogen chloride.
Hypochlorous acid.. Hydrogen hypochlorite.
Chlorous acid... Hydrogen chlorite.
Chloric acid... Hydrogen chlorate.
Perchloric acid... Hydrogen perchlorate.
And here another rule must be attended to, namely,
that when the name of a salt ends in ire, the name of the
acid or hydrogen salt with which it corresponds terminates
in ous, whilst salts in ate are derived from  acids whose
names end in zc.




Names of Carbon Compozunds.             83
One more example will serve to emphasise this rule:
H2SO3       Hydrogen sulphite or sulphurous acid.
NaHSO3    Sodium-hydrogen sulphite,
or acid sulphite of sodium.
Na2SO3      Sodium sulphite,
or disodium sulphite (neutral).
H2SO4       Hydrogen sulphate or sulphuric acid.
NaHSO4    Sodium hydrogen sulphate,
or acid sulphate of sodium.
Na2SO4      Sodium sulphate,
or disodium sulphate (neutral)
NAMES OF CARBON COMPOUNDS.
Dr. Hofmann, a few years ago, made a proposition with
the object of reducing to some degree of order the confused nomenclature of the very numerous compounds of
carbon and hydrogen. The number of atoms of carbon is
indicated by incorporating the Latin numeral into the name
and introducing a vowel into the last syllable, in order to
show the proportion of hydrogen.  In the following table
the words methane, ethane, propane, are based upon the
names of the radicles methyl, ethyl, propyl, which have long
been in use and are too familiar to be discarded:CH4    CH2
Methane.  Methene.
C2H6    C.2E14    C2H2
Ethane,   Ethene.   Ethine.
C3Hs    C3H6    C3H4    C3H2
Propane.  Propene.  Propine.  Propone.
C4H10   C4H8    C4H6   C4H4    C4H2
Tetrane.  Tetrene.  Tetrine.  Tetrone.  Tetrune.
C5H12    C5H10    C5H8    C5H6                C C5H2
Pentane.  Pentene.  Pentine.  Pentone.  Pentune.
The succeeding  terms would run: sextane, septane,
octane, nonane, decane, &c.
G 2




84:Chlemical Philosophy.
The radicles formed from these bodies by loss of
hydrogen are furnished with the termination-yl; thus
CH3 derived from methane by removing H is called
methyl, CH is methenyl, C2H5 ethyl, and so on.
Unfortunately, the names which have long been applied
to a great many carbon compounds are in the majority of
cases fanciful, and, even in allied compounds, have no
relation or resemblance to one another. They generally
bear some reference either to the original source of the
body, or to some more or less prominent characteristic.
Thus, formic acid is so called because it was originally
obtained from the bodies of ants (formica, an ant), acetic
acid because it was procured from vinegar (acetumo).  In
like manner succinic acid is the acid obtained by distillation of amber (succinum), and lactic acid from sour milk
(lac).
Attempts have been made to introduce some degree of
system into this crowd of heterogeneous materials by restricting the use of certain terminations. Thus, the hydrocarbons
of the aromatic series have names which all end in ene, thus,
benzene, toluene, anthracene.  Alcohols and bodies resembling them claim the terminal syllable ol; e.g., carbinol,
phenol, thymol.  Basic nitrogenous bodies are represented
by names ending in ine, thus, ethylamine, quinine, strychnine.
These conventions do serve their purpose to some extent,
though the whole matter is in a condition far from satisfactory.




Conditions of Chemical Change.         85
CHAPTER XIII.
CONDITIONS OF CHEMICAL CHANGE.-THEORIES REGARDING
THE NATURE OF CHEMICAL ATTRACTION.,
WE now proceed to discuss the general conditions under
which chemical changes are brought about, together with
some of the circumstances which have been found to
modify the exercise of chemical attraction.
I. Bodies act upon each other chemically only when,
according to the usual phrase, they are in absolute contact,
that is to say, when they are so near to each other that the
distance between them is immeasurably small.
In this respect chemical attraction differs from the forces
of gravitation, electrical and magnetic induction, all of
which are capable of operating through distances which are
appreciable by our senses, and measurable by our instruments. On the other hand, it agrees thus far with the force
to which we give the name cohesion, in virtue of which the
molecules of bodies are held together in masses, and with
adh/esion, which causes surfaces of various kinds after being
closely approximated to remain united.
2. In order that bodies may act upon each other chemically, one of them at least must be in the liquid or in the
gaseous state.
That is to say, chemical action cannot proceed to any
appreciable extent unless the several parts of the acting
masses are free to move, so that the mutual interpenetration
which chemical combination involves may not be interfered
with.
3. Moderate elevation of temperature generally favours
chemical combination, and is very frequently indispensable.
In many cases it is obvious that heat assists merely by
melting or vaporising one of the substances concerned;
and when it gives no assistance, this may always be explained by the fact that the compound which might be
formed is incapable of existing in the liquid or gaseous




86              Chemical Philosophy.
state, as the case may be.  Thus, iron and sulphur, or
carbon and sulphur, have no action on each other, unless
they are heated together very strongly. On the other hand,
many salts unite with water of crystallisation only at very
low temperatures; and nitric oxide, which is said to be
capable of forming a compound with bromine, can only do
so when the liquid is kept very cold.
But in a great many cases, as in the combination of
hydrogen and chlorine, or the explosion of gunpowder,
application of heat to one portion of the mass is sufficient
to cause the chemical disturbance in that part to be communicated to the whole. This is in reality a gradual operation, though sometimes proceeding very rapidly, and is due
to the transference of the heat which is disengaged by the
reaction of the first particles to the surrounding particles,
and so from one part to another throughout the mass.
Many bodies which combine together with the aid of
heat are separated again when the temperature is carried
too high. Mercury and oxygen afford an example of this.
These two elements combine together slowly when at a
temperature near the boiling-point of mercury; but the
oxide which is formed is decomposed again below a red
heat. In general, we may say that chemical attraction and
heat are opposed in their action and effects. Whilst
chemical attraction causes atoms to accumulate in molecules, heat tends to separate and scatter these atoms into
simpler groups.  And although, as in the production of
polymeric compounds, heat seems sometimes to be capable
of operating in a manner the reverse of that which has
been stated, it must be remembered that in all such cases
decomposition takes place at higher temperatures.
4. Heat is almost always generated when chemical combination takes place; and in every case, whether heat be
absorbed or disengaged, the thermal change is as definite
as are the weights of the materials engaged in the transaction.




Heat generated in Chemical Reactions.      87
Whenever much heat is evolved, it is tolerably safe to
conclude that the resulting compound is a stable one. The
explanation of this appears to be, that in order to separate
two bodies which are chemically combined, they must each
receive as much heat as they lost when they entered into
combination.
In some cases heat appears to be absorbed; it is so, for
example, when iodine combines with hydrogen, and when
sulphur combines with carbon. But in such cases, after all
due allowances have been made for the heat which
becomes latent in consequence of the change from solid
to liquid, or gas, the actual absorption of heat which is
observed must be attributed to the fact that heat is used up
in the decomposition of the elementary molecules which
precedes, or is simultaneous with, the formation of the
molecules of the compound. Thus, when hydrogen combines with iodine, the change is not simply
H + I ='HI
but is, strictly speaking, a double decomposition; thusH  } +I ) t  IH
Now, if the heat required to effect the separation of H
from H and I from I is greater than the amount of heat
generated when 2H  combines with 21, then the result will
be negative, no heat will be given out, but a certain amount
must be supplied from external sources.
If we compare together the thermal effects of causing
one gram of hydrogen to unite with equivalent quantities
of chlorine, bromine, and iodine, we get the following
numbers:H + C1 evolves..   22000 heat units*
(difference I356o0)
* One unit of heat is in this case the amount of heat required to
raise the temperature of one gram of water from o~ to I~O




88              Chemical Philosophy.
H + Br evolves..    8440 heat units
(difference 14480)
H 4 I absorbs.       6040 heat units.
These results agree with what we know of the chemical
behaviour of these elements. Chlorine displaces bromine,
and  bromine  displaces iodine from  combination with
hydrogen and the metals. Hydrogen, also, burns readily in
chlorine gas, but witl.. great difficulty in bromine vapour,
unless mixed with air; whilst with iodine, hydrogen will
unite only when strongly heated with it. In the thermal
differences given above we may also observe that iodine is
more widely separated from bromine than is bromine from
chlorine, and this corresponds with the well-known peculiarities of iodine, which, to some extent, isolate it from
he other two elements.
5. Not only is heat generated when chemical action occurs,
but in at least a great number of cases a definite amount of
electricity is developed. Imagine a plate of zinc plunged
into a solution of hydrochloric acid. The chlorine unites
with the zinc, and leaves the hydrogen to escape in bubbles
from the surface of the metal. If now a plate of platinum,
which is not acted upon by hydrochloric acid, is immersed
in the same liquid, and connected with the zinc by a wire,
it will be observed that the hydrogen is no longer disengaged from the zinc plate, but collects upon the platinum
in bubbles which rise to the surface of the liquid. If the
wire is cut in two, that part which proceeds from the zinc
can be shown, by a gold-leaf electroscope, to be charged
with negative electricity, whilst the wire from the platinum
is positive. If these wires are then attached to platinum
plates dipping into a solution of some salt, such as iodide
of potassium, which is easily decomposable, the elements of
the salt are separated, the metal going to the electrode
or terminal connected with the zinc or negative pole, and
the non-metallic element making its appearance at the




-Electrolysis.                   89
electrode belonging to the platinum plate* of the battery
that is, at the positive pole. This process of decomposition
by the voltaic current is called " electrolysis." It deserves
to be noticed that it can only take place when the body
to be operated upon is in the liquid state. Now, just as
the amount of heat evolved by a given chemical combination is definite and constant under the same circumstances,
so the electrical effect produced by contact of the same
bodies under the same circumstances is constant. When
electrolysis occurs in the body through which the current is
passing, the quantities of its constituents which are liberated
are always proportional to their chemical equivalents, and,
provided secondary actions are guarded against, these
quantities are also chemically equivalent to the materials
consumed in each cell of the battery. If, for example, in
each cell 32-5 grams of zinc are dissolved, the current
passing simultaneously through solutions of iodide of
potassium, bromide of potassium, acidulated water, and
sulphate of copper, would set free in the
Ist Cell, I27 grams of iodine,  39'I grams of potassium.t
2nd,,    8o,,  bromine 39',,
3rd,,    8,,   oxygen,  I,,     hydrogen
4th,,     8,,  oxygen, 3I'75,,        copper
The power of effecting electrolytic decomposition exhibited by different combinations of metals and liquids
depends entirely upon the chemical characters of those
bodies. The more energetic the chemical action the more
powerful will be the electro-rrmotive force of the combination. Chemical action is therefore closely connected with
electricity as well as with heat.
* The elements which make their appearance at the negative pole are
often referred to as electro-Jositive, whilst those which collect at the
positive pole are electro-neative. The metals are generally electropositive, the non-metals electro-negative.
+ Hydrogen and caustic potash would, of course, be the actual
products.




90              Chemical Philosophy.
We have now  to consider the influence of certain
physical conditions in modifying the chemical action of
bodies on one another.
6. When chloride of calcium and carbonate of ammonia
are dissolved in separate portions of water, and the solutions
are then mixed together, a precipitate of carbonate of
calcium is thrown down, and chloride of ammonium remains
in the mother liquid.  But if these two salts, calcium
carbonate and ammonium chloride, are mixed together in
the dry state and heated, a decomposition occurs which is
the reverse of the last, and by which the original compounds, chloride of calcium and carbonate of ammonia, are
regenerated.
Again, if acetic acid is poured into an aqueous solution
of potassic carbonate, effervescence ensues from the escape
of carbonic acid gas, and a solution of potassium acetate is
formed. But if potassium acetate is dissolved in strong
spirit of wine, a liquid in which carbonate of potassium
is insoluble, a stream of carbonic acid gas transmitted
through the solution is capable of decomposing the acetate,
throwing down a precipitate of potassium carbonate.
Experiments of this kind, which might easily be multiplied, show us that the character of a chemical reaction is
very largely dependent upon the circumstances under which
the operation is performed, so that a reaction which, under
one set of conditions, takes place in a particular manner
may be reversed when those conditions are suitably
altered.
We may accept it as very generally, though not quite
universally, true, that when we mix together two soluble salts,
which by double decomposition are capable of giving rise to
an insoluble compound, that insoluble compound will be precipitated until complete decomposition of one or both the
generating salts has taken place. If, for example, we take the
two salts, barium chloride and sodium sulphate, knowing beforehand that barium sulphate is insoluble in water, we may




Effects of Mass.               91
safely assert that a precipitate will be formed when the
aqueous solutions of these'two salts are mixed together.
Similar observations hold good with regard to mixtures of
compounds which, amongst them, contain the elements of a
gas or body volatile at the temperature of the experiment.
In every case this volatile body is formed. Conversely, if we
submit to pressure or to a very low temperature a mixture
of substances which, under ordinary atmospheric conditions,
evolves a gas, the chemical action is retarded, or sometimes
prevented altogether.
7. When several bodies capable of acting on one another
are mixed together, but the quantity of one of them largely
preponderates overthe rest, some curious results arefrequently
brought about. When, for example, a solution of bismuth or
antimony chloride in hydrochloric acid is mixed with a little
water, no change is perceptible to the eye, but the addition
of a larger quantity of water throws down a white precipitate of the oxychloride, BiOCi or SbOCL. These decompositions take place according to the following equations:BiCNl  +   OH2  =   BiOCl   +   2HC1
SbCl3 +   OH2  =   SbOCl   +   2HCl
That is to say, one molecule or i8 parts by weight of
water decompose one molecule or 3I6'5 parts of bismuth
chloride or, 228'5 parts by weight of antimony chloride; and
yet, if these proportions of the materials are brought into
contact, only a partial decomposition takes place, and the
reaction is completed only when a much larger quantity
of water is added.  In such a change as this we must
remember that there are two antagonistic agents at work,
namely, the water tending to decompose the chloride according to the equations given above, and the hydrochloric
acid which is generated by that decomposition tending
to reproduce the chloride. We may, therefore, consider
that there are four bodies in presence of one another, and
surrounded by water molecules.




92               Chemical PhiiosoJphy.
Taking one case,
We have BiCI13, OH2, BiOC1 and HC1.
We may safely assume that when the number of water
molecules present is augmented, the number of molecules
of chloride decomposed by water in a given interval will
increase, whilst the number of oxychloride molecules decomposed by the hydrochloric acid present will pari cpassu
diminish, until the whole of the chloride is destroyed and
precipitation is complete.
If now to the liquid holding the precipitate in suspension
we add hydrochloric acid in excess the action is reversed,
and the precipitate disappears again.
In such experiments as these the decomposition seems to
proceed continuously as the quantity of the acting body is
increased, any alteration in the proportions, however small,
apparently producing a corresponding alteration in the extent
to which decomposition takes place.
8. Experiments made by Bunsen show, however, that
this is not always the case.
When carbonic oxide and hydrogen are exploded with a
quantity of oxygen not sufficient to burn them completely,
the oxygen divides itself between the two gases in such a
manner that the quantities of carbonic anhydride and water
produced stand to one another in a simple atomic proportion. The results of Bunsen's experiments are given in the
following table, the numbers in which denote volumes:Composition of Gaseous  Quantities of CO and H.  Ratio of
Mixture.         consumed by detonation.  CO to H.
7257 CO  I8'29H  94 0    I2-I8 C0   6-ioH    2  I
59'93    26'71   13 36   I3'o6     1366 I66I
36'70    42'I7   21'3    Io'79     31'47      I  3
40-12    47'I5   12.73     4'97     20 49      I  4
The results were the same whether the explosion took
place in the dark, in diffused daylight, or in sunshine, and




Bunsen's Experiments on Mass.              93
were not affected by the pressure to which the gaseous
mixture was subjected.
From these and similar results, Bunsen deduces the two
following remarkable laws*:
" I. When two or more bodies B, B'... are presented in
excess to the body A, under circumstances favourable to
their combination with it, the body A always selects of the
bodies B, B', &c., quantities which stand to one another in a
simple atomic relation, so that for i, 2, 3.... atoms of the
one compound there are always formed I, 2, 3.... atoms
of the other; and if in this manner there is formed one
atom of the compound A B' in conjunction with an atom
of A B, the mass of the body B may be increased relatively
to that of B', up to a certain limit, without producing any
alteration in the atomic proportion.
"II. When a body, A, exerts a reducing action on a
compound BC, present in excess, so that A and B combine
together and C is set free; then if C can, in its turn, exert
a reducing action on the newly-formed compound, AB, the
final result of the action is that the reduced portion of BC
is to the unreduced portion in a simple atomic proportion."
In this case, again, the mass of the one constituent may,
without altering the existing atomic relation, be increased
to a certain limit, above which that relation undergoes
changes by definite steps.
9. We gather from  the foregoing considerations that
chemical attraction or affinityt is the effect of a force sui
generis, presenting peculiarities which distinguish it decisively from the rest of the physical forces with which we
are acquainted. The distinguishing characteristics of chemical attraction, briefly recapitulated, seem to be as follows:
* Watts' Dictionary, i. 860, to which the reader is referred for
further details of experiments upon the influence of mass.
+ The word affinity seems to have arisen from a notion current
among the early chemists, that when two bodies are capable of combining, there must be some resemblance or affinity between them, or
that they contain some principle common to them both.




94              Chemical Philosophly.
It operates only at inappreciable distances, the weights of
the reacting masses being definite and invariable for the
same substance. It is also, like ordinary adhesion, elective.
In other words, a given element seems, as it were, to choose
between two other elements presented to it, or to leave
one element in order to combine with another, as when
cinnabar is decomposed by metallic iron.  Lastly, the
manifestation of chemical attraction through the attendant
phenomena of heat, light, and electricity, is directly related,
in a quantitative manner, to the performance of mechanical
work.
Whatever may ultimately be concluded as to its real
nature, one thing is clear, namely, that it is in some way
intimately connected with motion of one kind or other in
the bodies concerned.
But in entering upon the discussion of this question
regarding the real nature of chemical action, it must not
be forgotten that we are venturing into a region which
belongs entirely to conjecture, and is-at least, at presentaltogether beyond the reach of observation. From the
very nature of things, no experimental test can be applied
to any hypothesis that may be framed in the hope of
explaining it. The student will therefore bear in mind
that any suggestions that may be met with in these pages,
in connection with this or kindred topics, must be accepted
with all the reserve which the obscurity of the subject
demands.
As already explained (in Chap. I. and elsewhere), there
are good reasons for believing that the molecules of liquids
and gases subsist unceasingly in a state of motion. Their
agitation is increased by heat, diminished by cold. In the
course of this dance in which they are engaged, and the
numerous encounters which must occur amongst them, it is
conceivable that some of the molecules get broken up into
atoms or atomic groups, which for a while wander about
until they encounter some other atom or atomic group with




Nature of Chemical A ttraction.       95
which they can unite.  If the original body was homogeneous, the molecules which are thus reproduced are of the
same kind as the original molecules. So long as this work
of reproduction goes on at the same rate as the destruction,
that is, so long as, in a given interval, the number of molecules decomposed and the number of molecules recomposed
is the same, no change occurs in the properties of the body,
because the average composition of the mass remains the
same. In a mass of hydrochloric acid gas, for example, it
is conceived that if it were possible to submit it to such a
scrutiny, the greater part of the mass taken at any instant
would be found to consist of molecules, each made up of
an atom of hydrogen and an atom of chlorine; but that
with these there would be associated a certain number of
free atoms of hydrogen and chlorine interspersed amongst
them. In the next instant, many of these free atoms would
be seen yoked together again, whilst their places would be
supplied by the disruption of fresh molecules.
Now, suppose an opportunity occurs for diffusion to take
place, either into a vacuum or into another gas, the lighter
hydrogen atoms, moving much more rapidly than the
chlorine, would pass away more rapidly into the new space,
so that the residual gas would be richer in chlorine than at
first. Dissociation has not actually been observed in the
case of hydrochloric acid, but other gases and liquids
exhibit phenomena of this kind in a very remarkable
manner. (See " Dissociation," Chap. XVII.)
Or imagine this hydrochloric acid dissolved in water,
and made to perform the part of the exciting liquid in a
voltaic cell.  The zinc attracts the chlorine and combines with it, and the hydrogen is disengaged. Now,
it is well known, that when pure or amalgamated
zinc is used, the action goes on very slowly, but that it
proceeds with great rapidity when the circuit is completed
by connecting the two plates with a metallic wire.
This seems to indicate that the zinc does not decom



96              Chemical Philosophy.
pose the hydrochloric at all, but that it probably combines
with the atoms of chlorine which it finds uncombined in the
liquid. Under ordinary circumstances, these must be supposed to come into contact with it only through the operation of diffusion, and hence the process of combination is
slow; but, under the influence of the electro-motive force,
the dissociated atoms of chlorine are driven towards the
zinc plate as fast as they become free, whilst the hydrogen
atoms, under the same influence, travel in the opposite
direction.
The hypothesis we are now discussing also affords an
explanation of the action of mass in determining the final
results ~of a given chemical action. The student will be
able to frame the conception for himself in any given case
after perusing paragraph 7.
Lastly, we may attempt to picture to ourselves the state
of things which obtains when two bodies, such as hydrogen
and chlorine, which have a great chemical affinity for each
other, are brought together. So long as they are cool and
in the dark, the process of combination is exceedingly slow,
so slow, in fact, that they seem to have no action on each
other. But once let the molecular agitation of even a small
portion of the mixed gases be exalted to the right pitch,
whether by heat or light, which is probably converted into
heat, and the breaking-up of the elementary molecules
proceeds rapidly throughout the entire mass, whilst new
molecules of the compound, which are probably more stable
than are those of either ingredient, are generated at a
corresponding rate. If, however, certain limits of temperature be exceeded, the vibratory or other motion of the
molecules may be sufficient to cause the disruption of
increasing numbers of them until decomposition of the
whole is complete.
It has been conceived that the chemical elements may
be formed of the same primordial matter distributed into
molecules which vibrate or rotate in different specific




Nature of Chemnical Attraction.       97
periods, and that these differences of movement may
correspond with the observed differences in external
qualities.
Adopting such a hypothesis, it must be admitted that
the molecules of a compound would almost certainly move
in a manner different from the molecules of its constituents. Also, that when two molecules approach each
other the movement of each must be more or less
modified in consequence of the proximity of the other. If
the motions proper to the one are in harmony with those
of the other, they may join to form a new unit moving in
a different path, and consequently give rise to a body
impressed with new qualities.
CHAPTER XIV.
COMBUSTION.
THE burning of wood, coal, charcoal, and other matters
commonly employed as fuel, is a process with which everyone is familiar. Chemists know that the production of fire
in the usual way is attended not merely by the consumption
or alteration of the fuel, but by changes in the surrounding
atmosphere, and that the presence of a sufficient supply of
air is an indispensable condition in the operation. They
explain the phenomena by stating that the process of burning consists essentially in the combination of the elements
of the combustible body with the oxygen in the air, so
much heat being developed that more or less of the solid
combustible and of the products of combustion are raised
to such a temperature that they emit light. Notwithstanding, then, that in ordinary fires the coals disappear and
seem to be destroyed, they do in reality only evaporate
away in the form of carbon dioxide and water, and if these
products could be collected and weighed, their weight would
H



98               Chemical Philosopy5y.
be found to be made up of the united weight of the carbon
and hydrogen of the coal, and the oxygen which is taken
from the air.
The phenomena of combustion may be observed equally
well when other materials are employed.  Thus copper
burns in vapour of sulphur, hydrogen will burn in chlorine,
whilst phosphorus and several metals become ignited spontaneously when introduced in the proper condition into the
same gas. In every such case, the resulting product consists of a compound of the body which is burned with one
or other of the constituents of the gaseous atmosphere which
surrounds it. From this it is evident that the terms " combustible " and " supporter of combustion," as generally employed, involve an error, if they are taken to imply any
difference of function; for that which in one experiment
occupies the position of combustible, may be made the
supporter of combustion or atmosphere in another. It is
easy to show, for example, that not only will a jet of hydrogen burn in oxygen gas, but that a jet of oxygen burns
equally well when surrounded by hydrogen.
These are the views universally accepted at the present
day. They serve to account for most of the facts, though
the precise explanation of the heat and light which are
developed is still unknown.
Previously to the discovery of oxygen by Priestley, and
the establishment of the modern theory of combustion by
Lavoisier at the close of the last century, a remarkable
theory had been for upwards of fifty years adopted by
chemists.  This was the celebrated theory of phlogiston*
proposed by Stahl.t  This phlogiston was supposed to be
a substance of great tenuity, which, by combining with
incombustible bodies, rendered them combustible. When
such bodies are burnt, it was imagined that: the escape of
the phlogiston in a peculiar condition of vibratory motion
* pXo7t-r6s, anything set onfire. F Died at Berlin, I734.




Theory of Phlogiston.              a
gave rise to the phenomena of fire. At the time t:e-iceas
originally introduced, little was known of the part which
the air plays in all ordinary burning. When accumulated
facts proved conclusively that bodies by burning increase in weight, some attempts were made to prop up the
theory by assuming that the presence of phlogiston gave
bodies lightness instead of weight. The merit of the idea,
however, lay not so much in providing an explanation of
certain special cases of combustion, as in referring all cases
of burning to a common cause, and in showing that the
property of combustibility is capable of being transferred
from one body to another.   Oxides of the metals, for
example, were regarded as ashes, or caxles, of the metals left
after the escape of their phlogiston, which could be restored
to them by contact with heated charcoal, a body which was
supposed to be specially rich in the hypothetical inflammatory principle.
Whilst we believe that the presence of no substance such
as phlogiston is necessary for the production of fire, and
that during the manifestation of the phenomena of combustion no loss of material occurs, yet it has been very justly
pointed out that bodies, when they burn, do in truth part
with something, and that is the potential energy or power
of doing work which belongs to a state of chemical isolation.
In order that combustion may commence in air the
temperature of combustible bodies must in general be
raised. The temperatures required in different cases are
very diverse.  Thus phosphorus, which liquefies at 44~, can
scarcely be melted in the air without inflammation. Carbon
disulphide vapour mixed with air takes fire if a glass rod
heated to about r500 is brought into contact with it.
Sulphur begins to burn at about 250o-far below its boiling
point-whilst carbon and many hydrocarbons require a red
heat.
The temperature produced when the process of burning
is once established is in general higher than that which is
H 2




ICO             Cihemnical Philosophy.
requisite for the commencement of combination. This difference is illustrated by the action of platinum upon a
mixture of hydrogen or coal gas and air. If a warm slip of
clean platinum foil or a coil of platinum wire is held in a
current of such mixed gases, the temperature of the metal
rises rapidly, in consequence of combination taking place
between those portions of the gases which are in immediate
contact with it, combination extending to the surrounding
mass only when the temperature reaches a certain point, and
the platinum is nearly white hot. Similar phenomena may
be observed in other cases when the heat evolved in the
early stages of the process is allowed to accumulate. The
spontaneous ignition of phosphorus, of finely pulverulent
iron or lead (pyrophori), and of heaps of oily rags, may
be referred to this cause.
The exact temperature of flame is difficult to determine
and is liable to vary.  The temperature of a hydrogen
flame, burning in air, has been estimated at about 2o8o0C,
but when the flame is fed with pure oxygen its temperature
rises to upwards of 4ooo0C. This is easily explained by
the fact that in atmospheric air the oxygen is mixed with
four times its bulk of nitrogen, which contributes nothing to
the chemical action, and which, being raised to the same
temperature, as the other gases present, consumes a great
deal of heat. A temperature still higher is produced when
a mixture of hydrogen and oxygen in due proportions is
fired in a closed vessel, so that the heated gases are not
allowed to expand. This expansion against atmospheric
pressure is work the performance of which involves the consumption of heat. The temperature produced by the explosion of oxygen and hydrogen in a closed vessel has been
estimated at about 525o~C.
But although the temperatures producible by the same
combustible under various circumstances are different, the
actual amount of heat evolved in the combustion of the
same weight of a given substance is always the same. This




[Heat of Combustion.                    IOI
statement can of course be accepted only on condition of
uniformity in the circumstances attending the experiment.
Thus it will appear from the table given below that, as in
the case of carbon, the different allotropic modifications of
the same substance may give rise to appreciably different
amounts of heat.
I-  Units of hzeat developed by combustion of equal
weigits of elements in oxygen.
Kilograms of water
Substance burned.         Product.    heated s~C by burning
I kilo of each substance.
Hydrogen...Water                  34034
Carbon
a Diamond...            Dioxide           7770
b Natural Graphite...,,              7797
c Wood Charcoal...                   80So
Sulphur (native)...   Dioxide           2220
Phosphorus (common)..  Pentoxide            5747
Zinc...    Oxide                              I330
Iron....Peroxide                         I582
II.-  Units of heat evolved by combustion of atomic weigfhts.
Kilos. of
Weight in               ~~~~water
Name of Element.   Weight in       Product.       heated'~C
Kilograms.                     by the combustion.
Hydrogen...       I    Water (liquid)..  34034
I    Hydrochloric acid (gas)  22000..  Hydrobromic acid (gas)   8440
Carbon
a Diamond... I2    Dioxide (gas).. 93240
b Graphite...    I2,,,,..  93560
c Charcoal...       I2,,,,.  96960
Sulphur (native)..    32    Dioxide (gas)..   71042
Phosphorus (common)       31    Pentoxide (solid).  178157,,. ~     3I    Pentachloride (solid). Io7740
Tin....   I8    Dioxide (solid)..  I35360
11.    I  Tetrachloride (liquid).  I22880




I02               Chemical Philosophfy.
The quantity of heat absolutely evolved also depends
partly upon the physical condition of the products of combustion. Thus the number 34034 which expresses the heat
evolved in the combination of one part by weight of hydrogen
with eight parts of oxygen, represents not only the heat of
chemical action, but the heat (amounting at the temperature of the experiment to about 5500 units) which is produced by the liquefaction of the resulting nine parts of
steam.
This relation of the amount of heat evolved to the
physical state of the resulting compounds is further indicated by the results exhibited in the following table:Substance burned.  Weight   Product.  Units of heat evolved.
burned.
Sn..   II8      SnO2                135360
SnO..   I34       SnO9    69584 x 2= 139168
Cu..    63'5    C 0                   38304
CuO..    71'5    CUO       I8304 X 2 = 36608
Graphite.    12      CO2                 9356o
CO..    28       CO2      67284 x 2 = I34568
Here we find that when solid tin or copper is converted
into its highest oxide, the amount of heat developed is,
practically speaking, twice as great as the amount of heat
developed in the conversion of the lower into the higher
oxide. In other words, the two successive stages of oxidation, both of which result in the formation of solid products,
are marked by the evolution of equal quantities of heat..The case of carbon is different. In the first stage of
oxidation the process involves the conversion of solid
carbon into gaseous carbonic oxide, whilst in the second
stage the carbonic oxide, which is burnt, and the carbonic
anhydride, which is formed, are both gaseous. There is no
change of state. Hence the quantity of heat which is
developed in the latter operation is nearly two-thirds
instead of only one-half, the total quantity evolved in




Problems on Combustion.             I03
the formation of the same weight of carbonic anhydride
from solid carbon.
In order, therefore, to calculate the actual amount of heat
obtainable by burning a given combustible, it is necessary
to take these and other circumstances into consideration.
The following examples, which are unencumbered by small
corrections, and in which it is assumed that no heat is lost
by radiation or conduction, will serve to indicate the general
nature of such calculations.
The combustion of I part by weight of wood charcoal
evolves 8o80 units of heat. That is to say, I kilogram of
charcoal would heat 8o8o kilos of water from o~ to Io, or
I kilo. of water from o~ to 8080o.  12 kilograms of charcoal
produce 44 kilos of carbonic anhydride, or I kilo produces
3'67 kilos, and if the heat produced by the combustion is
communicated to this quantity of carbonic anhydride, and
not to water, the temperature would be 8o8o or 22020, if
the specific heat of carbonic anhydride were the same as
that of water. But the specific heat of carbonic anhydride
is only'2I64, when that of water is I. Hence, the temperature of the carbonic anhydride is 2202 X    I  or'2 I64'
IoI75~, when the carbon is burnt in oxygen.
Now if the combustion is performed in atmospheric air,
which contains 77 per cent. of nitrogen, much heat is consumed in raising the temperature of this nitrogen.
The 2'67 parts of oxygen required for the combustion of
one part of carbon are accompanied by 8'93 parts of nitrogen,
the specific heat of which is'2438. Therefore, when the
combustion of carbon takes place in air, the temperature of
the resulting mixture of gases cannot be higher than
80o8o' = 2720~ C.
(3'67 x'2I64) +  (8'93 x'2438)
In practice, the temperature is not so high as this, partly
because some heat is lost by radiation, some by conduction




104                Chemical Philosophy.
through the solid unburnt charcoal, partly because an excess
of atmospheric air over and above that actually required
mingles with the products of combustion, and partly also
because, in all probability, the specific heats are not constant, but increase as the temperature is higher.
EXERCISES ON SECTION II.
I. Give the names of the elements represented by the following
symbols-Al, Sb, Fe, Mg, Hg, Mn, Ca, C, C1, I, N, P, K, S, Ag,
Na, Br, Cu, F, H, Pb, O, Si, Zn.
2. Write down the symbols and atomic weights of
Barium, boron, bromine;
Calcium, carbon, copper;
Magnesium, manganese, mercury, silver;
Phosphorus, potassium, lead;
Sulphur, sodium, silicon;
Iron, iodine, chlorine, oxygen, nitrogen.
3. Read these symbols and formulae, thus:N2 represents one molecule of nitrogen, consisting of two atoms;
0, 02, OH2, 20H2, HC1, H2, C12, NH3, H3PO4, H2SO4, FeSO4,
2FeSO4, A12(SO4)3, I20H2, I2A12(S04)3, CO2, 3C02
4. Write down the formulhe and molecular weights of water, ammonia, hydrochloric acid, carbonic anhydride, sulphuric acid, ferrous
sulphate, aluminic sulphate, phosphoric acid.
5. Write down the whole weight represented by each of the
following expressions: 2HgO, IoOH2, 3FeS, 3FeS2, 2CS2,
KC4HO6,  K2C4H406,  5C7HgN,  I2CH4,  KA1(SO4)2 + I20H2,
3[NH4Cr(SO4)2 + I 2OH2].
6. Name the following compounds:-BaO,  CaO, MgO, ZnS,
KC1, NaBr, AgF, H2S, HI, KCN, SSe, BN, H3P.
7. BaO, BaO2; Hg.0, HgO; FeS, FeS2; MnO,  Mn.03,
MnO2; FeO, Fe20O, Fe3O4; N20, N202, N203, N204, N205;
P2S3, P2,S; SnCl2,  SnC14;  FeBr2, Fe2Br6; Cu2Cl2,  CuC12;
CrCl2, Cr2CI6, CrF6; SbBr3, SbBr,.
8. KNO2, KNO3 (-ate); K2SO, K,2SO4 (-ate); KC1, KC10,
KC102,  KC103  (-ate),  KC104;  KI,  KIO3  (-ate),  KIO4;




Exercises on Section II.                 I05
NaHSO3,  Na2SO3;  Na2HPO4, Na3P4,  NaH2PO4; HSPO2,
H3PO3, H3PO4 (-ic); HC1, HC10, HBrO, HC102, HC103 II03,
HC104, HBrO4.
9. Write out the following equations according to the scheme on
page 66-:
(a) Mn02    +   4HC1    =  MnCl2  + C12  +  2H20.
(b) 2KI     +   C12        =  2KC       + 12.
(c) SO2  +  20H2 + Cl2 =  H2S04  +  2HC1.
(d) NaN03   +    H2SO4  = NaHS04 + HNO3.
(e) 2MnO02   +    2H2SO4 =  2MnSO4 +  2H20  + 0.2
(f) 2K2Cr207 + 8H2S04   =  2K2SO4 +  2Cr2(S04)3 + 8H.20
+ 302
Io. Write out the following equations according to the scheme oni
page 67:(a) 20H2    +    2 C12   =   4HCl +.2(b) CO.     +    C (solid)=    2 CO.
(c) 2CO      +   02       =   2 CO2.
(d) 2NH3         N.2      +   3 12.
(e) 2NH,    +    3 C12    =   N2    +   6 HC1.
(f) NH4NO3 (solid)        =    N20   +    2 H20.
1. Write out in symbolic equations(a) Ammonium chloride and calcium hydrate give ammonia, calcium
chloride, and water.
(b) Ammonium nitrite (heated) yields nitrogen and water.
(c) Common salt and sulphuric acid yield sodium, hydrogen sulphate
and hydrochloric acid.
(d) Copper and nitric acid yield *copper nitrate, nitric oxide and
water.
(e) Mercury and sulphuric acid yield mercuric sulphate, sulphurous
anhydride and water.
(f) Antimonious sulphide and hydrochloric acid yield antimonious
chloride and sulphuretted hydrogen.
I2. Ilow many grams of oxygen are required to burn 24 grams of
carbon and 32 grams of sulphur?
I3. How many pounds of zinc are there in 350 pounds of the sul.
phate, ZnSO4?
I4. How much sulphur will give IOO kilograms of sulphuric acid
H2S04?
15. How many pounds of black oxide of manganese are required to
yield, by the action of hydrochloric acid,  112 pounds of chlorine?
I6. How many pounds of chalk containing 96 per cent. of calcium
carbonate CaC03 will neutralise 250 pounds of sulphuric acid?




io6                Cihemical Phzilosopiay.
(In the following examples the gases are supposed to be at normal
temperature and pressure):I7. Find the weight of 20 litres of oxygen, of 50 litres of chlorine,
of 250 litres of ammonia,
I8. How many litres of oxygen are required to combine with
a. I2 criths of carbon
/. 2 grams of sulphur
7y. Io grams of carbon?
I9. IIow many litres of chlorine are required to decompose 12 litres
of hydriodic acid?
(2HI  +  C12  =  2HC1  +  I2)
20. How many litres of chlorine are required for the complete decomposition of Io litres of olefiant gas?
C2H4  +  2C12  =   C2  +  4HC1
2I. How many litres of hydrogen are obtained by dissolving 12
grams of magnesium in an acid?
22. What weight of potassium chlorate is required to yield 35000
cubic centimetres of oxygen?
23. What materials and what quantities would you employ in order
to obtain 50 litres of each of the oxides of carbon?
24. How much mercuric eyanide, Hg(CN)2, must be used to furnish
50 c. c. of cyanogen, C2N2, assuming that 60 per cent. of the cyanogen
is obtained in the gaseous form?
25. Red oxide of copper contains 88'8 parts of copper and 1I'2
parts of oxygen by weight; black oxide of copper contains 79-87 of
copper and 20-13 of oxygen. If the formula of the black oxide is
CuO, how should the red oxide be represented?
26. Water contains 88'8 of oxygen, II-I of hydrogen.   If its
formula is OH2, find the formula for peroxide of hydrogen, which
contains 94'I2 0 and 5-88 H.
27. Two hydrocarbons have the following composition:I.                    II.
Carbon. 85-7I  _C    92'3
Hydrogen.. I4.29  -       4  77
Find the formula for II.
28. Show that the composition of the oxides of nitrogen, of manganese, and of chromium, are in accordance with the law of multiple
proportion.
29. Examine the following equations, attach the name to each




Exercises on Section II.                   I07
formula, and classify the reactions according to Chapter X., p. 69 to
p. 7I, giving reasons in doubtful cases:(a) Na2     +      20H2       -    2ONaH   + H2
(b) Fe       +     H2SO4.H2()=    FeSO4.H20 + H2
(c) electrolysis)    2HC1     =    H2  + Cl2
(d) SO2     +    H202         =    H2SO4
(e) (heat)         3MnO02    -   Mn304  +  02
(f) OKH    +      HC1         =    KC1       + OHI
(g) 02H2    +     03 (ozone) =    OH2        + 202
(h) SO3    +      H2O  HSO4
(i) PH3     +     HI          =    PHI
(j) N205    +    H20          =    2HN03
(k) P205    +    3H20         =    2H3PO4
(1) 2KI     +    C12          =    2KC1    + I1
(1vi) C2H4   +    Br2         =    C2-I4Br,,
ethene
(n) NaNO3 +    H2SO4          =    HNO3    +.lNaHSO4
(o) (heat)        2KC103           KC1        + KCl  +   KC104 + 02
(p) MnO2   +    SOs           =    MnSO
(q) MnO2   +    2SOo          =    MnS20O
(r) C2H402  +   3C12          =    C2HCl302  +  3HC1
(s) K2S     +    CS2         =    K2CS3
(t) C6HHO +    HNO3          =    C6H4NO2HO   +   H20
Phenol.
(u) C21H-I402  +    PC]5     =  C2H3OC1  +   POC13  +   HC1
Acetyl Chloride.
(v) C1oH16  +    HC1         -  CloH17C1
(w) 31-IC1   +    HNO3       = 2H20  +         NOC1  +    C12
)x) As203   +   2HNO3         + 2H20  =   2H3As04 +   N203
(y) (heat)   NaNH4HPO4   =  NaPO3 +   NH3              +    H20
(z) (heat)    2Na2HPO  =       Na4P207+   H20
30. How many cubic centimetres of ammonia (measured at I5~ and
under 740 mm.) would be obtained from 5312 grams of ammonium
chloride?
3I. How many cubic centimetres of sulphur dioxide (measuredat 20~
and 740 mm.) can be obtained by the action of copper on 20 grams of
sulphuric acid?
32. What weight of water would be heated from  o~ to I~ by the
combustion of I gram of charcoal in oxygen?
33. What weight of water would be heated from o~ to I5~ by the
combustion of I gram of hydrogen in chlorine?




Io8               CChemical Philosophy.
34. Calculate the temperature of combustion of phosphorus burning in air.
35. Heat of combustion of hydrogen.      34034 units
Latent heat of steam.... 536
Specific heat of steam....'475
Specific heat of nitrogen.'2438
Composition of air, N77, 023 parts by weight.
With these data find the temperature of the hydrogen flame burning
in air.
Answer;34034 - (536 x 9)    =27o8oC.
(9 x'475) + (267 x'2438)




Io9
SECTION  III
CHAPTER XV.
EQUIVALENTS AND ATOMIC WEIGHTS.
WHEN a strip of metallic copper is immersed in a slightly
acid solution of mercuric chloride the mercury begins at
once to be deposited, and copper is at the same time dissolved. After the lapse of a sufficient length of time the
whole of the mercury will thus be thrown down and the
liquid will then contain nothing but copper chloride. When
this result has been brought about it will be found that for
every Ioo parts by weight of metallic mercury obtained,
3 I7 5 parts of copper are consumed.
If now a piece of iron is plunged into the solution of
copper chloride the copper salt is in its turn decomposed
and the whole of the metal recovered, iron being substituted for it. And if the weights of the metals which thus
exchange places are determined, it will prove that in order
to precipitate the 31'75 parts of copper in the liquid.28 parts of metallic iron must go into solution.
Moreover, it is found that whenever one of these metals,
mercury, copper, iron, is exchanged for another, the
weights concerned are always in these proportions. These
quantities are then said to be equivalent to one another.
But, further, when iron is dissolved in hydrochloric or
sulphuric acids, these 28 parts liberate from the acid one
part by weight of hydrogen.
In the three compounds, hydrochloric, hydrobromic, and




I I              C/zemical Philosophzy.
hydriodic acids, 35'5 parts of chlorine, 80o parts of bromine,
and 127 parts of iodine are united with I part by weight
of hydrogen, and it is also found that when chlorine,
bromine, or iodine takes the place of hydrogen, as they
frequently do in carbon compounds, the quantities which
replace I part by weight of hydrogen are represented by
the same numbers.
Again, the metals mercury, copper, and iron are capable
of combining with chlorine, bromine, and iodine, forming
various chlorides, bromides, and iodides; but the analysis
of these compounds reveals the fact that these are all made
up of their elements in proportions which may be represented as multiples of the numbers which have been found
to be characteristic of the same elements in other cases.
Thus there are two chlorides of iron, of which the first
contains 28 parts of iron united to 35'5 parts of chlorine,
whilst the second consists of 28 x 2 or 56 parts of iron,
with 35'5 x 3 or I06'5 parts of chlorine.
By such observations it is established that:ioo parts of mercury  are equivalent to each other
28,,  iron p       and to i part of hydrogen.
And that35'5 parts of chlorine
80,,   bromine   and to i part of hydrogen.
T127,,   iodine
Also thatIoo parts of mercury
or 31'75,,  copper          are at least sometimes
or 28,,  iron
equivalent to35'5 parts of chlorine.
or 80,,   bromine.
or 127,,   iodine.




Equivalents anzd Atomic Wezigts.        Ii 
The term  "equivalent," or "'equivalent number," was,
however, formerly employed to designate the weights of
the several elements, which are capable of entering into
combination with one part by weight of hydrogen, and by
some chemists it is still restricted to this sense.
The exact experimental determination of these combining proportions is a matter of great importance, for it is
in all cases the first step towards the establishment of the
atomic weights.  The methods used in these determinations vary according to the nature of the elements concerned,
and cannot be dwelt upon in a work of this kind.
It is unnecessary to draw up a complete list of the
equivalent numbers of all the elements, because when the
student has learnt by heart the atomic weights, he can
always calculate the equivalents by a very simple operation.
For the equivalent number, as must be obvious, bears a
very simple relation to the weight of the atom, being in
every case either identical with the atomic weight or an
aliquot part of it.
It must be remembered, however, that in actual practice
the determination of the equivalent precedes the calculation
of the atomic weight, for the former is a number which is
found by experiment, whilst the latter is to a certain extent
hypothetical.
It has already been mentioned that in the system of
atomic weights or ratios now universally employed by
chemists, the atomic weight of hydrogen is taken as the
unit.
The atomic weights of all the other elements are fixed by
reference to one or other of the following rules.
In those few instances in which it is possible tc make use
of all of them, it is found that the number indicated by the
application of any one of these rules is identical, or nearly
identical, with the number indicated by the others. This
is, of course, important, as showing that these different
considerations lead to the same conclusion; and, there



I 2              Chemical Philosophay.
fore, that our atomic weights belong to one and the same
system.
I. The atomic weight, as deduced from other considerations, is identical in a few cases with the specific gravity of
the element in the gaseous state.
This occurs in the case of the gases hydrogen, oxygen,
nitrogen, chlorine, and the volatile bodies, bromine, iodine,
sulphur, selenion, potassium, and probably sodium.
Mercury, cadmium, and zinc are exceptions to this rule,
as well as the non-metallic elements-arsenic and phosphorus. The three former produce vapours, the specific
gravities of which, as compared with that of hydrogen, are
the halves of their atomic weights, whilst the vapour densities
of the two last are represented by numbers which are equal
to twice the atomic weights.
I    Name of Elemelnt.  Specific Gravity of Gas or  Atomic
Vapour.           Weight.
Hydrogen..           I                     I
Oxygen..           6                  16
Nitrogen...        14                  1 I4
Chlorine...       35'5                   35'5
Bromine...        80                    0So
Iodine....     27                    127
Sulphur...        32  (at high temps.)    32
Selenion..       79 ( do.  do. ):   79
Potassium.39?           39
Sodium..          23                 23
Mercury...        00                1  200
Cadmium...  56                   I2
Zinc....        325?       1   65
Arsenic...     150                    75
Phosphorus...     62                    3 I
II. According to the system here adopted, and which will
be further dwelt upon in the next chapter, the bulk of one
part by weight of hydrogen is regarded as the volume of
the atom of that element, and is selected as the unit for
comparison of other volumes, atomic and molecular. Twice




Determination of A tomnic Wezgihts.          I 13
this bulk of hydrogen contains a molecule, and all molecules in the gaseous state occupy the same volume. Now,
according to the atomic theory, a molecule cannot contain
less than one atom of any element; and, consequently, if
we ascertain what is the smallest quantity of an element
contained in a molecule of any compound of which it may
be a constituent, we shall have determined the atomic
weight of the element.
The atomic weight, then, may be said to be the smallest
weight of the element ever found in two volumes of the
vapour of any of its volatile compounds, the bulk of one
part bv weight of hydrogen, at the same temperature and
pressure, being considered as one volume.
Suppose, for example, it is required to find by this rule
the atomic weight of oxygen, we have only to ascertain the
vapour densities of a number of compounds containing
that element and the weight of oxygen contained in each.
The results are then tabulated in the following manner:Specific Gravity,       Weiht of
that is Weight         Oe  o
Volatile Compounds containing   of: Volume of   Weight  OxTgen conOxygen.         G;as or Vapour at  of Twoolumes.    Twoae i
same Temp.             volumes.
and Pressure.
Water....                9           18        16
Carbonic oxide..       14          28         I6
Carbonic anhydride..      22          44         32
Sulphurous anhydride.     32          64         32
Sulphuric anhydride..     40          8o         48
Nitrous oxide...      22          44         I6
Nitric oxide...      5          30         I6
Alcohol....        23          46         i6
Ether....        37          74         16
Acetic acid...      30          6o         32
Etc.                       etc.                  etc.
Two volumes of the vapour of any volatile compound,
therefore, never contain less than i6 parts of oxygen, and
hence I6 is accepted as its atomic weight.
I




I I4               Chemzical Philosophy.
This is a rule of very general applicability, for although
a great many of the elements, carbon for example, are quite
incapable of being volatilised at any manageable temperature, they yield a large number of easily volatile compounds.
There are, however, many metals which are neither
vaporisable by themselves nor when in union with other
elements. In such cases this rule cannot be applied, and
information has to be sought in a different direction.
III. Law  of  Dulong and Petit.-" The specific heats of
the solid elements are inversely proportional to their atomic
weights." Whence it follows that the product of the multiplication of the specific heat by the atomic weight is a
constant number.
In the following table are given the specific heats of the
most important of the elements, together with their atomic
weights:Name of the Element.   Atomic    Sp. Ht. of    Sp. Ht. of
Weight.  Equal Weights. Atomic: Weights. 
Aluminium.,.     27'4'2143                 5'87
Antimony...   I22            o508          6'20
Arsenic.....    75'0814          6  I
Bismuth..  2IO'0308         6'47
Boron (crystallised)..      I       2300          2'53
Bromine (solid).. |'0843                  674
Cadmium..  i  2'0567         6'35
Carbon..     12
a Wood charcoa..'241o         2 89
/5 Natural graphite.'2020          242
-y Diamond..'I469         I'76
Cobalt...'To67                    6'29
Copper                     63..   -5'0952          6'04
Gold.          97.0324          6.36
Iodine  1...    I27'o54I         6 87
Iron....    56'II38         6'37
I,eadl..       207'0314          650
Lithium....       7'9408         6'59
Magnesium..    24'2499         6 -oo
Mercury (solid)..   200         0319          638
Nickel....    59'    I092               6 44




Sptecfc Heat azd A tomnic Weizght.            I 15
Atomic    Sp. IHt. of    Sp. Ht. of
ame of the  Weight.  Equal Weights. Atomic Weights.
Phosphorus...   3I
a Common..'1895                        5'87
/3 Red...'                698         5'26
Platinum.   197'5  i   0324   6'40
Potassium.       39I         1655         6'47
Silicon....
a Graphitoidal.                      I8I          5'07
p8 Crystallised..650                             4 62
y Fused..                   I38o         386
Silver..o8  0570     6'I6
Sodium.            23'2934         6'75
Sulphur (Octahedral)       32'I776         5'68
Tin..                    x I8'0562         6'63
Zinc....    65'0956         6'23
The greater number of the specific heats given in this
table were determined by Regnault.  A  glance down the
fourth column will show that, with three exceptions (boron,
carbon, and silicon), the amount of heat required to produce the same clhange of temperature in the different
elements is nearly the same in all cases when the quantities
operated upon are in the proportion of their atomic weights.
That the numbers representing the atomic heats are not
found to be exactly identical is due partly to unavoidable
errors in the estimation of the specific heats, and partly to
the fact that the different elements are not dealt with under
conditions which are strictly comparable with one another.
Thus, solid mercury and solid bromine, at the temperatures
at which the specific heats were determined, are much nearer
to their melting points than are the solids, copper and iron,
at the temperatures at which the same operation was per.
formed upon  them.   Other circumstances, such  as the
assumption of different allotropic forms by some of the
elements, tend to the introduction of further uncertainty.
But assuming the rule, with the exceptions already
named, we find that the atomic heat of a solid element
may be represented on the average by the number 6'2.
I 2




I 6             C(hemical Philosophy.
Nevertheless, since the determination of specific heats is
always attended by many sources of error, whilst the
equivalent or combining proportion can be fixed with a very
considerable degree of accuracy, the application of this
law consists essentially in enabling us to decide as to what
multiple of the equivalent is to be taken as the atomic
weight. In doing this, we are guided by the formula
6'2
At. Wt.
Sp. It.
Suppose, for example, it is found that 29'5 parts of tin
are equivalent to r part of hydrogen, and we require to find
the atomic weight.   The specific heat of tin is o0562,
therefore
6'2
At. Wt.  =               o562  110'3
The atomic weight of tin is, however, not taken to be
I0o'3, but rather such a multiple of 29'5 as comes nearest
to that number, and this is found to be 29'5 x 4 or II8.
IV. Law of Isomorp/hism.-If a crystal of common potash
alum is immersed in a saturated solution of the purple
chrome alum, the purple salt is deposited uniformly over
the colourless nucleus, so that the crystal increases in bulk
though it undergoes no alteration of form. The resulting
crystal may be transferred to a solution of ammonia alum,
or of iron, or manganese alum, and during every fresh immersion it receives a deposit of a different salt upon its
surface, the crystalline form, that of the regular octahedron,
being throughout preserved.
If instead of-thus causing successive layers of the various
alums to be superposed one upon the other, solutions of
any two of these salts are mixed together, crystals of the
same form are deposited containing the elements of both
salts.
The alums are double sulphates, all containing the same




Isomnorphism.                I 17
amount of water of crystallisation, and having a composition which may be represented by the general formula
M' M.'" (SO4)2 +  I20H
in which M' may be Cs, Rb, K, Na, Am, T1 or Ag, and
M "' may be Fe, Mn, Cr or Al.
If, therefore, any two of these compounds are compared
together, as for example
Potash Alum    -   KAl(S04)2. I 2 0H%
Soda Alum      -   NaAl(SO4)2. I20H2
it is obvious that atom for atom they have the same constitution, but the one contains potassium, the other sodium.
This exchange of an atom of one element for an atom of
another is in this case effected without producing any alteration in the crystalline structure of the resulting salt, and
when bodies thus agree in chemical constitution and in
crystalline form, they are said to be isomoorpihos.
From the examination of a great many instances of the
same kind, chemists have been led to infer that when two
bodies, composed of the same or similar elements, crystallize in forms belonging to the same crystallographic system,
they generally contain the same number of atoms united
together in a similar manner.
This statement must be considered to include cases in
which groups of atoms (compound radicles) take the place
and perform  the part of single elementary atoms.  The
compounds of ammonia with  acids, for example, are
isomorphous with the corresponding salts of potassium,
and a constitution is therefore attributed to the ammoniacal
salts similar to that of the potassic salts, the symbols NH4
being the representative of the metal in these compounds.
Thus in the following pairs of compounds there is the most
complete concordance in chemical characters as well as in
crystalline form.




I i8             C/i6/ical Phiiosophy.
Cubical.
AmCl (Am  =  NH4)          KCl.
Four or Six. sided Prisms (Trimetric).
Am2 S04               K2 SO4
Octahedral (Regular.)
AmAI(SO,), I20H2    KAI(SO4)2 I20H2
Am2PtC16                K2PtCI6
Although some of the relations between external crystalline form and chemical constitution are still involved in
obscurity, the existence of a great number of well-marked
cases of isomorphismn is a fact which is familiar to every
chemist, and occasionally the application of this principle
has led to the settlement of questions relating to atomic
weights, regarding which there had been previously more
or less of uncertainty.
For instance, alumina, the only known oxide of aluminium,
is believed to have the same constitution as ferric oxide,
because not only do the oxides themselves agree in crystalline form, but they are capable of replacing each other in
their compounds without disturbing the crystalline structure
of these bodies.  Now, since ferric oxide is universally
regarded as a sesquioxide, that is, containing in each molecule two atoms of the metal to three atoms of oxygen,
alumina is believed to be formed upon the same type, and
if the formula Fe,O, be employed to represent ferric oxide,
A1,,03 must be admitted as the formula for alumina. If
these considerations have to be applied to the determination of the atomic weight of the metal, we have only to
refer to the analysis of alumina to find that Ioo parts
contain
53'3 parts of aluminium,
and 46'7,,,, oxygen.
And since, according to the formula, we have 3 x T6 or
48 parts of oxygen united with 2x9 parts of metal, we can
easily calculate the value of x (=  27'4), which is the atomic
weight.
Other instances of a like character would readily present




IsomsoYrhis/m.                I 19
themselves upon enquiry. The following is an interesting
example:It is well known that the crystalline forms of sodium
nitrate and calc-spar are nearly identical, and a crystal of
calc-spar immersed in a saturated solution of the nitrate will
grow by uniform deposition of that salt all over its surface.
But arragonite (another form of calcium carbonate) is
also found in the same form as potassium nitrate. All
these, facts, then tend to prove that calcium carbonate probably contains the same number of atoms as the nitrates of
potassium and sodium, and that its formula should be
CaCO, if the
others are KNO3 and
NaNO, respectively.
But this formula cannot be used unless we assume that
the atomic weight of calcium is 40, a number which agrees
with the value deduced from other considerations.
V. In not a few cases the atomic weight selected for any
element in accordance with the foregoing rules may
receive support and confirmation from  a study of the
chemical reactions of some body containing the element
in question. For example, we have evidence that in a
molecule of water the hydrogen present is capable of being
expelled in two equal parts by the action of the metal
potassium or sodium, whilst the oxygen is not divisible in
any such manner by chlorine or any other element which possesses the power of replacing it. Hence the formula H2O.
Similar arguments might be applied in a slightly different
manner.  For example, 46 parts of formic acid  are
equivalent to 60 parts of acetic acid, since both saturate
the same quantity of any base, and represent two volumes
of vapour. The former contains I2 parts of carbon, the
latter 24, SO that, according to rule II., p. I I3, we believe 46
parts of formic acid contain one atom of carbon, whilst an
equivalent quantity of acetic acid contains two atoms.




I20             Chemical Philosoplay.
Further evidence of this may be found in the fact that
when formic acid is submitted to electrolysis all its carbon
goes to form carbon dioxide, whilst when acetic acid is
electrolysed half its carbon goes to form carbon dioxide,
whilst the other half combines with hydrogen and furnishes
ethane. The carbon of acetic acid is, therefore, divisible in
this operation into two equal parts, whilst that of formic
acid seems to be in the literal sense atomic.
CHAPTER XVI.
MOLECULAR WEIGHTS AND FORMULE.
WHEN a compound has been analysed, it is usual in the first
instance to represent its composition by the percentages of
the several elements of which it is made up. Acetic acid,
for example, containsCarbon.. 40'o
Hydrogen.....               6'6
Oxygen..  53'4
in Ioo parts. The next step is to endeavour to write a,formula which, whilst expressing the same facts more compactly, gives, at the same time, the number of atoms of
the constituent elements, and fixes the relative weight of the
molecule.
If the atomic weights of the elements were all equal, the
formula would be a mere repetition of the percentages; but
since they are different, the number of atoms of the several
elements contained in equal weights must be inversely as
their atomic weights.
The simplest rule for deducing the formula of a compound from its percentage composition is, therefore, to
divide the respective quantities by the atomic weights of the
elements.




Determination of Molecular Weights.       12I
Thus, if we divide the percentages of carbon, hydrogen,
and oxygen in acetic acid by the atomic weights of carbon,
hydrogen,and oxygen respectively, we arrive at these results.*
40
I 2..    *...=3'3
6'6
53'4
3'3
6..*. 3*.
So that evidently there are as many atoms of oxygen
present as there are of carbon, and there are twice as many
hydrogen atoms. The simplest formula, then, that can be
written for acetic acid is C H,20, but whether this is to be
taken as representing a molecule of acetic acid or whether
the true formula is some multiple of this, such as C2H,02 or
C3H6,O,, remains to be decided by considerations which
now require to be examined.
VAPOUR DENSITY AND MOLECULAR WEIGHT.
According to the law of Avogadro (Chap. VI.), equal
volumes of all truet gases, irrespective of chemical composition, contain under the same conditions of temperature
and pressure the same number of molecules. It follows
from this that the weights of equal volumes must be proportional to the weights of the molecules of which the gases
are composed.
This is the principle of the only direct method for ascertaining the relative weights of these molecules.
Comparing together, for example, equal measures of
hydrogen and hydrochloric acid gases, we find their respective weights represented by the numbers I and I8'25. But
the weight of hydrogen contained in hydrochloric acid is
exactly half the weight of the same element contained in an
* See Examples and Exercises, p. I65, No. 25.
t That is gases obeying the law of Boyle.




122               Czemical Philosophy.
equal measure of hydrogen, and if we assume that there is
one atom of hydrogen in a molecule of hydrochloric acid
(and by the theory there cannot be less than one atom), we
arrive at the conclusion that the molecule of hydrogen consists of at least two atoms.  But, further, we have every
reason to believe that whilst the molecule of hydrochloric
acid contains at least one atom  of hydrogen, it does not
contain more than one atom.  When metals act upon
hydrochloric acid  the hydrogen is expelled all at once,
and not in several portions, as in the case of water and
ammonia. So that assuming the atomic weight of hydrogen
as I unit of weight, the molecule of hydrochloric acid must
weigh 36'5 units.  The weight of hydrogen equal in bulk
to this, that is to say, one molecule of hydrogen or two unit
volumes, must accordingly be 2."
So that, in order to express the molecular weights of
these gases, we must double their specific gravities, and this
rule is applied to all gaseous and volatile bodies, with the
few exceptions referred to in the next chapter.
The application of this principle to those of the elements that are volatile leads to the conclusion that many
of them, like hydrogen, consist of molecules having a
duplex structure, whilst examples are not wanting of a more
complex as well as of a simpler constitution.  We come,
in fact, to the following classification:* The student will now perceive why 2 volumes and not I volume or
3 or 4 volumes is regarded as the standard volume of molecules. One
volume would be inconvenient, because we find that the molecule of
hydrogen can be divided into two equal parts, and this would necessitate
the use of fractions. Three volumes would be incorrect, because the
hydrogen molecule is divisible into two and not into three atoms. Four
volumes would also be inadmissible, because the same bulk, that is one
molecule of hydrochloric acid, would then be represented as containing
two atoms of hydrogen and two atoms of chlorine H2CI,. Whereas it
is a matter of fact that neither the hydrogen nor the chlorine in
hydrochloric acid is divisible into separate parts.




Molecular Weights of Elements.               I23
Afonrautomic fMolecules-                     olecular
Formula.      Weight.
Mercury..             Hg.    200
Cadmium..            Cd             I12
Zinc...        Zn.      65
Diatomic Afiolecuzlcs:Hydrogen..           H2.       2
Oxygen..               02.    32
Nitrogen....    N2.    28
Chlorine.             C].    7 r
Bromine.B..                    Br.   i6o
Iodine...    I2.   254
Potassium..    K2.    78'2
Sulphur (at 9oo0).    S2       ~    64
lri-a/omic Aiolecule.
Ozone....    03         ~    48
Te'ratomzic  ffolecules 
Phosphorus...    P4.   124
Arsenic....        As.   30oo
HeTxatomic Jfoleczule 
Sulphur (at 500~)..    S6.   192
Assuming, then, the equality in volume of gaseous molecules, and knowing that the molecule of hydrochloric acid
contains an atom of hydrogen, the conclusion that a
molecule of this element consists of two atoms is inevitable. But further evidence of this fact may be obtained
from other considerations unconnected with the question
o f the constitution of gases.
NASCENT STATE.
Thus, it is well known that bodies in the nascenzt state,
that is, at the instant of their liberation from compounds,
are capable of acting far more energetically than when they
are employed in the bodily form.   Hydrogen gas, for
example, is generally incapable of combining with other




I 24            Chemizcal P/ilosop/1/.
bodies; but when materials such as zinc and dilute sulphuric acid, which are capable of yielding hydrogen, are
employed, many decompositions and combinations may be
brought about which would be otherwise impossible.
Solution of sulphur dioxide may in this way be converted
into hydrogen sulphide and water by the action of zinc and
diluted hydrochloric acid, the nascent hydrogen attacking
and combining with both the sulphur and the oxygen. An
instructive experiment, illustrating the power both of nascent
oxygen and hydrogen, consists in electrolysing a solution of
hydrochloric acid coloured with indigo.  The liquid in
the neighbourhood of both poles is bleached; at the negative, because the hydrogen there liberated combines with
the indigo and forms a colourless compound; at the positive, because the chlorine, acting on the water, disengages
oxygen, which, whilst still nascent, combines with the
elements of the indigo, producing a pale yellow substance.
Neither oxygen nor hydrogen in the ordinary gaseous form
is capable of producing these effects, which are usually
supposed to be due to the superior powers of the atoms
whilst still in the free state and before they have partly
expended their energies by coupling in pairs or otherwise
binding together in molecules.
CATALYTIC ACTIONS.
Metallic copper is incapable of expelling hydrogen from
hydrochloric acid, even when boiled with it.  But by
adding hypophosphorous acid to a warm solution of sulphate of copper, a brown precipitate of cuprous hydride
is thrown down, and this compound, in contact with hydrochloric acid, evolves hydrogen and furnishes cuprous
chloride. This reaction can only be explained upon the
assumption that it is the attraction of the hydrogen in the
copper hydride for the hydrogen in the acid, superadded to
that of the copper for the chlorine, which determines the
metathesis:




Catalytic Decompositions.         125
Cu2 /H            H.- C1 
H       +,H   H-Cl 
Cu / C1  +   H-H
2~C1           H- H
A great many reactions of a similar character are known
chiefly among compounds containing a relatively large proportion of oxygen, part of which escapes in the gaseous
form. Thus, when silver oxide is placed in contact with
peroxide of hydrogen, the silver is reduced to the metallic
state, water is formed, and oxygen gas evolved.
Ag20O + H2O2 = Ag2 + HO + 02
In like manner, permanganic and chromic acids are decomposed by peroxide of hydrogen with evolution of oxygen
gas; and in these and similar reactions it has been proved
experimentally that half the oxygen comes from the peroxide, half from the acid or other body with which it is in
contact. The conclusion seems inevitable that these two
halves of the oxygen had an attraction for each other which
was sufficient to upset the equilibrium of the unstable compounds of which they previously formed a part.
SYNTHETICAL PRODUCTION OF MOLECULES.
The cases that we have hitherto had undoer discussion
have been chiefly those of the elements.  If we have
to examine into the constitution of compounds, we have
yet several sources of information which will help in the
solution of the problem of their molecular weights and
formulae.  One very important kind of argument is
deduced from a knowledge of the mode or modes in
which the compound may be formed, and of the products
of its decomposition under the influence of reagents.
For example, oxalate of sodium is formed when carbonic
anhydride gas is passed over heated sodium. The change,
however, might be represented by either of the following
equations:Na + CO2            NaCO,
or, Na2 + 2C02  =       Na2C204




126             Chemzical Philosophy.
But the doubt, if there were any, would be resolved in
favour of the latter alternative, by the observation that
when sodium oxalate is heated it yields carbonic oxide gas,
leaving a residue of carbonate.
Na2C204 - {   Na2 CO3
The most direct and rational explanation of this is, that
a molecule of the salt contains two atoms of carbon, as
represented by the formula.
Again, succinic acid may be built up from its elements
by a succession of processes, which are briefly represented
in the following series of equations, and from them we learn,
even if no other evidence were forthcoming, that succinic
acid contains at least four atoms of carbon in a molecule,
and hence must have the formula here assigned to it.
Carbon poles ignited by the electric current in hydrogen
gas give rise to acetylene.
C2 + H2 = C2 H2
Acetylene can be made to combine with hydrogen
yielding ethylene, the known specific gravity of which gas
renders impossible any doubt as to its molecular weight.
C2H2 + H2 = C2H4
Ethylene combines with bromine thus,
C2H4 + Br2 - C2H4Br2
This dibromide may be converted into a cyanide,
C2H4Br2 + 2KCN = C2H4C2N2 + 2KBr.
Lastly, this cyanide, under the influence of boiling alkali,
assimilates the elements of water, and yields up its nitrogen
in the form of ammonia.
Cyanide of ethylene.         Potassium succinate.
CN + 2KHO + 2H20    C2H4CO 2K + 2NH




~Molecular Weczr/Its of Acids and Bases.   I27
From this salt the acid may be procured by the action
of sulphuric acid.
Cs    CO02K         H
C2H4{CO2I:. +       SO4 s~  = C2H4 (C0211)2 +,2S04
Potassium    Sulphuric     Succinic     Potassium
succinate.     acid.        acid.        sulphate,
SATURATING POWER OF ACIDS AND BASES.
In the examination of acids and bases, and other bodies.
which are capable of entering into combination readily, or
of suffering the replacement of some of their elements by
simple reactions which do not involve destruction of the
molecule, the process for determining the molecular weight
is generally easy. Suppose, for instance, it is required to
determine the molecular weight of sulphuric acid, it is only
necessary to add to it various quantities of potash or soda
to discover that there are two, and two only, distinct and
definite sulphates of potassium  and sodium, and  that
even double salts are possible, in which the two metals
figure side by side, in place of the hydrogen of the acid.
So that sulphuric acid and its salts are representable by
such formula as the following:H  SO4  K}S04  I  SO4
} SO4
Na       F $ N_ } SO4
Basic derivatives of ammonia are dealt with in a similar
way. If we assume that the molecule of hydrochloric acid
is represented by the symbols HCI(= 36'5), the problem
is to find what weight of base will enter into combination
with 36'5 parts of hydrochloric acid so as to produce a
neutral compound.  In the case of ammonia itself, the
compound formed with hydrochloric acid is represented by
the formula NH3HC1, in which NH3 stands for I7 parts
by weight of ammonia.  In some cases, it is practically




I28             CZhemical Philosophy.
more convenient to prepare the double salts which the
hydrochloride of ammonia and all similarly constituted
compounds form with platinic chloride. Ammonio-chloride
of platinum has the formula 2(NH3'HCI).PtCJl, and that
quantity of the basic body which, in this compound, is
capable of taking the place of ammonia, NH3, is generally*
taken to be its molecular weight.
SUBSTITUTION COMPOUNDS.
In other cases, especially when the compound under
examination is neutral, and incapable of entering into
combination with other bodies of known molecular weight,
the results of " substitution" afford information in the
direction required. To take an instance, the hydrocarbon
benzene is a neutral liquid, neither acid nor basic, which
forms no compounds whose constitution throws any light
on the question of its molecular weight. It is volatile, and
its vapour density would tell all that we require to know;
namely, that to represent a molecule of it, we must use the
formula C6H6; but even if we had not this evidence to
appeal to, the same result would be indicated by the composition of the products which are formed from it under
the agency of chlorine.  Acted upon in this way, benzene
yields the following series of compounds:C6H6
C6H5C1
C6H4C12
C6H3C13
C6H2C14
C6HC15
C6C16
From these formulae, it is clear that the operation consists in the removal of successive atoms of hydrogen, and
* There are some exceptions, e.g., polyamines, which cannot be discussed here.




Molecular Formula.             i29
their replacement by atoms of chlorine; and consequently
that a molecule of benzene is formed of six atoms of carbon
associated with six atoms of hydrogen. This is a method
o'r investigation very frequently resorted to among carbon
compounds, and one of which examples will readily occur
to the student.
But notwithstanding the multiplicity of the rules which
serve to guide chemists in the selection of formule whereby
to represent molecules, there still remain a large number of
bodies which cannot be dealt with by any method at present
known. Hence many of the formulke commonly accepted
and employed in chemical works are at best expressions of
mere guesses enjoying various degrees of probability. Many
difficulties occur, for example, among metallic compounds.
The formula Cr03 is, generally used for chromic anhydride,
not on account of any direct evidence in favour of it, but
because of the existence of a volatile oxychloride CrOCl.
and the analogy of these two compounds with sulphuric
anhydride, SO3, and the corresponding oxychloride SOC1,,
also on account of the isomorphism of the chromates and
sulphates. Again, potassium permanganate is sometimes
expressed by the formula KMnO4, which recalls its isomorphism with the perchlorate KCIO4, but the double formula
K2MnO,8 has something to recommend it as satisfying the
law of even numbers.* So also doubts exist as to the
correct mode of representing salts like ferrous and stannous
chlorides, there being some probability that the usual
formulae FeCl2 and SnCI, should be doubled in order to
represent the reacting units or molecules of these compounds.
* See Chap. XIX.
K




I30             Chemical Philosophy.
CHAPTER XVII.
DISSOCIATION.
A LUMP of solid ammonium carbonate, or a solution of the
same salt, exposed to atmospheric air at common temperatures, loses ammonia, which escapes together with the
vapour of water, and the solution, originally alkaline,
becomes after a time neutral. The change may be thus
represented:
(NH3)2. o H2.Co   =  NH3.OH2CO2 +  NH3
Ammonium Carbonate.   Ammonium
Bicarbonate.
Or when a solution of ammonium oxalate, chloride or nitrate
is boiled, a similar escape of ammonia may be observed,
and the liquid acquires a distinctly acid reaction.
(NH3)2H2C204  =  NH3H2C204 +  NH3
Ammonium         Ammonium
oxalate.       acid oxalate.
And if in either of these cases the ammonia thus evolved
is brought into contact at a lower temperature with the
solution from which it was produced, it will enter again
into combination, and the original compound will be rege nerated.
Again, when calcium carbonate is heated strongly in a
vessel from which the air has been more or less completely
removed by the air-pump, it suffers decomposition into lime
which remains behind, and carbon dioxide gas which fills
the vessel, CaCO3 = CaO + C02, and this decomposition
proceeds until the evolved gas acquires a certain density or
tension, which increases as the temperature rises. If now
the whole is allowed to cool, the carbonic acid gas slowly
recombines with the lime, and a vacuum is once more
established.  Calcium  carbonate splits up in the same
manner when a current of steam, or other gas into which
the carbonic anhydride can diffuse, is passed over it whilst
under the action of heat.




Efflorescent Salts.                 13 
Decompositions like that of the ammonium  salts or of
calcium carbonate under the influence of heat, are examples
of what is known as dissociatiozn, or, as it is sometimes
more precisely termed, thermolysis.   The word dissociation has acquired by careless use some degree of ambiguity,
but in these pages its application will be restricted solely to
those cases of decomposition in which certain bodies are
resolved at an elevated temperature into simpler bodies,
which are capable of reuniting and reproducing the original
compound when the temperature is again allowed to fall.*
We shall now consider several cases of dissociation.
A crystal of sulphate of copper remains unaltered at
ordinary temperatures in moist air; but if moderately heated,
it loses its water of crystallisation, and crumbles down to a
white powder.
The same kind of decomposition takes place in many
cases at the temperature of the air, and under ordinary
conditions. Salts, which thus readily part with their water
of crystallisation and fall away to powder, are said to be
efforescent.  Sulphate, carbonate, and phosphate of sodium
afford examples of this kind of dissociation, which, however,
presents nothing remarkable beyond the fact of occurring
at comparatively low temperatures. In this respect, however, these phenomena are surpassed by those exhibited by
salts, which, under ordinary circumstances, are deposited
from solution void of water of crystallisation. As already
mentioned (Chapter II, p. I2), such compounds have been
found in every instance to combine with water when crystallised at temperatures below zero. The explanation of the
existence of anhydrous crystals is simply that dissociation of
the salt from the water occurs at or below the temperature at
which the crystals are deposited. And even those salts which
usually combine with water of crystallisation may be obtained
* The decomposition of ferric salts and other similar compounds, when
heated with water, ought not to be represented as cases of dissociation,
being in reality the effects of mass. See Chap. XIII.
K 2




132             C(zhezical Philosophy.
in a lower state of hydration, or altogether destitute of water
by causing them to crystallise at more or less elevated
temperatures. Sodium sulphate furnishes a case in point. The
crystals of this salt formed at ordinary temperatures contain
ten molecules of water of crystallisation combined with one
of the salt (Na2 SO,. I oOH2); when formed at I8~ they
contain seven molecules of water (Na2 SO4,.70H,), whilst a
solution heated to 340 yields crystals which contain no
water at all. Similar phenomena are beautifully exhibited
when the solutions of some coloured salts are heated.
Chloride of cobalt especially lends itself to this kind of
reaction. This salt forms crystals consisting of CoCI, +
60H,, and when dissolved in water it gives a red solution;
but if the temperature is raised even very slightly the liquid
changes in colour, becoming successively purple and blue,
and these changes correspond with the formation of the
compounds CoCl, + 40H, and CoCI, + 20H, respectively.
When the temperature is again allowed to fall, the solution
reassumes its original red, colour. This sort of dissociation,
however, is not confined to water presumably subsisting as
such in crystallised salts. Cases are by no means unknown
in which a more profound decomposition is effected by the
same agency of heat.
Thus, sulphuric acid of any strength, whether containing
excess of water or excess of sulphuric anhydride, when
evaporated till a liquid of constant composition is obtained,
leaves, not pure hydrogen sulphate, H2SO4, but a mixture
containing the elements of that body with about Ir per
cent. of water. And if pure hydrogen sulphate is heated
to between 300 and 400o~, it gives off vapours of SO3, so as
gradually to become reduced to the condition of common
oil of vitriol, the stable hydrate referred to above.
Furthermore, it is found that the specific gravity of the
vapour of this compound is only about one quarter and not
one half of the molecular weight as represented by the
formula HISO, that is instead of being, in accordance




Dissociation produced by Heat.       133
with the general rule (Chap. XVI, p. I 2 I), 49 times as heavy as
an equal volume of hydrogen taken at the same temperature,
it is only 4 2 241 times as heavy, or thereabouts.
The physical explanation of these phenomena is to be
found in a theory which has been already partly discussed
(Chap. XIII.). We have every reason to believe that the
phenomena observed are the results of two opposite and
reciprocal actions: the one of decomposition, the other of
recombination proceeding simultaneously. At low temperatures, when the composition of hydric sulphate is accurately
represented by the formula HSO4 or H20, SO,, the number
of molecules decomposed into HO and SO, is exactly
counterbalanced by the number of molecules which are
reconstituted in the same period by the reunion of these
two substances. As the temperature rises and the agitation
of the molecules in the mass becomes more vigorous, the
number of molecules which undergo decomposition progressively increases, whilst the recombination pari passu
continually decreases, till at length a point is reached in
which the SO3 molecules, and the OH2 molecules become
indifferent to each other, and recombination no longer takes
place. When the temperature is allowed to fall these processes are reversed, and for every degree of temperature a
certain definite relation subsists between the decomposition
and recomposition, so that a kind of equilibrium is maintained. The compensating process being at high temperatures annulled, decomposition is complete, the vapour
consists throughout of a uniform mixture of two different
kinds of molecules, and consequently, by the law of
Avogadro, it occupies twice the volume it would otherwise
fill if dissociation did not take place.  The chemically
reacting unit of sulphuric acid H2SO4 is apparently incapable of subsisting in the state of vapour, and thus no
conclusion respecting the chemical molecule of this body
can be drawn from the specific gravity of its vapour.
Ammonium chloride furnishes another instance in which




134            CChemical Philosophy.
the usual relation between the vapour density and molecular
weight is not preserved. The specific gravity of the vapour
of ammonium chloride is only about i 3 instead of 27 as we
should expect it to be. This anomaly is explained by the
assumption that the vapour evolved by ammonium chloride
is in reality formed of a mixture of ammonia and hydrochloric acid, which occupy double the volume they would
fill if bound up together in a single molecule undivided by
heat.
In like manner, under ordinary circumstances, phosphoric
chloride dissociates into phosphorous chloride and free
chlorine, and calomel in vapour becomes a mixture of
mercuric chloride and mercury.
That the vapour densities of all these compounds are
less than they should be according to theory, or, in other,words, the vapour-volumes of their molecules are greater
than the volume occupied by all other vaporisable molecules, is a fact which cannot be disputed. That these
anomalies may be ascribed to dissociation is also admitted
on all hands, but the direct proof that dissociation has taken
place in a given vapour is by no means easy to supply.
In the case of sulphuric acid and of ammonium chloride,
advantage has been taken of the difference in the diffusibility of the products into which these bodies are supposed
to dissociate. Thus the vapour of sulphuric anhydride is
much heavier and consequently less diffusible than vapour
of water, so that when sulphuric acid is heated for several
hours in a vessel with a capillary orifice, the water vapour
escapes more rapidly than the sulphuric anhydride, and the
latter gradually accumulates in the residue. Pentachloride
of phosphorus has also been resolved into free chlorine
and phosphorus trichloride, which may be to some extent
separated by diffusion. Phosphorus pentachloride has been
found to possess a normal vapour density when mixed with
a sufficient quantity of the trichloride to prevent dissociation.
In the case of sublimed calomel, we are enabled to con



Dissociation of Vapours.                    I35
vict this substance of having submitted to dissociation
whilst in the state of vapour, by appealing to the fact, well
known to manufacturers, that it invariably contains small
quantities of corrosive sublimate, and sometimes of metallic
mercury, which have escaped recombination during the
cooling down of the vapour.
The following is a list of the most important cases of
vaporous dissociation:Vapour
Nature of decomposition of dissociated Molecule.  Volumer
Phosphoric chloride. Phosphorous chloride.  Chlorine.    vols.
PC15       =      PC13        +        C12          4
Nitric oxide.
N202            (See Chapter XIX.)                   4
Nitric peroxide.
N204       =        NO2        +       NO2
(See Chapter XIX.)
Chloric peroxide.
C1204                                                3?
Sulphuric acid.       Water.        Sulphur trioxide.
H2SO4      =         20        +       SO3           4
Ammonium chloride.    Ammonia.       Hydrochloric acid.
NH4C1       =       NH3        +        HC1          4
Ammonium sulphydrate.    Ammonia.      Hydrogen sulphide.
NH4HS       =       NH3         +       H2S          4
Isoamylic iodide.    Amylene.         Hydriodic acid.
C5H111  CsH10                  +        HI           4
CHAPTER XVIII.
TYPES.-ATOMICITY.
BODIES which are capable of entering into chemical
reactions in the same manner, giving rise under similar
circumstances to new products, having similar properties,
are said to belong to the same type. The idea that the
chemical constitution of all known- bodies is modelled




I36             Chemical Philosophy.
upon a certain limited number of types supplies a means of
classifying them according to their modes of transformation. Thus water is capable, in a variety of ways, of
exchanging its oxygen and hydrogen for other elements
and groups of elements (compound radicles), and the
bodies which result from these exchanges retain, more or
less perfectly, the chemical deportment of water, and are
said to belong to the water type. Caustic potash, for
example, is referred to the water type because, like water,
it is capable of exchanging its oxygen for sulphur, and its
hydrogen for a metal, for an elementary atom like chlorine,
or for a group of atoms, such as ethyl or acetyl. As a
memorandum of the correspondence in these transformations, the two series of derivatives may be formulated in a
similar manner. Thus:
HIS                   K
H                   Kc i o
H                    K
C21130               C2H30 } O
&c.                  &c.
Formule of this kind are often spoken of as rational
formulae. But water is not the only type.  Hydrogen,
hydrochloric acid, ammonia, and marsh-gas are other
bodies which are often referred to as types of decomposition, each of which is imitated, more or less closely, by
a considerable number of elements and compounds. The
student will readily perceive that the list of bodies which
might, for special purposes, be selected as types, may be
extended and varied almost indefinitely.
This plan of registering in the formula some of the facts




Rational Formule.              137
which have been observed as to the possible transformations of a body, has led some chemists to infer that it is
possible to represent symbolically the relative positions
occupied by the atoms contained in molecules.  The
student will do well to approach such an idea with caution.
Rational or descriptive formule of various kinds are
valuable not only as memoranda of the possible modes of
formation and decomposition of bodies, but are positively
necessary to enable us to distinguish from one another
bodies having the same ultimate composition, but different
properties. But we know very little regarding the essential
nature of molecules, and still less of their constituent
atoms; and it is, to say the least, premature to attribute to
these formulae a meaning which has so little to support it.
Sulphuric acid furnishes a very good example of the kind of
fact and argument upon which rational formulae are based.
In this case, mere analysis tells us only that the compound
contains an atom of sulphur, two atoms of hydrogen, and
four atoms of oxygen, or SH204. But on examination of its
salts, we find that both the atoms of hydrogen are replaceable by metals; and to indicate this basic function of the
hydrogen, it is the custom to write it at the beginning of
the formula, thus,
H2SO4.
When sulphuric acid is distilled with phosphoric chloride,
it yields two products having respectively the formulae
SO3HCl      and       S02C12.
The former of these bodies results from the removal of an
atom of oxygen and an atom of hydrogen from the sulphuric
acid, whilst an atom of chlorine is taken up.
SO4H2 - OH + C1 = SO3HC1.
And this exchange is repeated when the second derivative
is produced.
SO3HC1 - OH + C1 = SO2C12.




138             ChzeCnical Philosophy.
Either of the new compounds will reproduce sulphuric
acid when dissolved in water. It seems, therefore, that
sulphuric acid is capable of breaking up into the groups
SO, and 2HO. And this is confirmed by the fact of the
production of sulphates by the union of sulphur dioxide
with peroxides, as in these instances:PbO2    +      S02    =       PbO2SO2
Lead peroxide.    Sulphur dioxide.  Lead sulphate.
(HO)2    +      SO2    =    (HO)2SO0
Hydrogen       Sulphur       Hydrogen sulphate,
peroxide.      dioxide.      or sulphuric acid.
Such reactions as these and many others are recalled
when we write the formula
HOSO
HO } 
or,
HSO
H}O
in which it may be regarded as a derivative from two
molecules of water, each of which has lost an atom of
hydrogen, so that the two residues are united together by
the group SO2.
-But there are many cases in which we write such descriptive formulae with the utmost confidence that they express
possible reactions, although such reactions, for various
reasons, may never have been observed.
The formulae that we now make use of are, to a great
extent, based upon certain assumptions regarding those
chemical properties of atoms which are referred to under
the name " atomicity."
In the formulae
ClH     OH2    NH3    CH4
we see one atom of chlorine combined with one atom of




Valency and A tomicity.            139
hydrogen; one atom of oxygen with two of hydrogen; one
atom of nitrogen with three of hydrogen; and one atom of
carbon with four atoms of hydrogen; and no compound is
known in which one atom of either of these elementschlorine, oxygen, nitrogen, or carbon-is united with a
larger,quantity of hydrogen than is represented here.
This difference of combining capacity is further illustrated
by the fact that when chlorine is made to act upon water,
ammonia, or marsh-gas, the hydrogen contained in one
molecule of each of these compounds is distributed into so
many separate molecules of hydrochloric acid. Thus,
|+ |   |    give         +
2 volumes of    2 volumes of  I volume
water vapour.    chlorine.   of oxygen.
OH2          C1             0         4 volumes of
hydrochloric acid.
2HCL
Again,
NH3   +    3C1   =    3HCl   +    N
2 Volumes.  3 Volumes.    6 Volumes.  I Volume.
The following succession of changes indicates the same
thing in the case of marsh-gas:I.   CH4   +      C12   =   CH3Cl  +   HC1
2 Volumes.   2 Volumes.    2 Volumes.   2 Volumes.
2.  CH3Cl  +    C12   =   CH2CI2  +   HC1
3- CH2C12  +    C12   =   CHCI3  +   HCl
4. CHC13  +    C12   =    CC14   +   HC1
Thus the four atoms of hydrogen which, by the carbon
in the marsh-gas, were united together into one molecule,




I40            Czhemical P/zilosopzy.
measuring 2 volumes, are separated into four molecules or
eight volumes of hydrochloric acid.
H  C1
H Cl
| C | H4 |    yields
H C1
H C1
In these cases, therefore, the combining value of an atom
of chlorine is equal to that of one atom of hydrogen; it is
univalent. We summarise these relations when we say that
chlorine is a monad, because we never find it linked to more
than one atom at a time. Hydrogen must also be a monad,
for in the production of the compound HC1, whatever is the
attraction of the chlorine for the hydrogen, there must be an
equal attraction on the part of the hydrogen for the chlorine.
As the result of similar observations, we find that the
atom of oxygen is diad, of nitrogen triad or pentad, of
carbon tetrad, the combining capacity or " valency " being
in each case measured by the number of univalent atoms,
such as hydrogen or chlorine, which one atom of these
elements can respectively combine with or replace.
The valency of the elementary atoms, though varying, is
limited, and in different cases attains a different maximum.
The term " atomicity " is employed to indicate the greatest
number of atoms of one kind or another with which a given
atom is ever observed to be united. To what the variation
of valency may be due it is impossible to say in the present




A4tomicity  of Elements.                 141
state of knowledge; it seems, at any rate, to have no very
obvious relation either to the atomic weight or to the
chemical energy of the element.*
The atomicity of a few of the rare elements is still in
obscurity, in consequence either of doubts regarding their
atomic weights or a want -of knowledge as to the composition of their compounds; but those which have been sufficiently studied admit of classification into the six divisions
displayed in the following table:ATOMICITY OF THE PRINCIPAL ELEMENTS.
NON-METALS.
Monads.   Diads.    Triads.   Tetrads.  Pentads.  Hexads.
F         O          B         C         N          S
1          ~Cl   i~ Si               P         Se
B r_                         _ _  _   _ _ _ _
Te
METALS AND METALLOIDS.
Ag        Hg        Au       Ir Ro  f   V        Os Ru
Cu             -
In      Pt Pd       As         W
Sb        Mo
H         Cd        TI
Li        Zn                   P b       Bi        U
Na        Mg
Sn                  Cr
K         Ca                  Ti         Ta        Mn
Rb        Sr                  Zr         Nb        Fe
Cs        Ba                   Al
Co
Ni
* Many facts seem to point to the conclusion that there is no absolute
measure of atomicity. The capacity of saturation of a given atom
depends upon the nature of the elements with which it is associated.




I42                Chzemical PhZilosophy.
An arrangement of this kind necessarily involves the
separation of many elements which, in properties, are closely
allied together, and the association of others which have
very little in common.  Thus we have to look for thallium
among the triads, although it has strong points of resemblance on the one hand to the alkali metals, and on the
other hand  to lead.  Oxygen, again, is separated from
sulphur, aluminium from chromium and iron, lead from
barium. This part of the subject will be again adverted to
in a later chapter.
Any portion of a molecule which is capable of being
detached and transferred to some other molecule by way of
decomposition is called a " radicle," whether it consist of a
single atom or of a group of atoms. The term " compound
radicle " is, however, not usually applied to a group unless
it makes its appearance in several different bodies. Compound radicles present different degrees of quantivalence,
just as do the atoms of which they are built up, so that they
are capable of linking together various proportions of other
elementary or compound radicles. This fact may be experimentally verified by such reactions as the following:Ordinary disodic phosphate is alkaline to test-paper,
silver nitrate is neutral. When these two salts are mixed
An atom of sulphur can take up no more than two atoms of hydrogen,
but it is capable of forming a compound with four atoms of chlorine, or
with three atoms of oxygen. In like manner, phosphorus forms the
compounds PC13 and PCI5, but its affinity for hydrogen extends only to
three atoms, PH3, though a fourth may be taken up if accompanied by
an atom of iodine - PHI. It has also been observed that the chlorides
corresponding with the highest oxides of many of the metals have not
yet been produced, and seem to be incapable of existing. Thus there
are the oxides CrO3, U03, As2Os, Ni203, but the chlorides Cr2C16,
UC15, AsC13, and NiC12 indicate the limits of the capacity of these
metals for chlorine. It is interesting to notice that in some cases in
which the chloride is missing, the corresponding fluoride is known.
The fluorides CrF6 and AsFs, the representatives of the unknown
chlorides CrCl6 and AsC15, have been described.




Valency of Cominpound Radicles.        I43
together in equivalent proportions a yellow precipitate of
phosphate of silver is thrown down, whilst the liquid
becomes strongly acid. The reason of this is apparent
when we express the metathesis in the form of an equation.
AgNO3
N2 PO    +    AgNO3          Ag)            Na4  N
One molecule.   Three molecules.    ne molecule   Two moleesof
One molecule.   Three molecules.   One molecule   Two molecules of
of silver phosphate.  sodium nitrate.
HNO3
One molecule
of nitric acid.
The group PO is trivalent, and so it holds together the
two atoms of sodium and one atom of hydrogen in one
molecule. But when the interchange occurs, these become
respectively united with three (NO3) groups, each of which
is univalent, and incapable of connecting itself with more
than one atom at a time. Three new molecules result, one
of which is nitric acid, the presence of which can be recognised by test papers.
The compound resulting from the union of two or more
atoms is called a saturated compound, when the atomicity
of each atom present is satisfied. This condition is fulfilled when one atom of a monad is combined with another
monad, or when two atoms of a monad combine with one
atom of a diad, or three of a monad with one of a triad,
four monad or two diad atoms with one tetrad, and so
forth.
Examples of various orders of compounds are shown in
the following formulae, in which the combining power of
each atom present is supposed to be neutralised by that of




I44            Chemnical Philosophy.
other atoms. In the notation here introduced, it must be
understood that the symbol placed on the left of a formula
represents an atom to which all on the same line are
directly united.  Those also which are connected by a
bracket, are united together. Thus,
CH3
CH3
means that there are two atoms of tetrad carbon united by
one-fourth of their combining power, whilst each retains
three atoms of hydrogen. The same relations are expressed
in this figure, or " graphic formula."
H H
I   I
H-C- C-H
H H
Of course the lines connecting the symbols are not
designed to represent any substantive bond or link, but
merely indicate the manner in which the combining capacity
of each atom is disposed of.
EXAMPLES OF CONSTITUTIONAL AND GRAPHIC FORMULiE.
Hydrochloric acid.
HC1       or         H —C1
Water.
OH2   or    H-O-H
Copper sulphide.
CuS      or         Cu-S
Hydrogen peroxide.
OH  or   HO —O-H
OH
C1
Phosphorie chloride.        |
PC15     or       C1 —P -C1
C1   C1




Graphic Formula.                I45
C]
Stannic chloride.         I
SnC14      or      Cl-Sn —C1
I
C1
O       0
Phosphoric oxide.     11      11
PO2      or        P- O-P
0                  II       11
P102                0       0
Acetic acid.         H 
CH3                  I   II
CHsH —C —C —O —H
CO(OH)          H-C-C-O-H
H
Aluminic chloride.        C1 C1
(A1Cl3 a1 
jAlCl3     or     CI-AI-A1-C1
{Al l3               II    I
C1 C1
Sulphuric acid    H- sO       
SO,2(OH)2  or              S
H-O/ % O
Potassium dichromate.  0        0
CrO2(OK)              11      II
O          or K-O-Cr-O -Cr-O- K
CrO2(OK)              II      II
0       0




146            Chemical Philosophy.
CHAPTER XIX.
UNSATURATED COMPOUNDS.
WHEN several compounds are formed by the union of two
elements in different proportions, it is very commonly
noticed that the change of valency or combining capacity
of the central atom to which the rest may be supposed to
be attached, takes place by pairs of units.
Thallium, tin, and phosphorus, for example, each form
two chlorides; nitrogen combines with three atoms of
hydrogen in ammonia, and with four atoms of hydrogen
and an atom of chlorine in chloride of ammonium. Sulphur
also yields compounds, in which one atom of that element
is combined with two atoms of hydrogen, with two atoms
of oxygen, and with two atoms of oxygen and two atoms
of chlorine. The formulke of these compounds are represented as follows:
TlCI  SnC]2  PC]3  NH3    SH2
TlCI,  SnC1l  PCI5  NH4C1  SO,
s 02,CI2
It will be observed that the difference in the first three
cases amounts to two atoms of chlorine, which represent
two atoms of hydrogen, the unit of valency. In the fourth
case, one atom of hydrogen and one atom of chlorine have
been added.  In the fifth, the oxide SO2 may be taken
to represent a hypothetical hydride SH4, whilst the oxychloride corresponds with the unknown compound S H,.
In each series, the advance in combining power is equivalent
to the assumption of two atoms of hydrogen. And so it IS
in a large number of other cases.
It seems, therefore, that, as a general rule, the index of
valency of any given atom is either an even or an
odd number; or, as it has been expressed, elements are




Xitric Oxide and Peroxide.          147
uniformly either "artiad" or "perissad." *  So that in all
saturated compounds, and in the great majority of unsaturated compounds, the sum of the indices of valency of all
the atoms present is an even number.
But there are not wanting exceptions to these statements;
and although the number is at present not great, the marked
characters of these exceptions is sufficient to destroy much
of the apparent significance of those more numerous instances which conform with them.
The following are some of the most notable instances:Nitric Oxire. — This most remarkable compound is a
colourless unliquefiable gas, almost insoluble in water, and
unchanged by heat.  It exhibits all the characteristics of
an unsaturated compound.  Thus it unites with oxygen,
with chlorine, with sulphuric anhydride, and with many
metallic salts.
It is composed of 14 parts of nitrogen with I6 parts of oxygen; its specific gravity is I5 (H = I), and consequently
its molecular weight is 30.  It therefore contains one atom
of nitrogen (perissad), combined with one atom of oxygen
(artiad), and thus it breaks the law of even numbers. This
difficulty might be avoided by employing the double formula,
N202 or N _ O
N =O
but that its specific gravity and iicondensability point conclusively to the simpler expression NO as the symbol of its
molecule.
Vitric Peroxide.-Below 9~ this compound seems to be
a colourless liquid, which solidifies at very low temperatures. But it cannot be volatilised without more or less
complete dissociation. At the lowest temperature, 40' 2, at
which its vapour has been examined, the vapour density
was found to be 2'588 (air = I), whilst the formula N04.
requires the vapour density to be 31 786.
d cprtos even, and reptwcr6s odd.
L 2




14( Chemical Philosophy.
When the temperature of the vapour is gradually raised
its orange colour deepens, till at about I80o it becomes
almost black, and the density is then I'589.  In this
condition the formula NO, represents two volumes of the
gas. This difference of constitution at high and low temperatures is further indicated by the fact that nitric peroxide
does not combine with chlorine in the cold, although it
does so when heated.
Uranium  Pentachloride.  (UCI5 Roscoe).-This compound has recently been described, but as it cannot be
converted into vapour without decomposition, its molecular
weight is unknown. It is not improbable. therefore, that
the molecule of the solid may really be
U2C110 or UC1,5
UC15
in which case it would present nothing unusual from the
point of view now under consideration.
Tungsten  and Miolybdenum  Pentachlorides, WC15 and
MioC5. —These  compounds afford very remarkable instances of the association of an artiad atom with an uneven
number of perissad atoms, and consequent infraction of
the law under discussion. Hexachlbrides of both these
elements, corresponding with their trioxides, exist; but
these chlorides when heated are split up into pentachlorides
and free chlorine. The pentachlorides are volatile without
decomposition, and the vapours exhibit normal densities.
Although nearly all unsaturated compounds are capable
of entering into combination, and many of them perform
the part of well-defined radicles, it does not follow that all
radicles should be capable of isolation, and the definition
of a radicle (p. 142) by no means involves this idea. On the
contrary, we have already examined phenomena (pp. I 24, 1 25)
which indicate that when a radicle of uneven quantivalence,
such as H, C1 or N is liberated from combination, its atoms
combine in pairs, and thus satisfy each other's attractions,




Compound Radicles.              i49
unless they find themselves in the presence of other radicles
with which they can immediately unite. Compound radicles
resemble elementary atoms in this respect. None of the
following groups, for example, are known in the free state,
the formulae  representing semimolecules, or what may be
termed chemical atoms of these radicles.
Hydroxyl (OH)' contained in acids, alcohols, and metallic
hydrates.
Potassoxyl (OK)' contained in potassium oxysalts.
Cyanogen (CN)' contained in cyanides.
Ammonium (NH4)' contained in the salts of ammonia.
Arsendimethyl (AsCH3) in kakodyl and its compounds.
Methyl (CH3)' in methylic alcohol and derivatives.
Amidogen (NH2)' in primary amines and amides.
Methenyl (CH)"' in chloroform and similar bodies.
When displaced from any of their compounds they do
not remain isolated, but unite in pairs, producing molecules
which in some cases are stable enough to maintain an
independent existence.
We have for example,
Radicle Name.    Corporate Name.
Hydroxyl. Hydric Peroxide (OH)2 or 0,H,
Potassoxyl. Potassic Dioxide (OK), or O0K1
Cyanogen. Cyanogen Gas  (CN)2 or C2N2
Arsendimethyl  Kakodyl  (AsCH,3  or As2(C,),
Methyl. Ethane         (CH3)2 or C2H6.
Methenyl. Acetylene      (CH)2 or C2H2.
The only free monad radicles known are the two bodies
already described, namely, nitrosyl, or nitric oxide, NO,
and nitryl or nitric peroxide, NO2. Instances of free diad
radicles are, however, more numerous. Thus we have,
* The dashes serve to indicate the usual valency of each group.




150 o           Chemical Philosophy.
Mercury..         Hg"
Cadmium...      Cd"
-Cairbonic oxide (carbonyl).  (CO)"
Sulphur dioxide (theionyl).   (SO2)"
Ethene            ~..      (C2H4)"
Ammonia...      (NHIL)"
Molecules of this kind are of the same order as those
referred to at the beginning of the chapter, but why in so
many cases the number of unemployed units of valency
should be an even number, has not yet been satisfactorily
explained.
An element in a free or unsaturated state may be compared to a body which has been raised to a height. In order
to lift a body into an elevated position energy in some
form must be expended, but the whole of that energy is
recoverable in the form of heat or mechanical force when
the body descends to its former level.  It is just the same
with a chemical element. Mechanical force, or its equivalent in the form of heat or electricity, is consumed *when
a chemical compound is resolved into its constituents, and
when these constituents come together again the same
amount of energy is called into action.
A body in an unsaturated state, then, like a stone on the
roof of a house, possesses a store of potential energy which
may at any time be called into activity.
Accepting the definition of a " radicle " given in the last
chapter, it is obvious that there are a great many commonly
recognized radicles which can hardly be expected ever to
assume a bodily existence apart from the compounds in
which they occur associated with elements of a different
chemical character.
In all the carbonates, for example, a group consisting of
one atom of carbon and three atoms of oxygen occurs, and
this group is capable of being exchanged for C12 or (OH)2
or O by double decomposition. It is, therefore, entitled




Compound Radicles.                 15 
to be spoken of as a compound radicle, although, by reason
of the large proportion of oxygen it contains, its condition
would be that of unstable equilibrium, even if it could
assume temporarily an isolated existence.
Similar remarks apply to such radicles as NO3 (of
nitrates), C103 (of chlorates), S04 (of sulphates), P04 (of
phosphates), and the rest, which under possible experimental conditions, have never yet been isolated.
But, after all, it is necessary to remind the student that
our system of notation is to be understood and employed
only in a unitary sense. Every molecule is, with reason,
regarded as one entire and undivided unit, whose actions
and reactions proceed, not from the affinities of this or
that element contained within it, but from the resultant of
all the different forces, exerted by its several constituent
parts.
A chemical compound may be compared to a musical
chord, constituted, doubtless, of many and complex elements, but communicating to the ear the impression of
singleness and harmony.
The doctrine of radicles, no less than that of atomicity,
and the graphic notation founded upon it, is at present to
be regarded solely in the light of a convenient, but not
absolutely necessary, system of recording and comparing
facts concerning the changes of composition to which
bodies are subject under the influence of chemical
attraction.
Note.-The following memoranda will serve to assist the
student in writing the formulae of many common salts. In
order to construct any required formula it is only necessary to
place a symbol or group of symbols, taken from under the
positive sign, side by side with a symbol or group taken from
under the negative sign, and to adjust the quantity of each so
as to comply with their respective habits of combination.
Thus let
R' represent a univalent radicle,
R",,, bivalent radicle,
R"',,,, trivalent radicle, etc.




152              Chemical Philosophy.
Then it is only necessary to remember that
R' combines with R',
2 R'              R",
or R",,,,
3R'               RI R"
3 R",,,, 2 R"', etc.
In this way the student will readily learn to compose the
unitary formulae of all the most commonly occurring compounds,
without risk of falling into any serious error.
_ _ W__ _                    __    0
~H   0 
+     t4
-t M
Wge - D




153
CHAPTER XX.
ISOMERISM.
SEVERAL bodies, though differing more or less in properties,
may have the same composition. In such cases they are
said to be isomeric.   The differences observed among
isomeric bodies sometimes extend only to their physical
characteristics, sometimes to their chemical properties.
Several cases require therefore to be considered.
PHYSICAL ISOMERIDES.
Sulphur, when crystallised from carbon disulphide, yields
rhombic octahedra, the specific gravity of which is 2o07;
whereas, if melted and allowed to cool, it crystallises in
oblique rhombic prisms, having the specific gravity r'98.
The prismatic variety soon changes spontaneously into the
octahedral, which is the stable form, at the same time
evolving heat.  Sulphur is said to be dimorp/ozuls, as it
crystallises in two forms, and these two modifications are
often spoken of as allotropic states of the element.
Many other examples might be cited of the same substance assuming different crystalline forms, the change of
structure being almost invariably attended by differences of
specific gravity and solubility. One form is generally less
stable than the other, and sooner or later, especially under
the influence of change of temperature, is converted into
the permanent variety.
When, as in these cases, two bodies chemically alike
exhibit slight differences of physical characters —such as
solubility, crystalline form, specific gravity, or action upon
light-they may be regarded as one and the same substance, though more or less disguised, and such bodies may
be distinguished as physically isomzeric.
Examples of physical isomerism are not difficult to find
either among mineral or carbonaceous compounds. Thus




I 54            (YzChemical Philosophy.
we have the two varieties of native calcium carbonate,
arragonite and Iceland spar, as well as the curious instance
of change of colour accompanying change of crystalline
structure in the two modifications of mercuric iodide;
whilst among carbon compounds, the varieties of tartaric acid
(described in Chapter XII., p. 74) may be referred to, beside
many members of the numerous class of hydrocarbons
called terpenes. These terpenes (formula CIOH16) constitute the chief ingredients in the essential oils of turpentine,
lemon, orange and bergamot, and others. They have the
same composition, and under the influence of chemical
agents yield similar compounds, but they differ in odour,
boiling point, and rotating action on a ray of polarised
light.
The difference in all these cases probably arises not from
any difference in chemical composition or constitution, but
from the various modes in which the molecules of the bodies
are associated together; such modification being connected
with some peculiarity in the circumstances attending their
formation.
That a change of molecular structure is accompanied by
a corresponding change of physical characters, is proved by
the well-known fact that when a piece of glass is strongly
compressed, either by mechanical means or by suddenly
cooling it from a high temperature, it acquires the power of
polarising light in a manner which it loses when the pressure
is relieved.
It has also been found that the rotatory power of solids,
when in solution, varies with the nature of the liquid (itself
optically inactive) in which they are dissolved. This is in
all probability due to the formation of molecular combinations, of various degrees of complexity, between the liquid
and the dissolved substance. (See Chap. II., p. Io)
POLYMERIDES.
Bodies containing the same elements united in the same
proportion, but having different molecular weights, belong




Polymzerism.                    I  5
to this class of isomerides.   The following are some examples:
Sulphur boils at 440o, and is converted into an orangecoloured vapour, the density of which, when taken at about
5000, is three times as great as it should be theoretically;
whilst at Io000 it is only 32 times as great as that of
hydrogen at the same temperature and pressure, thus confbrming with the ordinary rule. There seem, therefore, to
be two varieties of the gaseous sulphur molecule, one of
which is polymeric with the other. They may be represented by the formula
S, and S6.
Ozone, the molecule of which has been shown by various
facts and arguments to have the formula 03, or  02, may
also be regarded as polymeric with ordinary oxygen, 02.
The two varieties of nitric peroxide NO2 and NO2 have
already been described (p. I47). The latter is polymeric
with the former.
Of carbon compounds exhibiting similar relations, the hydrocarbons of the CnH2n, or olefine series, afford a prominent
instance. The formulke of these bodies are all multiples of
the first, methene CH2, which, however, is not known in
the free state. Being at once polymeric and homologous
with one another, they exhibit a regular gradation in their
boiling-points and specific gravities, and form  similar chemical compounds.
Name.             Formula.        Boiling Point.
I Ethene or ethylene.       C2H4             Gas,
liquefiable only under
great pressure.
Propene or propylene        C H            -17'8
1 Quartene or butylene.   C4H                3~
Quintene or amylene         C5H1o            35
Sextene or hexylene         C65H             68 —70
etc.                  etc.             etc.




i56             Chemical Philosophy3,.
Further illustrations are supplied by the two chlorides of
cyanogen CNC1 and C3N3C13; by cyanic acid CNOH and
cyanuric acid C3N303,H,3; also by the modifications of
aldehyd C2H40, acraldehyd (C2HO)2 or C4HIO2, and
paraldehyd (C2H10)3 or C6H,2C)03, and many others.
METAMERIDES.
We have now to consider a kind of isomerism, which
occurs very frequently among carbon compounds. The
nature of the phenomenon will be understood by comparing
together several such compounds as the following, all of
which are represented by the same empirical formula: —
i. Propionic acid, C3H602, is a crystallisable acid, which,
after melting, boils at I400. It is monobasic, forming one
salt only with each of the metals, sodium, potassium, and
silver. Its rational formula may, therefore, be written
thus: HC3H502.
2. Ethylic formate is a colourless aromatic liquid, which
boils at 560.  When heated with caustic potash, it is
resolved into ethylic alcohol and potassium formate. This
mode of decomposition is recorded when we write the
formula thus: C2H5.CHO2.
3. Methylic acetate is a colourless, volatile liquid, which
also boils at 560, but when decomposed by an alkali, it
yields wood spirit (methylic alcohol) and an acetate. So
its formula must be CH,.C2H302.
These three compounds, which have the same composition and molecular weight, but differ in the nature of the
products they yield, when decomposed or acted upon by
chemical agents, are said to be amtameric with one another.
Many cases similar to the last two may easily be found
amongst ethereal salts (compound ethers), ketones and other
bodies, the molecules of which consist of two compound
radicles united together by oxygen or a bivalent group.
Pairs of such bodies may be called reciprocal metamerides,
because the excess of carbon and hydrogen in one of the




letamerism.                  157
radicles is made up for by a corresponding deficiency in
the other. General formulae for such pairs of isomerides
among the compound ethers might be written thus:
CnH2n- 10    O  and  C"H2p _ 1-   O
CpH2p  1           CnH2,,  + 1
In many cases an extraordinary resemblance in physical
properties may be observed in comparing together two
bodies of this kind.
Another instructive example of metameric relations is
presented by the two classes of alcoholic cyanides.
I. Cyanides.-By distilling ammonium acetate with phosphoric anhydride (a substance which has an extraordinary
affinity for water), the ammonium salt is converted into a
volatile body, long known as acetonitril, and boiling at 770.
NH4C2H302 - 2OH2 = NC2H3
When acted upon by boiling alkali, it yields up its nitrogen in the form of ammonia, and regenerates an acetate.
NC2H3 + OKH + OH2 = NH3 + C2H3(OK)O
This reaction is explained by supposing that in acetonitril
or methyl cyanide the two atoms of carbon are in direct
union with each other, as represented by this diagram.
N -- C - CH3.
2. Isocyanides or Carbamines. —These compounds undergo
a different transformation when acted upon by hydrating
agents. Notwithstanding that they have the same composition as the cyanides, they are scarcely affected by alkalies,
though when boiled with diluted acids they are readily converted into formic acid and bases, in which the nitrogen is
associated with part of the carbon. For example, methyl
isocyanide treated in this manner yields methylamine and
formic acid.
NC2H3  +  2H20 = N  {CH  + CHO.OH
Methyl isocyanide.    Water.  Methylamine.   Formic acid.
In these compounds, then, the nitrogen probably forms




15 8            Chemical Philosop/O.
the link between the two atoms of carbon, in the manner
shown by the following graphic formula:CN - CH3
ALLOTROPYMany of the elements are known to exist in the form of
two or more modifications, which are very different in
physical properties and to some extent also in chemical
behaviour. One or two examples have been already referred to under the head of physical isomerism and of
polymerism. It is probable that a closer examination of
the different cases of allotropy known among the elements
would enable us to range them all in one or other of these
classes. At present, however, our knowledge will not allow
us to adopt with any degree of confidence a final decision
upon this point.
In the earlier part of this chapter, prismatic sulphur is
described as a physical or mechanical modification of the
octahedral form of the element, and this is probably correct.
But these two are not the only varieties of which this body
is susceptible. By heating melted sulphur to a temperature
of 2400 to 2500 it becomes extremely viscid, and if cooled
suddenly whilst in this condition the viscid consistency is
retained, and the product is a tough elastic solid quite
different in aspect from ordinary sulphur.  In this state it
is insoluble in carbon bisulphide, a liquid which takes up
octahedral sulphur very freely.
After keeping a few hours it becomes brittle and crystalline, and recovers its solubility in the usual solvents.'The
same change may be brought about in a few minutes by
plunging it into hot water, and in this way a most interesting phenomenon may be observed.  If a considerable
quantity be immersed in water a few degrees below the
boiling point, the water will be made to boil briskly by
reason of the evolution of the heat which is extricated
from the plastic sulphur during its conversion into the
crystalline.




A llot ropy.                 I59
In these transformations and in the insoluble character
of this plastic sulphur, we may trace a close resemblance
to the modifications to which certain carbon compounds,
such as aldehyd, cyanic acid, and other bodies, are subject,
and which are known to be the effect of polymeric changes.
It seems not unreasonable to consider that the production
of plastic sulphur may be brought about in the same way.
Phosphorus presents us with an example of a somewhat
similar kind. This element in its ordinary state is at
common temperatures a solid of waxy consistence, which
becomes brittle at low temperatures.  Its specific gravity is
I'82. It melts easily, dissolves in carbon disulphide, and
by sublimation or solution it may be obtained in brilliant
crystals in the form of regular octahedrons and dodecahedrons. When this body is heated to a temperature approaching 2500 it is slowly transformed into a dull red
powder or mass, of specific gravity 2'I4, which is no longer
soluble in carbon disulphide. It shows itself also in many
ways less inclined to enter into chemical combination than
common phosphorus, being far less easily inflammable and
oxidisable, and unaffected by hot alkaline solutions.
The explanation of this appears, at least very probably,
to be that the molecules of the ordinary phosphorus combine
together into more complex groupings to form the allotropic
molecules, and so expend part of their chemical energy.
How to reconcile with this view the fact that heat is
absorbed and not evolved in the process is, however, not
very clear.
Carbon is another element which assumes several distinct
forms, the relations of which are of great interest. We may
divide these various modifications into two distinct groups,
the crystalline and amorphous.
Crystalline carbon is dimorphous. In one form it constitutes the diamond, which crystallises in octahedral forms
of the regular system, and has a specific gravity on the
average of 3'3. The other is graphite, or, as it is frequently




60o              Chemical Philosophy.
called, plumbago or black lead, the crystalline form of which,
hexagonal plates, is quite incompatible with the form of the
diamond. The average specific gravity of graphite is 2'2.
If now we review the results which have been obtained by
burning the different forms of crystalline carbon, we find
that, allowing for slight experimental errors, the amount
of heat evolved by the combustion of equal weights of
diamond and graphite are practically the same. Twelve
grams of each substance burnt in excess of oxygen disengage enough heat to raise the temperature of about 93,300
grams of water one degree, or as it is usually expressed,
93,300 units of heat are evolved. The exact numbers in
each case are as follows:Diamond.         93240
Natural graphite.93560 mean
Graphite from iron.     93140o 93350.
The smallness of the difference observed would lead
one to the belief that graphite and diamond possess the
same atomic structure, and that they owe their peculiarities
to different arrangement of their molecules —that in short
they belong to the class of physical isomerides, were ii not
for some remarkable facts in connection with their behaviour
under the influence of chemical reagents. A mixture of
nitric acid and potassium chlorate has no action on the
diamond, even in the state of the finest dust, but under the
influence of this powerful oxidising mixture, graphite is con.
verted into a yellow crystalline substance, called by Sir B.
Brodie, who discovered it, graphic acid. This compound
contains CllH405 (Brodie), and when heated it decomposes
violently, leaving a black graphitic residue, which still
retains oxygen and hydrogen.
Amorphous carbon may be obtained by a great variety of
processes, and in each case the product exhibits more or
less distinctly marked peculiarities.
But neither vegetable nor animal charcoal, lamp-black,




Energy of Isomerides.          1xI
coKe, nor gas carbon yields by the action of potassic
chlorate any substance of the nature of graphic acid, but
only black soluble substances of indefinite composition.
Twelve grams of wood, charcoal give out 96,960 units of
heat when burnt so as to form carbonic anhydride, and
other kinds of charcoal when deprived as completely as
possible of hydrogen and oxygen, give numbers closely
agreeing with this. Taking the average heat of combustion
of crystalline carbon as 93,300 units, it is obvious that there
is too great a difference here to be fairly accounted for by
the hypothesis of experimental error, and consequently that
there is some essential difference in the constitution of
crystalline and amorphous carbon. The question whether
this difference is sufficient to indicate a polymeric relation
between these bodies remains to be answered. Silicon and
boron form allotropes, which are analogous to those of
carbon, and concerning which the same questions may be
propounded.
The study of a great number of cases has led to the
discovery that the formation of any two isomeric bodies
always involves the consumption of different amounts of
hear  Also, that when these bodies are burnt, or otherwise
similarly decomposed, the disruption of their molecules is
attended by the evolution of different amounts of heat.
Tlis is nearly equivalent to saying that in order to
produce equal weights of two isomerides different amounts
of work must be expended in the two processes, and that
different amounts of energy are stored up in the products.
How is this energy disposed of?   According to one
view, and adopting the molecular theory, we may reply that
the energy is employed in communicating to some atom or
atoms within the molecule a new kind of motion whereby
it acquires new chemical functions, and this change we
figure to our minds, and render intelligible by the hypothesis
that in the transformation of a body into its isomeride, the
position of certain atoms contained within the molecule is
M




162             Chemical Philosophy.
changed. We endeavour to represent this change of function by altering the arrangement of the symbols which go
to make up the formula of the body. Examples of this
will be found freely scattered through these pages; but as
an additional illustration, we might refer to an interesting case of isomeric change observed not long ago by
Hofmann.
It was found that methyl-aniline,
(C6H5)'-N/
\H
by protracted heating to a high temperature, is converted
into toluidine,
/H
(CH3)-(C6H4)"-N
In this metamorphosis an atom of hydrogen and an atom
of methyl, CH3, appear to exchange functions, and in order to
record this exchange the formulae are written in the above
or some similar manner, though it by no means follows that
we are to infer an exchange of place.
But in expending energy upon some part of a molecule,
it is not very probable that the energy of the molecule as a
whole remains unaltered. Still, it is conceivable that in
some cases it may be so, and a careful comparison of the
chemical properties of a great many pairs of isomeric
bodies would be of the highest interest, as in this way we
might arrive at a solution of the question, whether bodies
possessing a great store of energy are really more active in
their chemical behaviour than others in the production of
which a smaller amount of energy has been consumed.
EXERCISES ON SECTION III.
I. One atom of antimony is said to be equivalent to three, and one
atom of zinc to two, atoms of sodium. Explain this statement.
2. Distinguish between atomic, equivalent, and molecular weights.




Exercises on Section III.                I63
Give the atomic and equivalent weights of mercury, zinc, chlorine,
iodine, sulphur, iron, and copper.
Also write down the molecular weights of H2S, PC15, AsH3,
H2S04.
3. What weight of sulphuric acid can be precipitated by one gram of
barium chloride?
4. What is the weight and volume (at normal temperature and
pressure) of the hydrogen contained in Io grams of microcosmic salt,
NaNH4HPO4.4H20?
5. Enumerate very briefly the various methods by which atomic
weights may be determined; and indicate in the case of each of the
following elements the method or methods which would be applicableoxygen, chlorine, potassium, mercury, carbon,'sulphur, lead, silver,
arsenic, silicon, barium, copper, manganese.
6. The specific heat of iron is'I138. State approximately its
atomic weight.
7. The specific heat of cadmium is'0567, and its equivalent 56.
Give its atomic weight.
8. The equivalent of platinum is 49'4, and its perchloride has the
formula PtC14. Find its specific heat.
9. The formula of water was formerly written thus, HO, and
subsequently, for some years, H202 (assuming 0=8). Discuss both
these formulae, pointing out any inconsistencies you may detect in them.
Io. Complete the equation,
Ag2O + H202 =
Quote analogous reactions and explain the theoretical significance of
these!acts.
i i. The volume of the molecule of a compound body in the gaseous
state is double the volume of the atom of hydrogen. Examine the
truth of this statement; give the experimental facts upon which it is
based, and discuss any exceptions to it with which you are acquainted.
I2. Acetic acid contains C 40o'0, H 6'6, and 0 53'4 per cent.; and
chloracetic acid contains 37'5 per cent, of chlorine. Calculate the
molecular weight of acetic acid.
13. Explain the signification of the several formulhe for potassic
sulphate,
K2O. SO3; KO SO2; K2SO4
O O-K
and       S
O  O-K
M  2




i64                 CChemical Philosophy.
14. What is understood by the terms valency and atomirity respectively, and how would you ascertain the valency and atomicity of a
given element, for example, of carbon or phosphorus?
I5. What is the atomicity of each of the following radicles:S, O, C1, OH, NH4, NH3, NH2, NH, N, N2, PO, SO2?
I6. With the help of the table on page 152 write down the formulae
of the following salts:-Sodium fluoride, silver sulphate, mercuric
cyanide, mercurous phosphate, barium chlorate, bismuth chloride,
ferrous orthosilicate, cupric acetate, ferric nitrate, chromic oxalate,
stannic phosphate, calcium hypochlorite, etc. etc.
17. The density of the vapour of ammonium chloride is said to be
abnormal.   Explain  this statement, and give as far as you can
experimental evidence in support of this view.
i8. Define in a few words the terms allotroypy, metamerism, polymerisn.
I9. A current of electricity is passed simultaneously through solutions
of cupric and cuprous chloride.  How much copper and how  much
chlorine are liberated from the cuprous chloride for every molecule of
cupric chloride decomposed by the current?  In what relation do these
quantities stand to the quantity of zinc consumed in each cell of the
battery, secondary actions being neglected?
20. The atomic weight of silver being Io8, and its specific heat'o57,
another metal M, of which 70 parts unite with 35'5 parts of chlorine,
is found to have the specific heat'0306. What is the atomic weight of
this metal and the formula of its chloride?
2I. Give reasons for representing hydrobromic acid by a formula
similar to that of hydrochloric acid; HC1, HBr.
22. Alcohol, ether, and acetic ether have the following rational
formulae,
H            C2H5           C 30
What arguments could you draw from the existence of such bodies
in favour of the number I6 as the atomic weight of oxygen?  Why is it
probably neither 8 nor 32?
23. The formula of the molecule or chemical unit of ammonia is
NH3   What is the meaning of this formula, and what are the reasons
for choosing it?




Exercises on Section IIL.                I65
24. A compound is found by analysis to have the following composition:Carbon.. 52'8
Hydrogen..              I3'04
Oxygen.                34'78
I00'00
To find its simplest formula.
25. The analysis of a compound leads to these numbersCarbon.                                  3720
Hydrogen.                                7'90
Chlorine.                                54'95
o00'05
It is not often that the formula can be calculated so easily as in the
example given in the text and in the last exercise. It must be borne in
mind that in actual practice a slight loss is incurred in the estimation of
many elements. The number for hydrogen, however, generally comes
out a trifle too high. Oxygen is always estimated by taking the difference between the total weight of the body analysed, and the sum of the
weights of the constituents which have been actually weighed.
In the present example we proceed in the following manner:Divide the percentages by the atomic weights in the usual way:
37   3'  atoms of carbon.
12
7'9       79,,,, hydrogen.
I
5495  =   -154,,,, chlorine.
35'5
Divide the three quotients by the last, which is the least.
3-       2'01 atoms of carbon.
I'54
7'9  -  5-I2,,,, hydrogen.
1'54
I-54  _   I atom of chlorine.
I 54
Now  recollecting that the percentages found by analysis are not
exactly true, but only close approximations to the correct numbers, and
remembering that the hydrogen is generally in slight excess, we may




i66                Chemical Philosophy.
safely reject the two small fractions which occur in the above numbers and the formula then reads
CAHsCI.
To prove that this represents correctly the composition of the body,
it is well to recalculate the percentages on the basis of this formula.
This calculation is performed in the following manner:C2 =  I2 X 2 = 24
H5-=   I x 5 =   5
Cl=               35'5
64'5
Then 24 X      -  37020
64'5 
5 x 10
64-5  7'75
35'5 x Ioo
64'5
And these theoretical numbers are seen to be very close to those
obtained by experiment.
Theory.                             Experiment.
C 37'20..37'20
H 7'75.       7'90
C1 55'05.      54'95
26. What is the simplest formula you would assign to a substance
containingCarbon. 54'5
Hydrogen.                                9'2
Oxygen. 36'3
ill oo parts?
27. Also to the following body:Carbon..88 20
Hydrogen.I I'80
I00X00
28. And again to an organic base containingCarbon..63'78
Hydrogen..5.76
Nitrogen.                                 3'32
Oxygen..27'14
IOO'00




Exercises on Section II~.                  I67
29. From the following percentages calculate formule for the several
compounds:Mzag netic Iron Pyrites.
Iron....... 59'721
Sulphur.. 40'22
Iron Peroxide.
Iron....... 70
Oxygen.                                30)
Hydrogen Peroxide.
Hydrogen.                                 588
Oxygen..              94'12
Cryolite.
Sodium.. 32'79)
Aluminium......  I3'02
Fluorine....... 54'I9)
Mannite.
Carbon..39.'3
Hydrogen.                                77I -
Oxygen.                                52'98)
Benzoic Acid.
Carbon.6867
Hydrogen.                                 4'95'
Oxygen.                                26'38J
Caffeine.
Carbon..                                4905
Hydrogen                                   5'14
Nitrogen.                               28-61
Oxygen... I7'20
Cane Sugar.
Carbon.                                 4200oo)
Hydrogen.                                 646 
Oxygen..            5154
Uric Acid.
Carbon...357
Hydrogen...                              2 38
Nitrogen... 33'33
Oxygen...  28'58




i68                Chemical Philosophy.
30. Find the formula of nitrosoterpene from these numbers —
Carbon                              72.57)
Hydrogen.                           897 in Ioo parts,
Nitrogen.                           8'74 
and for nitrosoterpene hydrochloride from the following percentages:
Carbon.                                 59'58
Hydrogen.                                8-07
Nitrogen                                   7 20
Chlorine..745
3I. The silver salt of an organic acid was found by analysis to yield
47'1 per cent. of metallic silver. Determine its molecular weight. In
the formation of the silver salt fiom the acid, Io8 parts of silver take
the place of I part of hydrogen. Therefore,
Molec. wt. of acid- I - molec. wt. of salt - Io8
or       Molec. wt. of acid - molec. wt. of salt - io8 + I
In the example given 47-I parts of silver are contained in Ioo
parts of the salt.  So that Io8 parts of silver are contained in
Io8 X Ioo = 229 3 parts of the salt. This is, therefore, a number
47'~
identical with, or very near to, its molecular weight. The answer is,
therefore,
Molec. wt. required = 229'3 - I08 + I = I22'3,
Or, since the atomic weights of carbon, hydrogen, oxygen, and
nitrogen are all integers, the fraction must be discarded, and the
number becomes I22.
This corresponds with the formula of benzoic acid; verify it.
32. Aniline containsCarbon......                           774
Hydrogen                                 7'5  per cent.
Nitrogen.5.'o
and its platino-chloride contains 32'9 per cent. of platinum, to find its
molecular weight and formula.
As explained in Chapter XVI., p. I27, the platinum salts of nitrogenous bases are constituted on the same type as that of ammonia.
Hence we may rer resent the formula of platino-chloride of aniline thus,
2(Aniline + HC1) + PtC14
or                   Aniline + HCl + PtC14
2




Exercises onZ Section III.                I69
The first question then is, what weight of platinum salt is represented by this formula.  This is answered as follows:32'9 parts of platinum make Ioo parts of platinum salt,
therefore I97'5 parts or one atom of platinum  make I       97 5
32'9
parts, or 600.
We have now to subtract from this the platinum perchloride and
hydrochloric acid; half the remainder is the molecular weight of the
aniline.
600 - 339'5 - 73 = 187'
and                       187'5
--   93'7
Now, taking the percentage composition of aniline, we have to
calculate the proportions of the three elements contained in 93'7 parts
of the base.
100: 93'7:: 77'4  x
x = 72'5 carbon.
100: 93'7:  7'5  v
y = 7 o hydrogen.
oo: 93'7:: I5'0  z
z = I4'0 nitrogen.
Hence the formula is obtained by dividing these numbers by the
respective atomic weights.
72 5
2'5 = 6'o4 atoms of carbon.
7'0
7 atoms of hydrogen.
-'40 _ I atom of nitrogen.
I4
Hence, allowing for experimental error, which in this case is very
small, the molecular formula required is
C6H7N.
33.'I442 gram of anthraflavic acid gave'37I2 gram of CO2 and
~0448 gram of water. Calculate a formula.
34. A sulphide of tellurium and arsenic was analysed.'6347 gram
of the mineral gave'2584 gram of tellurium,'3978 gram of ammoniomagnesium arsenate (MgN H4AsO4. H20), and I 6453 gram of barium
sulphate. Calculate a formula.




170                Ch/nical PF/iz'osop/zy.
35. The analysis of trichloracetyl urea gave the following results:(a)'32IO gram gave'2060 gram of CO2 and'0453 gram of H20;
(b)'0825 gram gave'oIo9 gram of nitrogen;
(c)'I204 gram gave'25IO gram of AgCl.
Calculate the formula of the compound.
36. Analysis of uranium pentachloride:Weight of substance taken..'8955 gram,,,, U308 found....  6038,,,,, AgC,,...  I'4997,,
Calculate the formula.
37.'3807 gram of benzoic acid gave'9575 gram of CO, and'I698
gram of water. And *'4287 gram  of benzoate of silver gave'2020 of
silver. Calculate the rational formula of benzoic acid.
38.'5828 gram of platino-chloride of caffeine left after ignition'I43
gram of platinum. What are the molecular weight and formula of
caffeine, which contains
Carbon...... 49'05
Hydrogen.....  5'I4       cent
Nitrogen..            28~6I
Oxygen.                               7 20
39. The platinum  salt of a volatile organic base was found by
analysis to have the following percentage composition:-Carbon, 9'5;
hydrogen, 3'2; nitrogen, 5'7; chlorine, 42'0; and platinum, 39'0.
The vapour density of the base was found to be I'59 (air= I).
Calculate from these data its molecular formula.
40. The silver salt of an organic acid contained 62 44 per cent. of
metallic silver. It also contains I7-34 per cent. of carbon and I'73
per cent. of hydrogen. From these data endeavour to find a formula
for the acid.




17I
SECTION  IV.
CHAPTER XXI.
CLASSIFICATION OF ELEMENTS.
THE following list includes all but seven rare metallic
elements, the consideration of which, obscure as their
characters are, is unnecessary in a work like this. The
numbers in the table represent the specific gravities of those
which are solid, usually in the densest form in which they
are known.
I.-gNon-melallic or Oxygenic * Elements.
Gaseous.
0, N, F (?), C1 (liquid I'33).
Liquid.
Br. 2'96
Solid.
S (octahedral) 2'07
P (red). 2'2
Si (graphitic) 2'49
B (adamantine) 2'68
C (adamantine) 3'5
Se (crystalline) 4'5
I.      ~ 4'95
* Oxygenic = acid-producing.




172                Chemical Philosophy.
II. —Metadloids or Inpeyfect Afetals.
Gaseous.
H.
Solid.
Zr..   4I5
V.         ~   5'5
As.             5'9
Te..   625
Sb..   68
Sn.         ~   7'3
Mo..   8'6
Bi..   9.8
W..  I76
U..  I8'4
Ti,  Nb,  Ta?
III.-Afetais or Basigenic* Elements.
Li.'578
K.865
Na..'97
Rb..'52
Cs..?
Mg..   17
Ca..   x8
Sr..   2'5
Al..   2 6
Ba.   40
Cr..   68
Zn.             7'o
In..   7'2
Fe.             7'8
Mn..   8o0
Ni..   86
* Basigenic -, base-producing.




Classizfcation of the Elements.           173
Cd         8'7
Cu..   89
Co..   89
Ag..   o'53
Ro..II'O
Pb..  i'36
Ru..  11'4
Pd..  II8
T1..  II'9
Hg  (liquid at o~)   13'596
Au..  19'34
Os..  2I'4
Ir..  2I'I5
Pt..  2I'5
This division  into three groups is adopted here for
purposes of convenience, but the student must not infer
that it is absolutely necessary. Indeed, as he goes on he
will find that, as in all attempts to classify the things of
nature, it is impossible to define precisely a border-line
separating a given class of bodies from all others.
Division I.-Non-metals.
These elements, as a class, are characterised by no
generality of physical properties. Three are incondensable gases, fluorine (?), oxygen, nitrogen; one, chlorine, is
gaseous at ordinary temperatures, but liquefiable under
pressure; one, bromine, is a liquid; the rest are brittle
solids. Of these, iodine, sulphur, selenion, and phosphorus are fusible and vaporisable, the remaining three
are distinguished by infusibility (?), absolute fixity, even at
the highest attainable temperatures, by abnormal specific
heats (Chap. XV., p. I I4), and by furnishing, in the cases of
graphitic carbon and silicon, the only examples among the
non-metals of electric conductivity.
In their chemical characteristics, however, there is toler



174              Chemical Philosophy.
able uniformity. They all, except boron*, combine with
hydrogen; all, except fluorine, combine with oxygen, often
in several proportions, and their oxides are either neutral
and indifferent bodies, like carbonic oxide, or, the great
majority, anhydrides, which by uniting with water form
acids.
NON-METALS.-CLASS I.-THE HALOGENS.
Fluorine.. F  =  I9
Chlorine           C      355) C1  + I
Bromine            Br =80           =81.25
Iodine..          I   =  I27
These elements are characterised by a remarkable family
resemblance. The three last especially are constantly associated together in nature in the haloidt salts of potassium,
sodium, etc., and in the ores of mercury, silver, and other
heavy metals.  They also agree very closely with one
another in their general physical characters and chemical
deportment.
At ordinary temperatures chlorine is gaseous, bromine
liquid, iodine solid; but bromine and iodine are volatile and
yield heavy, coloured vapours, which, when largely diluted
with air, have nearly the same odour as chlorine. Each
forms with hydrogen a strongly acid compound, which
under ordinary conditions is a colourless, fuming, very soluble
gas, consisting of equal volumes of hydrogen and the vapour
of the halogen, united without contraction. The chlorides,
bromides, and iodides of the alkali metals crystallise in the
same form, and the isomorphous replacement of the one
halogen by another is observed in a great many other cases.
The following table exhibits the formula of all the known
oxides and acids of chlorine, bromine, and iodine, from
which it will be seen that although there are many gaps to
* Hydride of boron is not known, but the organo-boron compounds
B(CH3)3, B(C2H5)3 may be considered to represent it.
+ as = sea-salt = common salt.




The Halogens.                 I75
be filled up perhaps by future research, the correspondence,
so far as it goes, is complete:
Halogen Oxides and Corresponding Acids.
C120   C1203  C1204
HC1 HClO  HC102 HC10     HC103  HC104
HC103)
No oxide of bromine known.
HBr HBrO                      HBrO3 HBrO,
I205    1207
HI   HIQ (?)                  HI03   HIO4
The differences exhibited by chlorine, bromine, and
iodine are strictly gradational: chlorine, with the smallest
atomic weight, being most active, bromine next, and iodine
the least energetic of the three.  These differences are
manifested by their relative affinities for the metals and
hydrogen, chlorine displacing bromine, and bromine displacing iodine from such combinations.
Indications of the same differences are afforded by the
superior activity of chlorine as a bleaching agent, and by the
energy with which it replaces hydrogen in carbon compounds.
As in several other cases of nearly allied elements, to be referred to hereafter, the chemical activity diminishes in proportion to the increase of the atomic weight, and rise of
boiling-point and specific gravity. (Chap. IV., p. 22.)
The replacement of one or more atoms of hydrogen
in a hydrocarbon by an equivalent quantity of one of the
halogens, produces a neutral substitution compound; but if
a similar replacement is effected in the molecule of a body
which contains oxygen, the product not unfrequently presents well-marked acid properties.  This is the case, for
example, with some of the derivatives of phenol.
This oxygenic tendency of the halogens is also indicated
by the destruction of basic character in the amines or compound ammonias by the substitution of chlorine, bromine,




I76              CChemical P/ilosophy.
or iodine for their hydrogen, as is well shown by the chlorinated derivatives of aniline.
Aniline C6H7N, a powerful base.
Chloraniline C6H6C1N, less basic than aniline.
Dichloraniline C6H,C12N, feeble base.
Trichloraniline C6HC13N, neutral.
Zodine. —Iodine presents one or two peculiarities which
deserve special notice, as they serve to remove it to some
slight extent from immediate association with the kindred
elements, bromine and chlorine.  In the first place, its
affinity for hydrogen is decidedly less energetic than that of
either of the other two elements. This is indicated first by
the fact that iodine does not usually bleach vegetable colours;
secondly, that, acting alone, it is incapable of producing
ubstitution derivatives from carbon compounds.*  Whenever substitution of chlorine, bromine, or iodine occurs, the
hydrogen which is necessarily. eliminated goes to form the
corresponding hydracid. Now, in the case of iodo-substitution compounds, it has been shown that they are all
decomposed by the action of hydriodic acid, with reproduction of the original body and free iodine. Hence iodosubstitution compounds cannot be formed by the action of
iodine, unless precautions are taken to remove or to destroy
the hydriodic acid that may be produced. This is effected
in various ways, usually by the action of mercuric oxide or
iodic acid. The difficulty may also be got over in some
instances by substituting iodine monochloride for iodine.
Thus orcin acted upon by a solution of iodine chloride
gives triiodorcin and hydrochloric acid,
C7H802 + 3IC1 = C7H51302 + 3HC1.
Another distinguishing characteristic of iodine is the intense colour exhibited by the vapour of the element itself, by
its solutions in certain liquids, notably in carbon disulphide
* See also Chap. XIII., p. 87. Heat of combination of iodine with
hydrogen.




Special Properties of Iodine.         177
by its compound with starch, and by many iodides, the
corresponding chlorides or bromides being either colourless
or very pale.
Again, chlorine and bromine are more soluble in water
than iodine, and are even capable of forming at low temperatures crystalline hydrates, having theformulae C12I oH2O,
and Br2ioH20O, no such compound being formed by iodine.
On the other hand, the solubility of chlorine and bromine is
not appreciably increased by the addition of a chloride or
bromide to the water in which they are to be dissolved.
Iodine, however, is freely soluble in iodide of potassium,
and indeed produces in this way a black liquid which
probably contains an unstable triiodide of potassium, KI3.
This compound is not known in the solid state, although
analogous periodides are formed by the organic ammonium
bases, some of which form crystals of great beauty. The
following compounds, for example, containing the alkaloid
caffeine, were examined by the author some years ago.
(CsHION402H)I3
Caffonium triiodide.
(CsHo0N402C H3) I3
Methyl-caffonium triiodide.
(C8H10N402C2H5)13
Ethyl-caffonium triiodide.
The iodates exhibit some anomalies for which there is
no parallel among the chlorates. Thus, in addition to the
normal potassic iodate, KIO, there are two other well
crystallised salts, containing an excess of anhydride, for
which it is difficult to find analogues, except, perhaps, among
the chromates.
Iodates.                   Chromates.
KIO3
or K2,TO6                  K2CrO4
K2s120.I205               K2CrO4. CrO3
K21206. 21205            K2CrO,. 2 Cr03
N




1 78            G~zChemical Philosophy.
This tendency of iodine to accumulate in its compounds
is just one of those characters which belongs especially to
polyatomic elements, among which iodine seems, on the
whole, entitled to be placed.
FlZuoine. —This element has not at present been satisfactorily isolated, but there can be little doubt that it is a
gas, probably colourless, having properties similar to those
of chlorine or oxygen, but much more strongly marked.
Fluorine is connected with the other halogens by the correspondence of hydrofluoric with hydrochloric acid, and
by the isomorphism of the fluorides and chlorides. It is,
however, widely separated from them, partly in consequence
of its very extraordinary attraction for silicon, partly by the
non-existence of any oxide or oxyacid of fluorine. In spite
also of their general resemblance to the chlorides, bromides
and iodides, individual fluorides differ in many cases from
the corresponding chlorides. Thus fluoride of calcium is
insoluble, chloride of calcium very soluble and deliquescent;
fluoride of silver is soluble in water, chloride ot silver
totally insoluble; fluoride of potassium soluble in water,
but, unlike the neutral stable chloride, it yields an alkaline
solution which probably contains caustic potash and the
double hydrogen and potassium fluoride.
2KF + OH2 = KOH + KF.HF.
The tendency to produce double salts of this kind has,
indeed, led to the idea that fluorine may be really a
diatomic element, like oxygen, with the atomic weight
38. The formulae of the double fluorides would then be
comparable with those of oxygen compounds.
Fluorides.                Oxides.
H2F    analogous to      H20
KHF,,,,      KHO
KBF,,,,       KBO2
K2SiF3,,,,        K2SiO3
K2SnF,,,            K2SnO3
etc.                      etc.




A tomicity of the Halogens.       I79
These double fluorides, however, are not more numerous
or prominent than are the double chlorides, bromides, and
iodides, and it seems not unreasonable to explain their
existence by a similar hypothesis.  The atom F(=I 9)
may be occasionally trivalent. If so, the constitution of
fluoride of potassium and hydrogen may be represented as
KF = FH,
and that of the other fluorides, single and double, in a similar
manner.
Atomicity of the Halogens. —An atom of a halogen never
replaces, in a direct manner, more than one atom of hydrogen. It must also be admitted that, in the haloid salts of
these elements, 35'5 parts of chlorine and equivalent quantities of bromine, iodine, and fluorine, are almost always
combined with the metallic representative of one part by
weight of hydrogen. The halogens are therefore generally
univalent.  Nevertheless, many compounds are known, the
existence of which can scarcely be accounted for, except
upon the hypothesis of their occasional trivalent function.
Thus, in addition to the normal iodides and iodo-substitution compounds,
H'I, K'I, Hg"12, C2H3I'O2,
iodine forms the following compounds,
I"'C13, I"' (C2H302)3, KAgI 2,
which may be written as follows,
C1     C1\          /0O-C2H30
/          \\  /
I —C1 or      I-C1 I -O-C2H30  K-I=I-Ag
X           // 
C1      C1/ l         O —C2H30O
In their oxygenated compounds, chlorine, bromine, and
iodine present also a very marked resemblance to nitrogen,
which is most generally (perhaps always) a triad. Thus
we have hypochlorous and hyponitrous acids, both extremely unstable bodies, known chiefly in the form of their
N 2




80             Chemical Philosophy.
salts; chlorous and nitrous acids, also. very unstable;
chloric and nitric acids, both liquid, easily decomposable,
highly corrosive bodies, the salts of which are all soluble
in water. The nitrogen analogue of perchloric acid is at
present unknown. The formulae of these corresponding
pairs of compounds are as follows:
HC10       HClO  2      HC10O        HCIO1
HNO         HN02    HN03
A further correspondence is observable in their oxides,
C]20      C1203      C1204       I205
N20       N203       N204       N205
Now, if we admit that nitrogen is trivalent in these comr
pounds, the presumption that chlorine and its congeners
are also trivalent is, at least, worthy of discussion. The
following graphic formulae express the constitution of
chloric and nitric acids upon this hypothesis:O                   0
H —0-0Cl<l          H-0-N1 {
O                  ~O
Chloric acid.         Nitric acid.
As contributing evidence in support of the same view,
we might also point to the large number of permanent
and definite double compounds which the chlorides,
bromides, and iodides are capable of forming. We have,
for example, the well-known chloro-platinates of the alkali
metals,
2NaCl. PtC14
2KCl. PtCl,, etc.
besides the innumerable compounds of the alkali-metal
chlorides with the chlorides of magnesium, iron, aluminium,
mercury, copper, and other metals. Although a few of
these compounds are decomposed by water, the majority of
them are stable enough, and give no signs of alteration by
such treatment.   Their constitution is most reasonably




Molecular Compounds.             I8 
represented upon the assumption of the trivalence of the
halogen. The compound KI.HgI,, for example, may be
written in this manner.
K-I  l Hg
I
Many chemists regard these compounds as formed' by
the union of entire molecules of the constituent salts, combined together by some kind of adhesion differing from
ordinary chemical affinity.  Thus the potassio-mercuric
iodide represented above is considered to contain both
iodide of potassium and iodide of mercury, HgI, +  KI.
It is quite probable that such molecular compounds may
exist, but if they do, they might naturally be expected to
exhibit something of the appearance and external characters
of their proximate constituents.  In the case referred to
nothing of this kind is observed, the double iodide KHgI3
being a colourless salt, soluble in water, and totally unlike
the scarlet, insoluble, mercuric iodide, the elements of which
it contains.
NON-METALS.-CLASS 2.
Oxygen   -   0  =-   6
Sulphur   - S        32
Selenion   -   Se =  79'5       +   e - 80
eZellurium  -   Te =  128    2
These elements are associated together principally by
reason of the correspondence in composition and general
properties among their compounds with hydrogen,
OH2  SH, SeH, TeH,.
For although, taking any two adjacent terms of the series,
many points of resemblance may be traced out, yet there is
a very wide interval between the colourless incondensable




182             Chemical Philosop5hy.
gas oxygen and the silvery, metallic, crystalline, solid
tellurium.
Oxygen, standing apart from the rest in virtue of many
extraordinary qualities, deserves to be noticed first. This
element constitutes nearly one-half the total weight of the
earth's crust. Indeed, it seems to be the preponderating
element of the globe.
Leaving out of account the insignificant quantities of
metals and metallic sulphides embedded in the earth, it
may be said that the entire constituents of the earth's crust,
including the water of the ocean, consist of compounds
saturated with oxygen, and even supposing all organic
bodies to be completely burnt up, the atmosphere would
still contain a considerable quantity of unemployed oxygen.
Oxygen is entirely unmatched among the rest of the
elements, both as regards the number and varied character
of its compounds, and the important part which it plays in
relation to combustion and life.
Water, hydrogen protoxide, also exhibits properties which
are in every way remarkable as specially fitting it for the
-part it has to play in the economy of nature. The very
type of a neutral body, yet capable, under special circumstances, of acting either as a feeble base or a feeble acid;
an almost universal solvent of saline bodies; exhibiting the
anomaly of attaining maximum density at a temperature (4~)
above its solidifying point; standing alone amongst liquids
by reason of its great capacity for heat; having a vapourdensity little more than half that of its gaseous sulphur analogue (SH2), and yet liquefying and solidifying readily.
The higher oxide of hydrogen, 02H2, with its bleaching
and oxidising powers, resembles in no slight measure the
element chlorine.
The name oxygen, "acid producer," from  6~bc and
yrvvdaw, was given by Lavoisier under the mistaken
impression that this element contained a principle common
to all acids. This we know was an error. Nevertheless




Oxygen.                   183
the name was not ill-chosen, and for the following reasons:I. There are many hundreds of acids known, and yet,
with about six exceptions, HF, HC1, HBr, HI, HCN, HS,
all contain oxygen.
2. In proportion as the quantity of oxygen increases in
a series of acids containing the same elements, so the
acidity as well as the stability of the compound is almost
always increased. A. detailed examination of such examples
as the following would, if space permitted, confirm this
statement.
HCLO           HNO2  H2SO3            H3PO2
HC102          HN3O   H2SO4           H3PO3
HC103                                 H3PO4
HC104
Glycollic acid C2H403
Oxalic acid CH,20
3. The addition of oxygen to (neutral) aldehyds converts them into acids. Ex. gr.
Acetic.
Aldehyd    -   -          C2H40
Acid  -   -   -           C2H402
Benzoic.
Aldehyd    -   -   -   C7HO60
Acid  -   -          -    C7H602
4. In homologous series of acids, the lowest terms which
are richest in oxygen show a far stronger development of
the acid character than the higher terms of the same series,
which are comparatively poor in oxygen.  Compare for
exampleFormic acid, CH,02 containing 69'5 per cent. of oxygen.
Acetic acid, CH402,,    53'3,, 
with
Valeric acid, C5H1002,,    31 3,
and
Palmitic acid, C16H31102,,    12'4,,




184              Chemical Pzilosophy.
5. Many oxides of metals are basic, that is, saturate
acids with production of water and salt, but a few examples
will be sufficient to show that it is only those which contain
a comparatively small quantity of oxygen that possess this
power.  As the quantity of oxygen increases in a given
series, the basic character gradually disappears, and gives
place to a more or less decided acid-forming tendency.
EXAMPLES OF METALLIC OXIDES.
Basic       Hg20 K20  BaO  PbO   Mn           CrO       l
HgO                     Mn203 (feebly) Cr203 (feebly)
Indifferent    K202 BaO2 PbO, MnO2
_ _    04   _
Anhydric)                        MnO3          Cr03
Acids S                         M207
Allotropic o-xygen or ozone is a body which has for some
years attracted considerable attention. The experiments of
Brodie have finally decided the question of its constitution
in favour of the hypothesis long ago put forward by Odling.
Ozone is now proved to be allotropic oxygen, free from
hydrogen, and to have the formula 03. It may thus be
regarded as formed on the type of hydric peroxide, with
which body it agrees in many of its reactions.
Hydric peroxide.         Ozone.
H-o                 /0
I             0  1
H —O'o
S'uphur, Selenion. —These two elements resemble each
other closely. Sulphur is a yell6w, selenion a red solid,
exhibiting several modifications, some of which are crystalline and soluble in carbon disulphide, others amorphous and
insoluble.




Szdphur and Selenion.                    I 85
Principal Allotropes of Sulphur.
Spec. Grav.    In carbon disulphide.
I. Octahedral (native).  2,o7  soluble.
2. Prismatic (monoclinic)    i 98   transformed into i.
3. Plastic..  1-95  insoluble.
4. Amorphous..  I'95  insoluble.
(Precipitated from chloride or theiosulphates.
Principal Allotropes of Selenion.
I. Monoclinic (native).   4'4  soluble.
2. Crystalline (form?).   48   insoluble.
3. Vitreous...   4'3  insoluble.
4. Amorphous..          insoluble.
(Precipitated from selenious acid.)
The following is a comparison of the most important
compounds of sulphur and selenion: —
H2S         SO2         SO3         H2SO3           H2SO4
Gas.       Gas.        Solid.    Crystallisable at    Oily liquid.
low temperatures.
H2Se        SeO2        SeO3         HSeO3         H2SeO,
Gas.       Solid.     Unknown.   Crystalline solid.   Oily liquid.
The selenates are isomorphous with the sulphates.
The relations of sulphur to oxygen are shown in the
following synopsis of some of their compounds: —
SULPHIUR.                      OXYGEN.
SH2 gas, feebly acid            OH2
S2H2 oily liquid                02H2 oily liquid
CS red solid?                  CO colourless gas
CS2 volatile liquid             CO2 gas
COS gas          5
ACIDS.
C2H30.SH theiacetic. B.P.930    C2H30.OH acetic. B.P. I20o
SO2. OH. SH theiosulphuric       SO2. OH. OH sulphuric




i86              Clemical Philosoplhy.
SULPHUR.                  OXYGEN.
BASES.
SKH                        OKH
SK2                        OK2
SCa                        OCa
CS(NH2)2 sulphurea         CO (NH2)2 urea
SALTS.
M'2CS, sulphocarbonate     M'2CO3 carbonate
M'3AsS4 sulpharsenate      M'3AsO4 arsenate
M'CNS sulphocyanate        M'CNO cyanate
ALCOHOLS.
S  C2H5 mercaptan. B.P.36'.   0 C2H5 P P. 780.4
ETHERS.
S(C15)2. PB.P. 720         O(C2H5)2 B.P.350'6
It is necessary, however, to add that many oxygen compounds are known for which at present there are no corresponding terms in the sulphur series, the oxides of nitrogen,
for example. On the other hand, sulphur has a power of
accumulating in a manner which is not exhibited, at least
to the same extent, by oxygen. Consequently, many polysulphides exist for which there are no corresponding oxides.
Thus we have K2S,, CaS5, FeS2, (C2H,)2S2 and (C2H5)2S3
Atomicity of Oxygen and Szd/zhur.
We know of no instance in which oxygen is other than
diad, if we except the very curious but imperfectly studied
sub-oxide of silver, which is said to have the formula Ag4O.
There are, indeed, two other examples which may be
brought forward if we choose to do a little violence to
commonly received theories. Carbonic oxide is generally
represented as containing unsaturated carbon, C = 0, but




A tomicity of Oxygen.            187
admitting the essentially tetrad character of the oxygen, we
should represent it as constituted thus, C _O.  Again,
water of crystallisation is almost universally considered by
chemists to exist as water in the salts in which it occurs, the
molecules of the salt and the molecules of the water retaining their individuality, being united only by some kind
of adhesion.  This is, perhaps, the most rational view,
considering the facility with which such water is detached,
and the fact that it constitutes no essential part of the
chemically reacting unit of the compound in all the most
characteristic of its transformations. Nevertheless, the idea
that this water is held to the salt by the two extra units of
combining power of the oxygen is not wholly preposterous.
At any rate, it does away with the difficulty of supposing,
that in the constitution of a given chemical compound, two
different kinds of chemical attraction come into operation,
and with the frequent impossibility of referring with any
degree of confidence particular cases to the category of
atomic or molecular combinations. One example of the
application of this hypothesis will suffice. Zinc sulphate
contains ZnSO4.70H2, or ZnSOOH2.60H2. This may be
expressed in the following manner:O      0-II
Zn  11 S  II
O I 0-H
0
/  X —"
H2 =0    0 = H2
H2=0   0 H2
I      I
H2  0         =0 H2
At present, however, and for all practical purposes, it is
safest to consider oxygen as invariably diad.




188                Chemnical Philosophy.
With regard to sulphur, the question is somewhat different.
This element forms three well-marked classes of compounds.
In the first of these it is bivalent, and may be regarded as
the strict representative of oxygen. Examples of this type
of compound have been already given, SH2, S2H2, CS2,
COS.
In the second class the sulphur alone is quadrivalent:
ex. gr. inll*
iv ( C1              Cl        SO"
i Cl  S{ C?  and
t C1
Sulphur tetrachloride. Theionyl chloride.  Sulphur dioxide.
In the third class of sulphur compounds represented by
sulphuric anhydride, SO3, the central atom seems to be sexiO
valent sulphur;   II.  It is true that the constitution
O=S=O
of this and all the rest of the sulphur compounds might
be represented  on  the hypothesis that sulphur is diad,
~O -S   0  but the analogy of sulphur with chromium, and
especially the isomorphism of the sulphates and chromates,
seem to establish the title of sulphur to be regarded as a
hexad. The several oxides, hydrates, and oxychlorides are
therefore represented in the following manner:vi o"           o//            10"               (o0"
S O           vi            Vi O      v         iO 0
O    "    S (OC1         S                    j (OH)  S (H)'
C1             C1                t(OH)'
Sulphur trioxide.  Sulphuryl     Sulphuryl     Sulphuryl hydrate, or
chloride.     chlorhydrate.   common sulphuric acid.
* Sulphur terrachloride is not well known except in combination
with metallic chlorides, but the corresponding compounds of selenion
and tellurium have been prepared.




Sulphuric Acids.                        I89
(0"                       0
(OH)'                    0
v  (OH)'                   1 (~,(OH)'
(OH)                   l(OH)'
Glacial or tetrahydric sulphuric  Ferrous sulphate
acid.                (dried at ioo~).
0 Hg
0
S (OH)6                   S O/Hg
/aHg
Orthosulphuric acid     Trimercuric orthosulphate
(unknown).              Turpeth mineral.
SO2.0H                   SO2.C1
0                        0
SO2.OH                  (SO2.C1
Pyrosulphuric, or Nordhausen   Pyrosulphuryl
sulphuric acid.            chloride.
NON-METALS.-CLASS 3.-BORON.
A  triad  element, but exhibiting  marked analogies with
tetrad silicon. The following are their chief points of
agreement:BORON.                          SILICON.
Element, (a) amorphous brown  Element, (a) amorphous brown
powder or (b) quadratic octahe.    powder, or (b) hexagonal plates
drons
S.G. 2-68                         S.G. about 2-5.
BH3 wanting                       SiH4 spontaneously inflammable
gas.
B(CH3)3, B(C2H5)3 etc.            Si(C13f)4, Si(CH5)4, etc.
BiC13  B.P. I7~                   SiCl4  B.P. 59
BF3 gas                           SiF4 gas
BF,. HF or BHF4                   SiF4.2HF or SiH2F6
known only in solution: salts    scarcely known except in solucrystallisable.            tion: salts tolerably stable.
II3BO3                            H4SiO4
crystallisable.                   exists only in solution.




g19               Chemical Philosophy.
BORON.                       SILICON.
HBO2                         H2SiO3
B203 (fused, S.G. 1'83)      Si 02 (fused S.G. 2'2)
M' B03 orthoborates          M'4Si04 orthosilicates
M'O02 metaborates            M1'2SiO3.metasilicates
Many anhydroborates          Many anhydrosilicates
M'B02 +:  B203              M'2SiO3 + x SiO2
Ex. fused borax 2NaBO2 + B203  Ex. orthoclase felspar
K2SiO,       + 2SiO2
A12(SiO.3)3
Boron is one of the few elements which combine in a
direct manner with nitrogen. Boron nitride has the formula
BN or perhaps B3N3.
NON-METALS. -CLASS 4.
Carbon, C  =   T2.
Silicon, Si =   28.
Allotropes of Carbon.
a. Diamond, octahedral, S.G. 3'3 to 3'5.
b. Graphite, hexagonal, S.G. about 2-2.
c. Charcoal, amorphous.
Allotropes of Silicon.
a. Adamantine (?)
b. Graphitic, hexagonal, S.G. about 2'5.
c. Amorphous.
Carbon is remarkable as being the essential element of
organic nature, and silicon as being one of the most
abundant constituents of the earth's solid crust.  Regarded
as chemical elements, they present problems of special
interest in their nonconformity to the law of Dulong and
Petit (p. II4), and in the multitudinous array of compounds
which they have furnished to the chemist. The carbon compounds will be specially considered hereafter; it therefore
only remains to point out in this place a few of the remarkable coincidences of composition so frequently observed




Carbon and Silicon.                  9 I
between the compounds of these two  elements.   The
number of bodies in which silicon replaces one or more
atoms of the carbon in some already familiar compound is
rapidly increasing, and still further developments in this
direction may be looked for.
In comparing carbon and silicon compounds together, it
cannot be said that the similarity so often noticed in their
constitution extends to their properties.  In most cases
corresponding carbon and silicon compounds are very
different bodies. A few examples only can be referred to:Carbon Compounds.      i       Silicon Compounds.
CO                            SiO unknown.
CO,, colourless gas, soluble to  SiO2, crystalline solid, fusible only
some extent in water.         at a very high temperature,
totally insoluble in water. Hydrates slightly soluble.
CH14, permanent gas.          SiH4, spontaneously inflammable
gas.
CHC13, heavy liquid, unactedupon  SiHC13, liquid, decomposed by
by water.                     water.
Orthosilicic and orthocarbonic acids, HSiO4 and HCO,,
are unknown, and but very few of their compounds have
been studied.  Of orthosilicates we have olivine, Mg"2SiO4,
ferrous orthosilicate, Fe"2SiO4, the ethyl salt (C2H5),SiO,,
and a few others. Orthocarbonates are represented by such
compounds as the ammonium-hydrogen orthocarbonate
(NH4)2H2CO and the ether (C21H5)4CO0. Ordinary carbonates are derived from metacarbonic acid H2CO3, a body
which has not been isolated, though the corresponding
silicic acid has been obtained as a solid glassy mass. All
carbonates are decomposed on the addition of even the
weakest acids, whereas the majority of the silicates resist
the attacks of any but hydrofluoric acid.
Atoimicily of Carbon and Silicon.
Silicon is always tetrad. Carbon is tetrad in all its known
compounds, except carbonic oxide, CO, and possibly the
isocyanides.  The unsaturated character of carbonic oxide




192             Chzemical Philosofphy.
is shown by the readiness with which it enters into combination with chlorine, with oxygen, with potassium, with
caustic potash, and with cuprous or platinous chloride.
CO + CI2 = COC12 carbonyl chloride
CO + O  = CO2  carbon dioxide
nCO + Kn - KCO,
CO + KHO = KCHO2 formate, &c.
From the characteristic reactions of the isocyanides (see
Isomerism, Chap. XX., p. 157)' it appears that in these
bodies an atom of carbon is linked to the hydrogen, or other
positive radicle present, by the atom of nitrogen. Now, according as we regard the nitrogen in these compounds as
discharging a triad or pentad function, we must attribute
to the carbon a replacing value equal to two or four atoms
of hydrogen. Either of these two formula, then, may be
adopted:-,/  ///,iv              V
C= N —R  or C-N — R.
The one may be regarded as a derivative of ammonia,
the other of marsh-gas.
In favour of the former view, we may adduce the combination of such a cyanide as prussic acid with hydrochloric,
hydrobromic and hydriodic acids, forming crystalline compounds analogous to sal-ammoniac, and with hydrogen forming methylamine; also the combination of the isocyanides
generally with oxygen to form isocyanates, and with sulphur
to form isosulphocyanates. These combinations may be represented thusN{C     N{C/   N{C            NC  = S         CN{I3
HC1
Hydrocyanic Carbylamine  Cyanic Acid.   Sulphocyanic  Methylamine
Acid or Hydrochloride.           Acid.
Carbylamine.
If the second hypothesis is adopted, we must imagine




Nitrogen and Phosphorzus.          193
that the union of the carbon to the nitrogen is partly
unlocked before these compounds can be found. Written
on the marsh-gas type the formule then becomeC~lv NH IV~ C(HCI   C      J S//        H3
(   )    Z(NH)" CIH     (NH)"    ((NH)" /  (NH2)'
On the whole, the former mode of representation seems
the more natural.
NON-METALS. —CLASS 5.-NITROGEN AND PHOSPHORUS.
Nitrogen  N= I4 N +       Phosphorus P=3I P + Sb
Phosphorus P = 3   -   =32'5 Arsenic  As=75  -=76'5
Vanadium V= 5I   2        Antimony Sb=I22   2
Phosphorus P =I )3 P+As   Phosphorus P=3I P+Bi
Vanadium yV=5  -- =53  Antimony Sb=I22  1 —=120'5
Arsenic   As= 75   2      Bismuth  /i=2Io   2
Diagram of numerical relations typical of chemical relations:
NI4                     iV5 I-As75
P3I                      As75 —SbI22
V5I                     tSbI22- Bi2Io
Nitrogen is a permanent incondensable gas, which has
never been obtained in any other form.
Phosphorus, on the contrary, is, in its ordinary condition,
a crystalline solid, though capable of passing into several
allotropic modifications. These have already been referred
to (Chap. XX.)
The specific gravity of nitrogen agrees with the molecular formula N,, whilst the vapour density of phosphorus
corresponds with P4.  This peculiarity of phosphorus is
shared by the semi-metal arsenic.
Both nitrogen and phosphorus are intimately concerned in the processes of animal life, phosphorus being
especially abundant in the form of phosphates in bone, and
in nervous tissue.
0o




194               Chesmical Philosophy.
Nitrogen is connected with phosphorus mainly by reason
of the resemblance between ammonia NH3, and phosphine
PH3, and the bases formed from them by substitution of
hydrocarbon radicles for the hydrogen. The element itself,
as a permanent incombustible gas, is wholly unlike the
solid inflammable phosphorus, whilst its oxides and acids,
in their volatility and easy decomposability, form a strong
contrast to the corresponding compounds of phosphorus.
The accident of agreement in their crude or synoptic
formulae by no means implies identity of constitution, and,
for the reasons just alluded to, it seems not improbable that
the atomic structure of the nitrogen oxides and acids is, at
least in many cases, different from that of the phosphorus
compounds.
The following are almost the only known examples of
concordance in the crude formulae of the oxygen compounds:Nitrogen                  Phosphorus
Trioxide.                  Trioxide.
N203                      P203
Pentoxide                 Pentoxide.
N205                       P205
Nitric Acid.           Metaphosphoric Acid.
HNO3                      HPO3
The  compounds  N20,  NO, N204,  HNO, find  no
parallels in the phosphorus series, whilst H3PO4, H3PO3,
H3PO2, H4P207, the salts of which, at least, are highly stable
and definite, are altogether unrepresented among the
nitrogen  compounds.   Again, whilst phosphorus  yields
two chlorides, PC13 and PC15, and an oxychloride,
POC13, nitrogen forms no chloride,* but two oxychlorides,
NOCl,a yellow  gas, and  NO2C1, a volatile  liquid.  It
has been the very general custom to represent all the
* The so-called chloride of nitrogen, obtained by the action of
chlorine upon sal-ammoniac, probably contains hydrogen. Its formula
is supposed to be NHCI2, but in consequence of its dangerously
explosive properties, it has been only imperfectly examined.




A tomicity of Nitrogen and Pho.sphorus.     95
compounds of nitrogen by formulae framed in the same
manner as those of the phosphoric compounds, without
regard to their dissimilar properties. But by assuming
that nitrogen is trivalent in these bodies, we render some
account of these discrepancies. On this view the formulae
given above for nitric acid and anhydride may be expanded
in the following manner:Nitric Pentoxide.         Phosphoric Pentoxide.
N         /O0             =P-O —P=O
NO- N   I                  II    II
0/           \         0               0
Nitric Acid.            Metaphosphoric Acid.
0O                          O: —PO —H
I  N-O —H                      II
0/                             0
The general fact that bodies in'which oxygen is more or
less united with oxygen are rather unstable lends some
further countenance to this view.*
In the ammonium compounds on the other hand (p. 2 5 ),
there can be little doubt that nitrogen is pentad, chloride
of ammonium, for instance, being NVH4Cl.
Phosphorus.-The two chlorides PCI, and PC1, may be
taken as the types of the two chief classes of compouns
which phosphorus is capable of forming.
The trichloride PCI, is a colourless volatile liquid, which
combines in a direct manner with chlorine, with cxygen, or
with sulphur, yielding the pentachloride, PCI, the oxychloride, PC130, and the sulphochloride, PC13S, in which
compounds the phosphorus appears to be saturated.
The best known iodide is anomalous.  It has the
formula P214, and corresponds with a liquid hydrogen
compound PH,, to which the spontaneous inflammability
of phosphoretted hydrogen is stated to be due.
* The formulae might also be written O=N-O —O-O-N=O and
O=N-O —O-H, though for several reasons the form given in the
text is preferable.
0 2




I96             Chemical Philosophy.
Phosphine, or phosphoretted hydrogen, PH3, is a highly
inflammable gas, which so far imitates ammonia,* that it
combines with hydriodic acid to form a salt, phosphonium
iodide PH4I, crystallisable in cubes.
The compounds in which phosphorus plays the part of
a pentad are more numerous. Beside the pentachloride
PC15, and pentoxide P205, and intermediate PC130, we
have the following series of oxidised bodies which may
be considered as derived from phosphine, either by addition
of oxygen or by substitution of oxygen for its hydrogen.
H
Phosphine                   /
H3P      or             P-H
H
Phosphine Oxide.
H3PO, unknown, but represented by
C2H5
Triethyl-phosphine Oxide.     /
(C2H5)3PO    or      O = P-C2H,
C2H5
OH
Hypophosphorous Acid (monobasic)
H3P02    or         O=P-H
H
Oil
Phosphorous Acid (dibasic).    /
H3PO3    or         O=P-OH
H
* See also Amines, p. 252




Pzosphzoric Compounds.            197
OH
Orthophosphoric Acid (tribasic)
H3PO4   or          O=P-OH
OH
Pyrophosphoric Acid (tetrabasic).  OH
H4P207    or        O -P     H
I  O= 1
OH
OH
Metaolhosphoric Acid (monobasic).  i/
HPO3                 O=P
OH
With the exception of the first two compounds in the list,
phosphine and its oxide, these bodies are all powerful
acids, syrupy, glassy, or crystalline, according to circumstances. They cannot be made to evolve oxygen at any
temperature as nitrous and nitric acids do. Under the
influence of heat, hypophosphorous and phosphorous acids
emit phosphine, leaving a residue of phosphoric acid;
2H3PO2 -  H3PO4 + H3P
and 4H3PO3 = 3H3PO4 + HP,
whilst orthophosphoric acid by the same treatment loses
water, though the dehydration can by no means be carried
beyond the production of metaphosphoric acid.
2HsPO  -  HO = H4PO27
and H3PO4 -  H20 =  HPO,
or HIP2O7 -  H20 =  2HPO3




198             Chemnical Philosophy.
By boiling either of these products with water the orthoacid is regenerated.
Metaphosphoric acid is convertible, by the action of
heat, into several polymeric acids, Hn Pn O3n.
Phosphorus is closely connected with the semi-metals
vanadium and arsenic, by reason of the very general
isomorphism of the phosphates with the arsenates and
vanadates.  This relationship  will be adverted to in a
subsequent chapter.
CHAPTER XXII.
DIVISION II.-METALLOIDS.
THE name melalloid obviously belongs to bodies which have
the form or appearance of a metal, or which in some way
resemble a metal. Hence the impropriety of applying this
term, as is not uncommonly done, to such elements as the
halogens, to nitrogen or phosphorus.   The name is intended in this volume to designate a body which, although
resembling a metal in most characteristics, yet lacks some
one or more of the features which true metals generally
present. It applies to a somewhat miscellaneous set of
bodies. The question of the propriety of including hydrogen
amongst them will be discussed presently, but, leaving this
one element out of consideration, the test by which the submetallic character of most of them may be detected is the
formation of more or less definite oxisalts. Their oxides
are for the most part well-marked anllydric acids, but each
of them is capable of producing at least one oxide which
possesses basic tendencies more or less pronounced. Still
putting hydrogen aside, the metalloids are comparatively
imperfect conductors of heat and electricity, and generally
brittle. Tin is, however, both malleable and moderately
ductile.




Acids or Hydrog-en Salts.           199
METALLOIDS. —I. HYDROGEN.
Hydrogen is isolated from all other known bodies by
reason of its extreme lightness: its specific gravity being
only'o693 when air is I. Chemically it differs from all the
non-metals in manifesting no tendency to combine with
metals; only one such compound, cuprous hydride, Cu2H,,
being known. On the other hand hydrogen is, of all the
elements, most ready to lend itself to that peculiar state of
mechanical combination with metals known as " occlusion "
(Chap. V., p. 3i.) Palladium, for example, absorbs hydrogen
largely, and the charged metal is indistinguishable in appearance from pure palladium, though its conducting power is
slightly diminished. This body was regarded by Graham,
who discovered it, as the realisation of the long-cherished
idea of the essentially metallic character of hydrogen. But
its title to recognition as a metal or metalloid is based more
securely upon the resemblance which acids or hydrogen salts
bear to metallic salts. In spite of a considerable number;of gaseous and liquid exceptions, the majority of known
acids are crystallisable bodies, often combining with water
of crystallisation, and utterly indistinguishable to all appearance from ordinary saline compounds. Among common
acids, for example, the following are crystallisable:Sulphuric acid.. H2SO4 (below - 35~)~
Glacial sulphuric acid. HSO5 (below 9go.)
Phosphoric acid.      H3PO4.
Boracic acid.      H3BO3.
Oxalic acid..      H2C204 + 2H20.
Formic acid.. HCHO, (below  ~o).
Acetic acid.. HC2H30, (below I7~).
Tartaric acid...   2C4H40.
Citric acid.         H3C6HO7 + H2O.
Gallic acid..      HCH5O05 + H20.




200             Chemical Philosophy.
But the resemblance between hydrogen salts and metallic
salts is not limited to external form. They agree in chemical
reactions. When an acid is acted upon by a metal, and
hydrogen is expelled, the metathesis is entirely comparable
with the exchange of a more positive for a less positive
metal of the ordinary recognised class. For example, zinc
decomposes hydrogen sulphate just as it decomposes copper
sulphate; in each case the less positive metal is displaced,
and a new salt formed.
(H2SO + Zn = H, + ZnSO4.
I CuSO4 + Zn = Cu + ZnSO4.
Again water, that is, hydrogen oxide or hydrate, H20 or
H(HO), not unfrequently imitates metallic oxides or
hydrates in their chemical reactions. In order to produce
exactly the same effect in any given case, it is only necessary
to use the water in larger quantity or at a higher temperature.
The following are examples of this mode of action:IMgClI +  HgO    =    MgO   +  HgC12.
MgCI2 +  H20    =    MgO   +   2HC1.
(Fe2C16 + 6NaHO = Fe,(HO)6 + 6NaC1.
Fe2CI6 +- 6HHO  =  Fe2(HO)6 + 6HC1.
I(CH,)'"St,' + 3NaHO  =  C3H5(HO)3 + 3NaSt.
j Stearine.                 Glycerine.    Sodium
Stearate
(C3H5)"'St, + 3HHO   =  C3H5(HO)3 + 3HSt.
Stearine.                Glycerine. Hydrogen stearate
or stearic acid.
Water and acids also undergo electrolysis in the same
manner as metallic salts, e.g.,
1 St = C18H3502.




Tellurium.                    201
(2HHO   =   H2  +   (HO)2*
2HC1    =   H2  +   C12.
CuC12   =   Cu  +   Cl,.
Lastly, it might be pointed out that the sour taste of
hydrogen salts and their reddening effect upon vegetable
blues are no more to be regarded as indications of peculiar
constitution than is the alkaline reaction of many normal
metallic salts or the neutrality of others.  In such a series
as the following we see a gradual transition from strongly
alkaline to strongly acid properties.
Na3PO4      Na2HPO4        NaH2PO,       H3PO4
Very alkaline.  Alkaline.      Acid.       Very acid.
Enough has been said to indicate to the student that,
despite many peculiarities which are in all probability connected with its remarkable physical characters, hydrogen
displays, from a chemical point of view, features bearing an
unmistakeable metallic impress.
METALLOIDS.-2. TELLURIUM.
Tellurium in many respects closely imitates sulphur and
selenion. Thus it forms a gaseous telluretted hydrogen,
H,Te, the representative of hydrogen sulphide, also oxides
TeO, and TeO, and acids H2TeO, and H2TeO, corresponding respectively with sulphur dioxide and trioxide, and
with sulphurous and sulphuric acids.  But tellurium  is
decidedly metallic in appearance, is thrown down from
solution in the form of sulphide by sulphuretted hydrogen,
and its lower oxide saturates acids forming salts, of which
the sulphate Te(SO4)2 and nitrate Te(NOa), are sufficient
examples.
* This has lately been shown to be the first phase in the electrolytic
decomposition of water, the oxygen which is evolved being due to a
secondary reaction, thus,
2HHO =  H2 + (HO)2 = H2 + H20 + H     0 + O.




202                Chemical Philosopyy.
METALLOIDS. —3. TIN, TITANIUM, ZIRCONIUM.
Save for the distinctly metallic character of elemental tin
and production by both tin and titanium of an inferior
diatdic class of derivatives, these elements bear a strong
family resemblance to silicon. That the metallic function
is more or less distinctly developed in them, whilst it is
altogether imperceptible in silicon, is also evidenced by the
existence of various oxysalts, sulphates, phosphates, etc.,
of tin, titanium, and zirconium, similar compounds of
silicon being entirely unknown.
The most characteristic compounds of these elements are
the chlorides, oxides, and acids in which they play the part
of tetrads. The following are some examples compared
with corresponding silicon compounds.
Silicon S=28. I Tin Sn=II8. ITitanium Ti=5oi   Zirconium
Zr=89'5.
SiCI4,         SnC14           TiCI4           ZrCl14
liquid, B.P. 50C liquid, B.P. I20~  liquid, B.P. J35~    volatile solid
SiFnF4 ST                      TiF4            ZrF4
liquefiable gas   fuming liquid?  fuming liquid  solid
M',SiF6       M'SnF6            2 I'TiF6      M'2ZrF
Filuo.ilicates  Fluostannates  Fluotitanates  Fluozirconates
Corresponding salts isomorphous.
SiO2    I    SnO,       I      TiO       I     ZrO
isomorphous
in tinstone.     in rutile.      artificial.
Atomic/ty of ]Ti and A/liez  Elements.
As already mentioned, tin forms, in addition to the welldefined tetrad compounds, some of which are represented
in  the  foregoing  table,  an  oxide, Sn.On, a  chloride,
SnXCln, a sulphide, SnS,n, and some other compounds in
which it seems to be diad. In the absence of direct




Vanadium, Arsenic, Antimony, Bismuth.   203
evidence in favour of the one view or the other, it is
customary to represent the oxide as SnO, and the chloride
as SnCl2. In favour of the formula SnO, we have its
analogy to that of carbonic oxide, CO, though there is
absolutely no resemblance in the properties of the two
bodies to support such a view.
For stannous chloride representing it as a dichloride we
have no example among the compounds of carbon and
silicon. On the whole, it seems, therefore, probable that the
tin retains its quadrivalence in both the oxide and the
chloride, and their formulae in accordance with this view
would be written thus:Sn2CI4  or C1, = Sn = Sn = C12
Sn20,, or O = Sn = Sn = 0.
METALLOIDS. —4. VANADIUM, ARSENIC, ANTIMONY, BISMUTH.
(For numerical relations of atomic weights see Non-metals,
Class 5, p. I93.)
These  elements are all quinquivalent in their most
advanced states of combination, the tendency to assume
this condition being the least prominent in the case of
bismuth, which is of the whole class the most decidedly
metallic.
Vanadium is remarkable for the extensive series of
oxides, chlorides, and oxychlorides to which it gives rise,
the different stages of oxidation of its dissolved compounds
being characterised by a strange variety of colour, a yellow
or red tint belonging to the higher, and a blue or green tint
especially to the lower oxides, which thus resemble some
of the compounds of chromium, molybdenum, and tungsten.
Oxides of phosphorus, arsenic, antimony, and bismuth,
except BiO,, are white.
The known oxides of vanadium have the formula VO,
VO or V202, V2O3, V VO2 or V204, and VO2, thus running
parallel with the series of nitrogen oxides. There are three




204              Chemical Philosophy.
chlorides, VC12 or V2C14, VCI3 and VC14. The last is a
brown liquid boiling at 1540 and yielding a vapour of
density 96 5, which is half the molecular weight represented
by the formula VCI4. Vanadium, therefore, does not maintain the usual persistence of odd or even atomicity throughout its combinations (see Chap. XIX).
The vanadates M'3V04 are isomorphous with the phosphates M'3P04. Metavanadates and pyrovanadates also exist
corresponding with meta- and pyrophosphates, besides a
certain number of anhydro-meta-salts M"(VO3)2.V205 and
M"(V03)2. 2V205.
The analogies of arsenic, antimony, and bismuth will
be most satisfactorily brought out by tabulating the formulle
of their leading compounds. For the sake of comparison,
the corresponding compounds of phosphorus are placed
side by side with them.
The vapour densities of phosphorus and arsenic lead to
the molecular formulae Pi and As, for those elements; but
antimony and bismuth, though easily fusible metals, are
not volatile enough to give vapour at any manageable
temperature. Their molecular formulae are therefore unknown.
Phosphorus     Arsenic      Antimony       Bismuth
Hydrides and Ethylides.
PH3    I   AsI3            SbH3    t
P( C2H5)3  As(C2Hs)3  i  Sb(C2H5)3  Bi(C2H,)3
Chlorides.
PC1a         AsCl; i      SbCI3          BiC13
~PCI-                  SbCl 
Oxides.
P203        (As2O3)2   1    SbO3    I    Bi2O3
Sb2O4         Bi2O4
P200         As O     {   Sb2O5    ~     Bi205




A rsenic, Antimony, and Bismuthz.       205
Phosphorus.   Arsenic.   Aintimony.    Bismuth.
Acids.
HP1120         _             _      I     _
HAsO2?       HSbO2         HBiO2
H2PHO3       H3AsO3  _I
HP03         HAsO03       HSbO3          HBiO3
H3PO4        H3AsO4       H3SbO4?
H4P207       H4As207      H4Sb2O7
Sulphides.
P2S 
AssS.2                     Bi2S,2
P2S3        As2S3 a       SbsS3         Bi2S3
P2S5  As2S5               Sb2S5
A few peculiarities exhibited by particular compounds
occurring in this table demand notice.
i. The faculty of neutralising acids possessed in so remarkable a manner by ammonia is displayed by phosphoretted hydrogen towards one acid only, hydriodic acid,
and altogether vanishes in the arsenic and antimony
hydrides.
2. The ethylated and corresponding methylated compounds are characterised not so much by a power of
combining with acids, as by a tendency to unite with
0, S, C12, J1, &c., whereby the central element is brought
to a state of saturation.
3. No pentachloride, pentabromide or oxychloride, or
bromide of arsenic is known.
4. The vapour density of arsenious oxide (white arsenic)
is i98. Hence its molecular weight is I98 X 2=396 and
formula As406. It is usual to represent the oxides of
phosphorus by the formulae written in the table; but if the
trioxide could be obtained in a pure state and its vapour
density determined, it is by no means improbable that its
molecular formula would also be doubled.
White arsenic occurs in commerce in the form of white
masses with conchoidal fracture, often exhibiting a banded
appearance, due to the presence of the isomeric vitreous




206              Chemical Philosophy.
and porcellanous varieties. The former is converted into
the latter by pulverisation or by protracted boiling with
water. Either variety is but slightly soluble in water, and
the solution gives only a feebly acid reaction. Porcellanous
arsenic probably owes its opacity to crystalline structure.
By careful sublimation at regulated temperatures white
arsenic may be obtained either in the form of transparent regular octahedra or in prismatic crystals of the
trimetric system.  It is, therefore, isodimorphous with
antimonious oxide.
White arsenic is a feebly basic, as well as acid, anhydride.  Its best known salt is the  double  tartrate
K(AsO)CHO6, which is analogous to the potassio-tartrates
of antimony, K(SbO)CH,06 (tartar-emetic), and boron
K(BO)C4H406.
5. Phosphoric oxide is volatile and stable, but AsO5,
SbO,, BiO,5, are resolved by heat into oxygen and a lower
oxide.
6. Arsenious acid appears to be normally tribasic, the
yellow  silver  salt containing  Ag3AsO3.  It therefore
differs from phosphorous acid, which is dibasic.
Phosphorous Acid.  Arsenious Acid.
OH             OH
O-=P-OH          As-OH
H              OH
7. The orthophosphates and arsenates correspond in
every respect with each other, and in many cases with the
orthovanadates.  In addition to the ordinary salts of the
common phosphate type, M'H2PO, M'2HPO4 and M'3PO,,
there is a remarkable series of compounds occurring as
minerals in combination with a chloride or fluoride.




Niobilm and Tantalum.              2o7
Apatite      3[Ca3(PO,4)]. CaF, or Ca.(PO4),F
or       3[Ca,(PO4),]. CaCI, or Ca5 (PO4)C1l
Pyromorphite 3[Pb3(PO4)2]. PbCI2 or Pb5(PO4),Cl
Mimetine     3[Pb,(AsO4).]. PbC12 or Pb,(AsO4),CL
Vanadite     3[Pb,(VO4)2]. PbCI, or Pb5(VO,),C1.
Orthoantimonates and orthobismuthates are unknown, unless we consider the intermediate tetroxides in that light:
antimonious orthoantimonate Sb204 or Sb"'(SbvO,) and bis-,1i~thous orthobismuthate  Bi204 or Bi"'(BiVO4).  Several'-hantimonates are, however, known; the sodium
mple, Na3SbS4.9H20, crystallising readily.!Iphides of arsenic, antimony, and bismuth act
~x. eti ainhydrides, and in combination with the sulphides of silver, lead, copper, and other metals, constitute
several interesting minerals.
METALLOIDS.-5. NIOBIUM, TANTALUM.
These two rare metals may be regarded as about equally
related on the one hand to tetrad tin and its allies, and on
the other to hexad molybdenum and tungsten. With the
latter elements, indeed, there is not only considerable
analogy of properties, but a curious coincidence of atomic
weights.
MovI 96            NbV  94
WVT I84            Tav  i82
Niobium and tantalum are pentad elements, and are characterised by the production of pentachlorides in the form of
yellow, fusible, volatile bodies, the vapour densities of which
accord with the formula NbC5, and TaCl, respectively.
The metals themselves are only known in the condition
of black powders, which resist the attacks of most acids,
except hydrofluoric acid, in which they are slowly soluble
with evolution of hydrogen. Their fluorides unite with
the fluorides of the alkali-metals, forming soluble and crys



20c8            Chemical Phzilosop/hy.
tallisable double salts, 2KF,NbF5 and 2KF,TaF5.   But
niobium, at least, is also specially characterised by the
production of oxyfluorides, which, in combination with the
fluorides of the more positive metals form salts which are
isomorphous with the fluotitanates, fluostannates, fluotungstates, and fluozirconates.  The following pairs of compounds, for example, crystallise in the same form:3KF. HF. NbOF3
3KF. HF. SnF4
2NH4F. NbOF3
2NHF. WOF2,
3NH4F. NbOF3
3NH4F. ZrF4.
2NHF. CuWO,F4
2NH4F. Cu TiF6.
It thus appears that groups of elements of different
atomicities may occasionally replace one another isomorphously, provided the valency or atomicity of the various
groups is the same.
The radicles which are thus at the same time chemically
and isomorphously equivalent to one another in the foregoing compounds are as follows:
(TiF2)", (ZrF2)", (SnF2)", (NbOF)", and (WO,)".
Such relations are of precisely the same character as
are those of the group (NH,)', ammonium, to the single
metallic atom  K', and precisely the same conclusions,
neither more nor less, can be deduced from them.
METALLOIDS. 6.
Molybdenum      Mo =  96
Tungsten        W   = I84
Uranium         IU  -= 240.




Molybdewim, Tun; zgsten, Uraniumn.    209
These elements are heavy metallic bodies: tungsten and
uranium being especially distinguished both by high atomic
weight and specific gravity.
They appear to be hexads, though, of the three, tungsten
alone produces a hexchloride, and that unstable. In the
formation of trioxides and volatile dioxydichlorides (general
formula M'OCI2,) they resemble chromium, but they differ
from that metal in furnishing neither sesquioxide nor monoxide analogous to Cr2O, and CrO.
The infraction of the law of even numbers occurring in the
case of the chlorides and bromides of tungsten, and in the
dlorides of  anium, has been already adverted to (Chap.
T   tc Su denum, like the rest, seems to form a pentaoe sulnl,, r
The trioxides, MoO3, W03, and U03, are generally
acid anhydrides. Some of the tungstates of the form
M)'2WO seem to be isomorphous with the corresponding
chromates and sulphates, but the majority are extremely
complex.
These trioxides also exhibit a feebly basic function, the
oxysalts of uranium being the most stable. The nitrate, for
example, a yellow crystallisable salt, has the formula
(UO2)"(NO3)2.6H20, and the sulphate and  oxalate are
formed upon the same type.
Molybdenum and tungsten agree in forming trisulphides,
MoS, and WS3, which are unrepresented among the compounds of uranium and of chromium.
P




210              Chemical Philosophy.
CHAPTER XXIII.
DIVISION III.-METALS.
The following are the principal characteristics of the
metals as a class:
I. Metals are all solids at the temperature of the air,
with the sole exception of mercury, which is ordinarily
liquid, but solidifies at - 4oo.  The solid metals differ very
much in their fusibility, the metals of the alkalis melting
most readily (potassium at 62~'5), whilst platinum and its
congeners require the highest temperature of the oxyhydrogen flame.
2. Metals combine great opacity with high reflective
power, exhibiting the appearance which is sufficiently well
known as metallic lustre. Gold in very thin leaves transmits a greenish light.
3. That all the common metals are heavy is a fact familiar
to everyone; but an inspection of the table of specific
gravities given at the commencement of the section will lead
to the conclusion that there are many which are much lighter
than the familiar iron, copper, lead, and silver, whilst there
are several which are even heavier than gold. As a curious
fact, the heaviest solid known is a metal, platinum (sp.
gr. 2I'5), and the lightest known solid is also a metal,
namely, lithium, whose specific gravity is only'578.
But it is of much greater importance to observe that in
many cases groups of metals which are associated together
by reason of community of chemical properties, have also
specific gravities which nearly approximate to one another.
As examples of this may be cited the alkali metals,
lithium, potassium, sodium, rubidium; the alkaline earth
metals, magnesium, calcium, strontium, barium; the metals of
the iron family, chromium, iron, manganese, nickel, cobalt,




General Characters of Metals.           21 I
and copper; silver, lead, and thallium; and lastly, the noble
metals, gold, iridium, and platinum. Possibly other similar
associations might be discovered, but for the fact that
several metals are scarcely known in a compact form, and
in other cases the numbers may be more or less incorrect
in consequence of the presence of impurity in the specimens
operated upon.
4. Metals are the best conductors of heat.  They differ
widely among themselves in this respect, as the following
rough comparison shows.  Silver, as the best conductor, is
placed at the head of the list. The numbers may be taken
to represent the relative lengths of bars of equal diameter
which, by applying a common source of heat to one extremity, would become equally heated in the same time.
Silver...... oo000
Copper....         736
Gold......  532
Iron......   1 9
Lead......   85
Platinum.....   84
Bismuth (an imperfect metal).   I8
5. Metals are the best conductors of electricity.  The
metals given in the foregoing list stand in nearly the same
order as regards electric conductivity.
6. Metals are almost always malleable, though in a few
cases this happens only at slightly elevated temperatures.
Frozen mercury is malleable.
7. Many metals are ductile.  It by no means follows,
however, that malleability and ductility should be manifested in the same degree by a given metal. Gold, for
example, is by far the most malleable metal, though in
point of ductility it is surpassed by platinum.
8. The oxides of metals are very generally basic.
The basigenic character belongs exclusively to the metals
and metalloids. But the oxygenic function is discharged
P 2




2 I2            Chemical Philosophy.
almost equally well by particular elements in all three
divisions.
The transition from metal to non-metal is, therefore,
not accomplished by any sudden break, and the student
must be prepared to encounter great, and, perhaps, insurmountable difficulties in any attempt to establish a line of
demarcation between them.
The following series of oxides will serve to indicate how
gradually the one character disappears as the other is
developed. The symbols in black type represent basic
oxides, those in Roman represent oxides which go to the
negative side of the salts into which they enter.
From Non-metallic to iJetlallic.
SO3  W03 Cr03  FeO3
SO2 Wvo2
Cr03  Fe2O3  Ni203
No Salts,
CrO    FeO    NiO   CuO
Extremely
oxidisable.            CU2O  Ag20  K20
Salts unstable.
METALS.-I. THE METALS OF THE ALKALIS. MONADS.
Lithium Li  -   7  Li+K
Sodium Na   =  23            23
Potassium K  =  39'I    2
Potassium K  =  39'i K+Cs
Rubidium Rb =  854 -- =86
Ccesium   Cs =  I43-   2
Salt Type, AM' Cl.
These are soft, white, light, easily fusible, and somewhat
volatile metals. They oxidise rapidly in the air, but differ




Metals of the Alkalis.             213
materially from one another in this respect, their affinity
for oxygen increasing with the atomic weight.  This is
shown especially by the spontaneous inflammability of
rubidium, and the impossibility of obtaining coesium by
decomposition of its carbonate with charcoal.  They all
decompose cold water with formation of a soluble
hydrate and evolution of hydrogen gas. But as a consequence of the inferior energy of lithium and sodium, the
heat developed by their action upon water is not sufficient
to cause the ignition of the escaping hydrogen, whereas the
hydrogen disengaged by potassium  inflames instantly, and
continues to burn with a purple light, due to the accompanying vapour of the metal.
The hydrates of these metals are all fusible, scarcely decomposed by heat, but volatilising at high temperatures,
very soluble in water, and the solutions caustic to the
skin, alkaline to litmus, and absorbing carbonic acid from
the air. The hydrates also saponify oils and fats, and the
resulting soaps are alkaline and soluble.
The chlorides, sulphides, sulphates, phosphates, and
carbonates of the alkali metals are all soluble in water,
and their chlorides and sulphates yield perfectly neutral
solutions.  The sulphates  combine with  the  sulphate
of aluminium and the allied metals, generating highly
characteristic double salts, called " alums," which crystallise in octahedra. A crystallised lithium-alum has not yet
been described, but this is probably owing to its great
solubility.
The alkali salts all communicate intense and characteristic
colours to the Bunsen flame, and the spectrum (p. 41) of
the light so produced exhibits in each case a comparatively
small number of bright lines.
As indicated by the atomic weights, the alkali-metals
may be divided into two sub-groups.  In the one we have
potassium, ccesium, and rubidium, which are distinguished
by greater chemical activity and inferior solubility of




2 I4            Chlemical Phzilosophy.
their salts, notably of the platinochlorides and acid
tartrates.
K2PtCl6          KHC4H406
Rb2PtC16         RbHC4H406
Cs2PtCI6         CsHC4H406
Sodium and lithium, though entirely comparable with
potassium, are yet distinguished from it by much feebler
chemical energies, by the solubility of their platinochlorides, acid tartrates, and alums, and by different minor
peculiarities. Sulphate of sodium, for example, crystallises
with ten molecules of water, whilst sulphate of potassium
is anhydrous. The carbonates also differ, that of sodium
forming large efflorescent crystals containing oH,20, the
carbonate of potassium  occurring in  small deliquescent
granules, containing usually about one or two molecules of
water.
Both sodium and potassium yield unstable peroxides,
Na,02 and K02,, and potassium is even capable of passing
to a higher stage of oxidation, the product being a tetroxide,
K204. No salts corresponding with these oxides exist.
METALS.-2. METALS OF THE ALKALINE EARTHS. DIADS.
Calcium    Ca =   40
Strontium   Sr =  87'5  Ca + Ba = 88.5
Barium     Ba   133
Sat l  ype, l " Cl2.
These three metals, obtained by the electrolysis of their
fused chlorides, are yellowish, hard, and fusible, and extremely oxidisable. They are, however, less oxidisable
than the metals of the alkalis, though they are still capable
of decomposing cold water.  They also communicate
characteristic colours to flame, and yield spectra (p. 41)
which are easily recognisable but somewhat more complex
than those of the alkalis generally.
The hydrates of the alkaline earths are white substances




Mietals of the A Ikaline Earths.      215
which are decomposed by heat into water and the anhydrous
oxides. They are far less soluble in water than the alkalis,
and are proportionately less caustic, alkaline, and attractive
of carbonic acid.  They saponify fats, but the resulting
soaps are generally insoluble in water.
The salts of barium, strontium, and calcium are, as a rule,
perfectly neutral.
The chlorides and sulphides are soluble in water, but the
sulphates, phosphates, and carbonates are insoluble.
Oxide of barium or baryta exposed to a current of air
or oxygen at a heat short of redness, absorbs oxygen and
becomes converted into a peroxide, BaO2.  The corresponding peroxides of strontium and calcium are obtained
as white precipitates by adding solution of hydric peroxide
to lime or strontia water. All three are resolved by ignition into oxygen gas, and a residue of the protoxide. They
are also soluble in hydrochloric acid, yielding peroxide of
hydrogen and the chloride of the metal, e.g.,
BaO2 + 2HCI = BaCl2 + 02H2.
Hence it is presumable that they have the same constitution as hydric peroxide, the oxygen atoms partly satisfying
each other's attractions, according to the following graphic
formulae:Hydric peroxide.  Baric peroxide.
H O                   0
I           Ba"/  I
H-O                 XO
Of the three elements, barium, with the highest atomic
weight, is decidedly most basylous. Strontium standing next
is, in respect to some characters, morenearlyrelated to barium
than to the third member of the series, calcium. Thus
the nitrates of strontium and barium crystallise in anhydrous
octahedra, isomorphous with lead nitrate, Pb(NO,)2. The
sulphates of these two metals are also anhydrous, and are
practically insoluble in water. and acids.




21 6            Chemical Philosophy.
Nitrate of calcium is a deliquescent salt, soluble in
alcohol and crystallising in prisms, which contain
Ca(NO )2.4H0O.  Its sulphate, in the form  of gypsum,
combines with two molecules of water, is perceptibly soluble
in water, and much more freely so in hydrochloric acid.
On the other hand, strontium agrees with calcium in the
production of a deliquescent chloride, which is soluble in
alcohol, whilst the barium chloride is insoluble in alcohol.
The crystallised chlorides have the following formulae:Chloride of Calcium    CaC12. 6HO.,,    Strontium  SrCl2. 6H2O
Barium     BaCl2. 2HO
METALS.-3. ZINC GROUP. DIADS.
Miagnesium  Mg      24
Zinc        Zn _  65 Mg ~ Cd  68.
Cadmium    Cd = I12'    2
Salt Type, MzC2.
These elements are rightly associated together in consequence of a very obvious seriation of properties, notwithstanding that they are far less intimately related to one
another than, for example, the metals of the alkaline-earth
family. Magnesium is a white metal, zinc and cadmium faintly
bluish-white, and all three are volatile. Their volatility
somewhat strangely increases in proportion as the atomic
weight increases, whilst their basigenic power diminishes.
That zinc is decidedly more positive than cadmium is
shown by its power of precipitating cadmium in the metallic
state from its solutions. That magnesium is more positive
than the other two is shown by its power of decomposing
water when heated with it, or more readily if previously
coated with pulverulent copper.* Also by the precipitation
* Zinc and cadmium, when coated with spongy copper, are also
capable of decomposing water slowly.




Magnesizuz, Zinc, and Cadmium.        217
of both zinc and cadmium when metallic magnesium is
introduced into solutions of their salts.
Magnesium, zinc, and cadmium are all easily combustible
in air or oxygen, the combustion of the former two being
attended by the emission of a dazzling light. Each metal
forms one oxide: MgO, a white unalterable powder; ZnO,
a white powder, becoming yellow when heated; CdO, a
yellowish-brown powder.  These oxides are insoluble in
water, that of magnesium only showing a faint alkaline
reaction when placed upon wet test paper. The oxides and
hydrates are readily soluble in solutions of ammoniacal salts.
The hydrates are easily resolved, by being heated, into
water and the oxide, and the carbonates in like manner
give up carbonic anhydride, leaving a residue of the oxide.
The chlorides are volatile, deliquescent solids.
The sulphide of magnesium is an earthy substance which
is decomposed even by water.
MgS + 20H2 = Mg(OH)2 + SH2
and is therefore not precipitated on the addition of a
soluble sulphide to a magnesian solution.
Zinc sulphide (native =  blende) is a white precipitate
easily soluble in diluted mineral acids, and hence only imperfectly precipitated by the action of hydrogen sulphide
upon the solution of a zinc salt.
Cadmium sulphide (native = greenockite) is a yellow
precipitate thrown down by hydrogen sulphide from acidified solutions of cadmium salts.
The sulphates of these metals are, perhaps, the most
characteristic of their salts. They are soluble in water, and
have the following formulze:Magnesium sulphate  MgSO4.7H20
Zinc sulphate. ZnSO. 7 HO0
Cadmium sulphate. CdSO4.4H20
The magnesium and zinc salts crystallise in four-sided




2 8             Chzemical Philosophy.
prisms, isomorphous with the corresponding nickel sulphate.
All three combine with potassium sulphate, generating
double salts, which crystallise with six molecules of water:
MgSO4. KSO. 6H20
ZnSO4. K,SO4. 6H,20
CdSO4. K2SO4. 6H20
Sulphates isomorphous with these are produced by several
other metals, such as copper and iron, and will be referred
to in the proper place.
A general review of its properties indicates that magnesium forms a connecting link between zinc and calcium.
From the latter it differs chiefly in the insolubility of its
hydrate in water, and the solubility of the same compound
in ammonium  chloride.  Sulphate of magnesium  is also
distinguished from sulphate of calcium by its ready
solubility.
METALS. 4.
Silver Ag =  Io8.      Mercury Hg =  200
Salt type, M'CI.      Salt types, M",Ci,,
and MA" Cl,.
Silver is a white metal, very malleable and ductile, fusible
at a red heat, and volatilising in blue vapour at very high
temperatures. It is the best known conductor of heat and
electricity. The only known monad metal beside the metals
of the alkalis, it is undoubtedly related to them, for its
sulphate is isomorphous with anhydrous sodium sulphate,
and by combination with aluminium sulphate it yields a true
alum crystallising in octahedra. In almost every3 other
respect, however, it differs from them.
Thus its specific gravity is high, Io'5, and it is quite untarnished by pure air, whether dry or moist, the blackening
so often observed being due to the formation of a film of
sulphide. Pure silver has the singular property of occluding.




Silver anzd Mercury.             219
when fused in the air, a considerable volume of oxygen,
which escapes as the temperature goes down and the metal
solidifies. Silver has no action upon water or steam at any
temperature, and is quickly reduced from its salts by zinc,
iron, or even mercury. The oxide is said to be sufficiently
soluble in water to give a faintly alkaline reaction, but it
differs entirely from potash or soda in appearance and in its
decomposability by heat. The chloride, iodide, sulphide,
carbonate and phosphate of silver are all insoluble in water,
and the sulphate very sparingly soluble.
Silver forms a peroxide, Ag202, even less stable than the
potassium peroxide. There is also a suboxide, AgiO, the
composition of which is, however, not very well established.
The relations of silver to the alkali-metals on the one
hand, and to mercury on the other, are about equally
distant. This connection is obscurely indicated by comparison of their atomic weights, from which it may be seen
that silver is intermediate between sodium and mercury.
Na 23 + Hg 200 = III'5 (Ag = Io8).
2
Mercury, like silver, is a white metal, crystallising in the
regular system and volatilising, though much more readily.
Both are easily reducible to the metallic state, and the
metals are both  incapable of decomposing water or
hydrochloric acid. Mercury, however, is slowly oxidised
by heating in air or oxygen. Both are soluble in nitric
acid, and when boiled with concentrated sulphuric acid
sulphur dioxide is in each case evolved, and a sulphate produced in the form of a sparingly soluble crystalline powder.
Silver oxide, Ag20O, and mercurous oxide, Hg20, are both
nearly black, insoluble in water, and resolved by heat into
oxygen gas and the metal.
Silver chloride, AgCl, and mercurous chloride, Hg2Cl2, are
both obtainable by precipitation as white powders, which




220             Ckzemzical P/zilosopiy.
blacken, the silver salt quickly, the mercury salt slowly, on
exposure to light.
The iodides are both yellow and insoluble. The chief
distinction between silver and mercurous salts lies in the
volatility of the latter.
As regards the formulae of the mercurous compounds,
their analogy to the compounds of silver points to the
adoption of similar formulae.
Ag - 0 - Ag.          Hg - O-Hg.
Ag - C1, etc.          Hg - C1, etc.
But, inasmuch as the low vapour-density of calomel,
II7-75=  200 + 35'5, has been traced to dissociation
(Chap. XVII.), this, and consequently all other mercurous
compounds, must be represented as containing diad
mercury, thus:
Hg
Hg2O  =-    I  O
Hg
Hg —C1
Hg2CI =
Hg-Cl
The mercuric salts, in which the mercuric atom exercises
its full combining capacity, are almost without parallel as
regards properties among metallic salts.  Their formulae
are written upon the same type as those of the salts of
cadmium and zinc, but it cannot be said that the resemblance
extends much farther.
In one respect, however, metallic mercury agrees with
the metals cadmium  and zinc. Notwithstanding its much
greater volatility, its vapour density, like that of the other
two, is the half of its atomic weight. Hence, whilst the
molecules of all other volatile elements are polyatomic,




Mercuric Cornpounds.            22 I
those of the three metals referred to are simply monatomic,
the atom and the molecule being represented in each case
by one and the same symbol or formula,-Zn, Cd, Hg.
The following are the most important mercuric compounds. All are volatilisable.
Oxide. HgO. A red or yellow powder darkening by heat,
and decomposed below redness into Hg and 0O.
Chloride. HgCl2. White, crystallisable, soluble in water,
alcohol, and ether. Vapour density 200 + 71 =   355.
4midochlaoride. HgNH2Cl. Commonly known as'White
Precipitate.' Representable either as an amidochloride,
igN=H2
Hg"/
\C1
or as chloride of mercuric-ammonium,
Hg
NH2
C1
The former view is preferable, considering its formation
from mercuric chloride by ammonia.
Hg/Cl    NH2H  =         NH2 +  fHC1
Hg-C  +                  -- Hg   +  {NH3
CL NH3         C        (NH
Iodide. HgI2. Red or yellow, dimorphous, insoluble in
water, soluble in ether and in solution of potassic iodide.
Sulzlhide, HgS. Cinnabar, dark red masses of hexagonal
prisms. Artificial vermilion, bright red powder.
Su/zhate. HgSO4. Colourless, minute, prismatic crystals,
characteristically decomposed by water with formation of
yellow'turpeth mineral,' or trimercuric orthosulphate
Hg302SO0 or Hg 3(SO6) V




222              Chemical Philosophy.
METALS.  5.
Indium, In =  I 1 34.
Yr7iad. Salt type, A "'CZ3.
Thallium, T1 = 204.
2riiad. Salt types M' C and Jf'" Cl3.
Lead, Pb = 207.
Tetrad.  Salt type, M/" Cl2.
INrDIUM is a very rare metal, occurring in minute quantities in zinc ores.  It is white, soft, fusible, but not particularly volatile.  It resists oxidation up to a temperature
somewhat beyond its melting point, I76~, but at a red heat
it takes fire in the air and burns with a blue flame and
brownish oxide smoke. It is readily attacked by nitric acid,
but dissolves slowly in dilute hydrochloric or sulphuric acid.
Oxide of indium, In2,0, is a pale yellow powder, becoming darker when heated.  The sulphide is an orange yellow
precipitate, insoluble in acetic, but soluble in mineral, acids.
Chloride of indium, InC13, is volatile at a red heat, but undergoes decomposition when heated in the air, and the solution
of the chloride also leaves on evaporation an oxychloride.
The spectrum of indium is highly characteristic, consisting only of two bright blue or indigo bands.
THALLIUM was originally discovered, and occurs most
abundantly in certain kinds of copper and iron pyrites. It
is a very soft, nearly white metal, which tarnishes quickly
but superficially in the air.  It closely resembles lead in
physical properties, except that it -is softer and streaks
paper more readily.  It melts at 2940, and volatilises at a
red heat. It is capable of burning brilliantly in oxygen.
Thallium forms two classes of compounds, those in which
it is univalent being the more stable.
The following are the most characteristic of thallozs
compounds:Chloride. TICI. A white curdy precipitate, resembling




T/zallium and Lead.                223
silver chloride, but crystallisable from boiling water, and
combining with platinic chloride to form a yellow, slightly
soluble, crystalline compound, TI2PtC16.
Oxide, T2I0, anzd Hydrate, 27110.  These compounds
are soluble in water, the solution being strongly alkaline,
and reacting in the same manner as caustic potash. The
hydrate differs from the alkali hydrates in losing the elements
of water when exposed over oil of vitriol in a vacuum. The
residual oxide is almost black, and after fusion crystalline.
Suikhale. T,1S0. A soluble colourless salt, isomorphous
with sulphate of potassium.  It combines with aluminium
sulphate, forming a true alum, TlAl(SO,4). I 2H,O.
Suicpzd'e. TlS. A brown precipitate, soluble in acids.
Thal/ic Compouznds.
Chloride. TlCl3. A soluble salt, crystallisable by evaporation of its solution in a vacuum. It melts and evolves
chlorine at high temperatures.  It combines with the monochloride, forming two compounds, T14C]6 or 3T1Cl. T1C]3,
and TI,C1, or TlC1. TIC13. It forms similar compounds
with the chlorides of potassium and ammonium.
Oxide. TI,03. A dark red powder, insoluble in water,
and reduced by heat to thallous oxide. Thallium is readily
obtained from its salts either by electrolysis, by the action of
zinc, or by fusion with cyanide of potassium.
The ordinary spectrum of thallium is the simplest known.
It consists of a single bright band in the green.
LEAD is a bluish metal, so soft as to streak paper, and
possessing very little tenacity. Its specific gravity is I 1'36,
a number very near to, and intermediate between, that of
thallium, I  9, and of silver, Io'53. The latter metal is very
constantly associated with it, occurring as sulphide in,
probably all, galena. Lead melts at 3260, and at a red
heat volatilises freely.  A  freshly cut surface tarnishes
rapidly in air, but oxidation, except of the melted metal,
does not proceed to any appreciable extent.




224              C/zemnical Philosophzy.
Lead forms two oxides of definite composition, namely,
litharge, PbO, and the peroxide, PbO2, besides several
mixed oxides containing the elements of these two in
various proportions. The red oxide is generally 2PbO,
PbO2, or Pb30,,. When plumbic peroxide is acted upon
by hydrochloric acid, it does not, like barium peroxide,
give rise to hydrogen peroxide, but to a tetrachloride,
PbCl4. In these two compounds, therefore, lead is quadrivalent, and their formula must be written as follows:C1       C1
O- = Pb  =          l \Pb/C
C1/   \C1
The tetrachloride is a very unstable compound, and
no oxysalts corresponding with it are known.
The ordinary salts of lead are typified by the di/chloride,
PbCI2, a white, sparingly soluble salt, crystallisable from
boiling water.
The oxide, PbO, is a dull yellow, the hydrate, a white
powder. Both are slightly soluble in water, the solubility,
like that of lime, being increased by the addition of sugar.
The solutions are strongly alkaline, and absorb carbonic
acid from the air.
Sulphate, PbSO4. In the form of lead-vitriol, isomorphous
with heavy spar, BaSO4.
Carbonate, PbCO3. As white lead ore or cerusite, isomorphous with witherite, BaCO3.
Nzitrate, Pb(NO3)2, crystallises in octahedra, isomorphous
with barium nitrate.
Suzphide. PbS. Black insoluble powder or crystallised in
shining cubes (galena).
Relations of Thallizum, Lead, and Indium.
The relations of thallium both to potassium and lead are
exceedingly well-marked. With potassium it agrees in the
alkalinity of its oxide and the characters of the p]atinochloride, acid tartrate, sulphate, and alum.  But it differs




Relations of Thallium, Lead, and Indium.  225
from potassium and resembles lead in the ready reducibility of the metal, in its high atomic weight and specific
gravity, and in the characters of the sulphide, monochloride,
and other salts.
Lead, in its turn, has strong points of resemblance on the
one hand to thallium, and on the other to barium, as indicated by the isomorphism of many lead and barium salts.
The relations which are thus manifested by these metals
serve to bring into view the connection which they undoubtedly have with other metals, the characters of which
have already been discussed. Without attempting to trace
out any further parallels of the same kind, a task which the
student can very well perform without assistance, we may
just indicate the direction in which analogies will be most
readily detected by arranging the symbols of the most
important of these metals in the following order:(Hg2)"
or  Hg'   Hg"   Pb"-TI'
Ag     Cd    Ba       Cs
I     Zn    Sr       Rb
Na   Mg -Ca   K
Regarding the position of indium among the metals, some
diversity of opinion exists. There appears to be little doubt
of its near approximation to cadmium, both in respect of its
atomic weight and the characters of its oxide, sulphide,
chloride, and other compounds. But in the extreme simplicity of its spectrum it is comparable only to some member
of the group of alkali metals, especially to sodium.
In order to indicate these relations we may range the
symbols of these metals in a series, the parallelism of which
to the thallium series will be at once apparent.
Atc. Wt.  Sp.Grav.      Atc. Wt. Sp. Grav.
Na   23'97         K    39'i'86
Cd  II2      8'7        T1 204  II'9
In   II3'4   7'2        Pb  207  II'3
Q




226              Chzemical Philosophy.
METALS.-6. IRON-COPPER GROUP.
SUB-GROUP A.
Aluminium, Al = 27'4.
Tetrad. Salt type (M2)"' Cl6.
Chromium, Cr = 52'5.
Hexad. Salt types 1" Cl2 and (z2)v" Cl6.
Manganese, Mn =  55
Iron,      Fe =  56
liexad.  Salt types, M"C2l and (If2)v"C 6
Cobalt, Co = 58'7 )
Nickel, Ni = 58-7 J
Tetrad. Salt lype, M"C42.
These are white or grey metals of very high melting
point. Their specific gravities follow one another in the
same order as the atomic weights, and are almost directly
proportional to  them.  Including  copper, the  solitary
member of the next sub-group, they may be placed in the
following order, which we shall show presently represents
very fairly their mutual chemical relations:
Atomic Weights.   Specific Gravities.
27'4    Al    2'6
52'5    Cr  6'8 to 7'3
Atc. Wt.   Sp. Grav.    Atc. Wt.   Sp. Grav.
55  Mn  S8o (about)   56    Fe 7-8
58'7 Co  8-9            58'7   Ni 86
63'5  Cu 8'9
The metals of sub-group A resist atmospheric oxidation
to a considerable extent, except moisture or carbonic acid
be present. Several of them especially iron and manga



Metals related to Irmoz.           227
nese, are capable of decomposing steam at a red heat with
evolution of hydrogen.  They all dissolve more or less
rapidly in dilute acids.
Iron, cobalt, and nickel are strongly paramagnetic,
chromium and manganese feebly so.
A synoptical view of the chemical relations of these
metals to one another will best be obtained, since they are
somewhat complex, by tabulating the formulae of their most
characteristic compounds.
Few observations upon the following table are necessary.
As in all similar groups of intimately related elements, each
one seems to affect a particular state of oxidation or combination in which it attains a condition of chemical, repose
or equilibrium.  Thus the aluminic salts are absolutely
irreducible to any lower state of oxidation; chromic
salts reducible with great difficulty; ferric salts are easily
transformed into ferrous, whilst manganic salts evolve
chlorine  or oxygen  by mere ebullition.  The isomorphous relations of these metals to one another are of great
interest, and are exhibited not only by the alums and other
artificial salts, but in the constant association of these
metals in nature, as, for example, in the replacement of
aluminium by iron in clays, and in the silicates from which
they are formed, in the replacement of the elements of
ferric oxide by chromic oxide in chrome-iron, FeO.Cr2,O,,
and by the occurrence of metallic nickel in meteoric iron.
Several members of the group exhibit peculiarities which
are well worthy of study, but in this place can receive only
passing notice.  Such, for example, are the metameric (?)
modifications of alumina and of chromium salts, indicated
in the latter case by curious changes of colour and solubility; also the remarkable influence of a small quantity of
carbon in increasing the fusibility and hardness of iron,
nickel, cobalt, and, perhaps, others of the group; also the
production of an extensive series of ammoniacal bases by
cobalt, not by nickel.
Q2




228
Al    1               Cr                     Fe
FeO
CrCI2                 FeCI2
CrSO47H20?           FeSO4. 7H20
No Aluminous Com-    pale blue                pale green
pounds known.
CrSO4   6H20          FeS04   6H20
Eo~ S   6-~o          K2SO4
isomorphous with
All Aluminic Compounds Colourless.   Cr304                  Fe3O4
A1203                 Cr203                  Fe203
A12C16                Cr2Cl6                 Fe2Cl6
volatile.              violet;                green:
volatile at red heat.  volatile at red heat
A12(S04)3             Cr,(S04)3              Fe2(SO4)3
A1K(SOA)2 12H20    xCrK(SO4)2  I2HO          FeK(SO4)h. I2H20
modified
- (FeS.)
CrO2C12
CrO3
CrO4K2                FeO4K2
CrO4Ba                FeO4Ba
5 CrO4K2
CrO3
or
{CrO2.OK
CrO.2. OK
CrO41.K
2CrO3
or
CrO20K
CrO.OK
Cr206(OH)2?




229
Ni                      Mn                      Co
NiO                    MnO                    CoO
NiCI2                  MnC12                  CoC12
NiSO4. 7H20            MnSO4. 7H20             CoSO4. 7H20
bright green           pale pink               bright red
NiSO4  6HO            MnSO4   6H2O           COS04  6HO20
Ki4           SO         M         nSO4
corresponding magnesium salts.
--  (        Mn304                   C0304
NiMAO                  Mn203                  C02O3
No Salts               Mn2Cl66                 Co2C16
decomposed by heat     decomposed by heat
Mn2(S04)3?             Co2(SO4)3?
MnK(SO4)21I2H20 
cubes or regular octahedrons.
(NiS2)    MnO2
MnOCl2
-MnO4K2
MnO4Ba
Mn206(OH)2
Mn20O(OK)2




23c             Clzezmical PRilosopzy.
Atomzicily of the Iron Grozp.
Manganese forms a sexfluoride, MnF6, and a class of salts,
the manganates, which are said to be isomorphous with the
chromates and sulphates. Hence manganese is sexivalent,
and the formulI of manganic perfluoride and potassic
manganate may be written as follows:
F      F              KO        O
X /\v
F-Mn-F                      Mn
F       F             KO       0
MiIanganese also forms a dioxide in which it is quadrivalent,
O = Mn = 0,
and a chloride corresponding with ferrous chloride in which
it may be considered to be quadrivalent,
Cl2Mn = MnC12.
or bivalent, C1 - Mn - C1. There is no direct evidence for
either the one formula or the other, and, consequently, the
latter is very commonly accepted for the sake of simplicity.
The permanganates must be represented by the formula
M'2Mn208, on the assumption of the hexad character of
manganese, so that potassic permanganate becomes0
11
O -= Mn —OK
I
O-Mn-O-OK
11
O




A tomicity of Zroln, Mangazese, Clzromizum.  23I
or,
0
O = Mn-OK
I
O-Mn-OK
O
The alleged isomorphism of the permanganates and
perchlorates remains unexplained.
By reason of the correspondence between the ferrates
and manganates, iron must also be regarded as hexad;
although in ferric, and probably also ferrous compounds, it
is ordinarily tetrad.
Thus the vapour density of ferric chloride agrees with
the half of the molecular weight denoted by the formula
FeC16, although the formula FeCl3 would equally well record
its composition. The existence of iron pyrites, FeS., the
analogue of SnS2, also supports the hypothesis of the
quadrivalence of iron in these compounds.
C1 C1
Il Il
C —Fe —Fe — C1        S  F~ e - S"
l   l
C1 C1
Chromium is also tetrad in its ordinary chromic salts, but
hexad in the chromates, as well as in chromium sexfluoride,
trioxide, and oxychloride.
C1 C1             KO        0
C1-Cr-Cr-C1                 Cr
C1 C1             KO 
Chromic chloride.    Potassium chromate.




232             Chemical Phzilosophy.
Aluminium manifests no tendency to form more than one
chloride or oxide. It has been supposed triad, with the
formula A1C13 for the chloride, but the isomorphism of
alumina with ferric oxide and the vapour density of
aluminic chloride point conclusively to the formulae A1,03
and Al2C16 for these two compounds.
0                   C1 C1
0 = Al —Al = 0       C1-Al-Al —C1
Aluminic oxide.
C1 C1
Aluminic chloride.
Cobalt and nickel are bivalent in their ordinary salts, but
they form unstable peroxides, CoO2 and Ni2,O,, which lose
oxygen at a red heat. These compounds, and the salts of
the oxycobaltamines are almost the only known tetradic
combinations of these two metals.
SUB-GROUP B.
Copper, Cu = 63-5.
DZiat. Salt-types, (M2)" CG2 anzd lf" CE.
This metal is distinguished from the members of the
series just considered by its red colour, by its very superior
conductivity of heat and electricity, by its diamagnetic
properties, by its ready oxidation when heated in dry air,
and by the easy reduction of the metal from its oxide and
salts, also by its indifference to diluted sulphuric or hydrochloric acid. It is further characterised by the formation
of no oxide superior to the oxide CuO, by the production
of a suboxide, Cu20O, and corresponding series of salts, also
by the facility with which all its compounds unite with or
act upon ammonia. By the last two characters it indicates
its relationship to silver and mercury.
The following are the most important of the compounds




Relations of Cuprous and C'upric Salts.   233
of copper, together with the formulae of compounds with
which they manifest greater or less analogy:Cuprous.        Mercurous.        Argentic.
Cu2O             Hg20O            Ag20
or               or
Cu"             Hg"               Ag'
Cu" /           Hg"               Ag'/
Cu-Cl            Hg-C1
Ag-Cl
Cu-CI            Hg-C1
Cu-H
[I             No hydride.      No hydride.
Cu-H
Cu2S isomorphous with Ag2S and with CuAgS.
Very few cuprous oxysalts are known, for when cuprous
oxide is acted upon by acids it generally splits up and
yields a cupric salt and a residue of metallic copper.
Cupric.                  Iron and Manganese.
CuO                         FeO
CuSO4. 5H20 isomorphous with FeSO4. 51H20
and with MnSO.s5H20
KCSO4 j6H20 isomorphous with FeSO4.6H20
CuS                         FeS
FeCuS2
or Fe2Cu2S4                   FeS2
Copper pyrites.                Iron pyrites.
Considering the very general blue or green colour of the
cupric salts, and their dissolution in excess of ammonia to
form a blue liquid, copper approaches more nearly to nickel
than to any other member of the iron group. And although
a pentahydrated nickel sulphate corresponding with common
blue vitriol has not been described, yet of the isomorphism of
copper and nickel in the form of sulphate there can be no




234              Czewicral P/hilosoy/ y.
doubt.  For when a mixture of cupric and nickel sulphates
is crystallised, the crystals which are deposited, though containing both nickel and copper, have the form of nickel
sulphate, and contain seven molecules of water when the
nickel is in excess.  If, on the other hand, the cupric
sulphate is in excess, the crystals have the form of blue
vitriol and contain five molecules of water.
Through the isomorphism ofthe double potassio-sulphates,
the iron-copper group of metals is connected with the zinc
group previously described.
METALS.-7. PLATINUM GROUP.
SUB-GROUP A.-GOLD.
DTiad. Salt t02es (AIM) cl2 and MCG.
Gold is a metal of familiar yellow colour and high specific
gravity. In consideration of its inalterability in the fire or
by the action of acids (save nitro-muriatic acid, hence called
aqzua regia), gold was regarded by the old chemists as the
type of a " noble " metal.  It forms two classes of salts, of
which the aufous are somewhat unstable, readily undergoing
decomposition into an auric compound and metallic gold.
The sodio-aurous theiosulphate    (S203)4.H20 is one of
the most definite.
The trichloride is the most important auric compound.
It is a red, crystalline, deliquescent body, which  forms
yellow  crystallisable compounds with  the  chlorides of
hydrogen   and  the  alkali-metals,  HAuCli,  KAuCl,
NH4AuCl4, etc.
Gold is reduced from its solutions with the greatest readiness by nitrous acid, sulphurous acid, ferrous salts, and all
reducing agents, including many organic substances. Its
oxides and chlorides also yield up the metal when strongly
heated.




Platinzum Group.                 235
SUB-GROUP B.
The members of this group range themselves very naturally into two divisions, each of which exhibits a striking
uniformity of atomic weight and specific gravity. With one
of these gold allies itself, in virtue of its high specific gravity
and atomic weight, as well as its resistance to oxidation and
solution in acids; also by the yellow colour of its compounds and the tendency of its highest chloride to form
double salts. With the other, silver is, perhaps, in a similar
manner, remotely connected, though in this case it must be
admitted there is but little in the properties of the metal to
sanction such an arrangement.
Symb. At. Wt. S.G.          Symb. At. Wt. S.G.
Silver    Ag - IoS   Io' 5  Gold    Au  - I97   I9'3
Monad                      7Triad
Palladium Pd - Io6'5 II'8  Platinum  Pt. - I97'5  21 5
Rhodium  Ro - 104'5  I I  Iridium   Ir  - I97    2I'I
RutheniumRu - I04'5 I1'4  Osmium   Os - I99    2I'4
Tetkrad                   T7et;ad
The true platinum metals are white metals which require
for their fusion the highest attainable temperatures of the
oxyhydrogen flame. It is stated, indeed, that osmium has
never been really melted; hence the numbers by which their
respective specific gravities are denoted can only be approximately correct.
They differ somewhat in their behaviour towards re-agents.
Palladium, for example, is slowly dissolved by nitric acid,
whilst some of the others resist the action even of aqua regia.
Ruthenium and osmium, on the other hand, although slowly
attacked by aqu  regffia when in the compact state, undergo
oxidation somewhat rapidly when heated in the air.
They all, with the exception of rhodium, form more or
less stable tetrachlorides, which are generally yellow and
easily reducible by heat, either to the metallic state or to




236              Chemical Philosophy.
lower chlorides. These compounds also unite with the
chlorides of the alkali-metals, forming crystallisable salts,
which in some cases as, for example, the platinochlorides of
potassium and ammonium,
2KCl.PtCl4 or K2PtC16
2NH4C1.PtCIl or (NH,)2 PtC16
are almost insoluble and highly characteristic. But besides
these tetrachlorides, the platinum metals yield dichlorides
and sesquichlorides, which are in special cases of greater
importance. The dichloride of palladium, for example, is
one of its most important compounds, and far more stable
than the tetrachloride, which is known only in a state of
solution, and in its double salts. Rhodiumrn again forms
only one chloride, and that the sesquichloride.
The following are all the definite chlorides of these
metals which have been described:Dichlorides.     Sesquichlorides.  Tetrachlorides.
PdC12               _                PdC14
PtC12               -                PtCl4
Ro2C16
IrCl2?           Ir2C6              IrC14
RuCl2             Ru2oC6             RuCI4
OsCl2             Os2C16            OsC14
Ruthenium and osmium are specialised by the existence
of potassium  salts called  rutheniate and osmiate, which
contain the elements of potassium oxide, together with a
trioxide of ruthenium or osmium.
K20.RuO3 or K2RuO4.
K20O.OsO3 or K0OsO4.
The trioxides, or anhydrides, are not known in an
isolated state.  But the most remarkable compounds of
these metals are the tetroxides RuO4, OS04, which are
highly volatile bodies, soluble in water to form acid solu



Exercises on Section I Vr.               237
tions, but apparently incapable of generating salts. The
constitution of these compounds is not well understood.
They are the only tetroxides known.
EXERCISES ON  SECTION IV.
I. What experiments and reasoning would lead you to regard
chlorine as a monad element?
2. In what respects does iodine resemble and differ from bromine?
3. Discuss the title of hydrogen to be considered (a) a non-metal;
(b) a metalloid.
4. Classify the solid elements according to their specific gravities.
5. What are the properties which distinguish chlorine, and the elements most like chlorine, from potassium, and the elements which are
most like potassium?
6. Compare and contrast the elements phosphorus and arsenic, both
in their physical and chemical properties.
7. What are the special characteristics of carbon which distinguish it
from other elements?
8. Describe as many as you can of the modified forms of carbon,
sulphur, and phosphorus.
9. What acids are known consisting of hydrogen, oxygen, and phosphorus? Give their graphic constitutional formule, and point out the
basicity of each.
Io. What differences and analogies exist between the corresponding
compounds of potassium and sodium?
II. To what elements does mercury present the greatest resemblance? Point out instances of analogy in properties.
I2. Show by a comparison of their compounds in what respects the
elements carbon, silicon, tin, and lead may be said to constitute a
natural group.
13. Give the formule, simple and constitutional, of the following
compounds:-common salt, caustic potash, sulphuric acid, baryta,
barium peroxide, lead peroxide, iron peroxide, silver phosphate, silver
arsenate, silver arsenite, kakodyl, and tartar-emetic.
I4. Give a full account of the two oxides of nitrogen known as nitric
oxide and nitric peroxide respectively.
15. Give instances of compounds which do not comply with the law
of even numbers.




238                Chemical Philosophy.
i6. What are the chief points of resemblance and difference between
the corresponding calcium and magnesium compounds?
I7. Assign a place among the elements to each of the following
bodies:-copper, nitrogen, boron, thallium, vanadium, gold. Give full
reasons in each case.
I8. The constitution of phosphorus compounds has been explained
on the assumption of the trivalence of the phosphorus atom. Discuss
this view.
9x. By what arguments would you support the number 32 against
the number I6 as the atomic weight of sulphur?
20. Caustic potash is often said to be formed upon the water type.
Explain this.
21. What is an alum? At the temperature of I8oS common aluml
loses gt of its water. How may this fact be brought to bear upon the
construction of its formula?
22. Classify some of the better known elements according to their
atomicity, mentioning and explaining as far as possible doubtful cases.
23. Write the'formuloe of orthosulphuric acid and some of its salts.
An ortho-acid may be defined as one in which the characteristic element (sulphur in this case) is saturated with hydroxyl.
24. One gram of phosphorus yields 2'903 grams of phosphoric
anhydride.  Will this enable you to determine the atomic weight of
the element? and, if not, what further data are necessary?
25. How much phosphorus is contained in I20 lb. of bone-ash,
consisting of Ca3 (PO4)2, 88'5 parts, and Ca CO0,  I'5 parts, in Ioo?
26. What weight of phosphorus is contained in io litres of phosphorus vapour at Io040? Pressure normal.
27. What weight of phosphorus is contained in Io litres of phosphine
at 12~? Pressure normal.
28. No volatile compound appears to be known of which two
volumes of vapour contain less than 55 parts by weight of aluminium.
Why is not the usual atomic weight of aluminium doubled?
29. Calculate the percentage composition of cryolite, Na3A1F6.
30. How many kilograms of litharge can be obtained from 40'5 kilos
of lead, and what volume of oxygen measured at 20o and under 765 mm.
is absorbed in the process?




239
S ECTIO  N V.
CHAPTER XXIV.
CLASSIFICATION OF COMPOUNDS.
ACIDS, BASES, SALTS.
Acids. —The only element common to all acids is hydrogen, and the characteristic property of this hydrogen is its
ready exchangeability for a metal. It does not appear that
this is due to any peculiarity of the hydrogen itself, but
to the fact that, in all well-defined acids, it is associated
with other elements of very eager affinities. Acids may be
divided into two classes according as oxygen does or does not
enter into their composition. The latter class is necessarily
very small, and the following list includes nearly all the unoxidised acids known:
Hydrofluoric acid       HF.
Hydrochloric acid       HC1.
Hydrobromic acid        HBr.
Hydriodic acid          HI.
Hydrocyanic acid        HCN
Hydrosulphuric acid    H2S.
It is important to remark, however, that although such a
distinction may for certain purposes be occasionally recognised, there is no essential difference between acids which
contain oxygen and those which do not.
Now, it is obvious that the activity with which hydrochloric acid, for example, produces salts when in contact
with suitable metals, is due to the elective attraction of the




240             Chemical Philosophy.
chlorine for the metal, and that hydrocyanic acid, a molecule of which contains the same quantity of hydrogen, does
not act upon metals with the same promptitude, because
the cyanogen group has not the same attraction for metals
as chlorine.
The chemical energy of an acid, then, does not depend
upon the hydrogen which it contains, but upon the electronegative character more or less strongly marked in the
elements with which the hydrogen is associated. It is a
very remarkable fact, however, that the existence of hydrogen in a body, in the position which it occupies in acids,
namely, in the position of a base, does confer upon the
compound one very general characteristic, and that is a sour
taste. The sourness of soluble hydrogen salts would, perhaps, have been considered no more remarkable than the
astringency of iron salts, the bitterness of magnesium, or the
sweetness of glucinum compounds, but that it constitutes a
character which serves to differentiate real acids from bodies
such as ammonia, alcohols, and metallic hydrates, which all
resemble acids so far as to exchange, under certain circumstances, part of their hydrogen for metals.
Oxyacids are bodies of very varied character.  Some
considerations concerning them have already been dwelt
upon in the article on oxygen (Chap. XXI., p. i83).
With regard to the properties of acids in general, sufficient
has already been written in the article on hydrogen (Chap.
XXII., p. I99). It now only remains, therefore, to discuss
that property of acids known as " basicity."  This inquiry
amounts in some cases to a determination of the weight of
the molecule, and reference has already been made to an
example of the kind in the article on molecular weights (Chap.
XVI., p. I27). The question is, however, one of great importance, and deserves to be examined a little more in detail.
The basicity of an acid depends upon the number of
atoms of hydrogen directly exchangeable out of each molecule for metallic atoms' an acid containing one such atom




Basicity of Acids.             24I
of hydrogen being called monobasic, one containing two
such atoms is dibasic, or three, tribasic, and so on. We may
infer the basicity of a given acid from the number of distinct
salts it is capable of yielding with each metal. Monobasic
acids generally give one salt, dibasic acids, as shown in the
case of sulphuric acid, give two, and tribasic acids give
three. Mere inspection of the formula of an acid does not,
however, furnish sufficient information upon which to form
a judgment regarding the basicity of the acid and the
number of salts it is capable of producing; for it by no
means follows, from the existence of two or three atoms of
hydrogen in the molecule, that it should be dibasic or tribasic.
The basicity depends not upon the total number of hydrogen
atoms present, but upon the total number endowed with
this particular power of metallic exchange.
The three following acids of phosphorus afford an
example of this. Each molecule contains three atoms of
hydrogen, but the first is monobasic, the second dibasic, the
third tribasic:
Hypophosphorous acid. H3PO2 or HPH,20
Phosphorous acid. H3PO3 or H.PHO3
Phosphoric acid. H3PO4
In this and similar cases it has been generally observed
that increase of basicity, as well as of sourness and general
chemical acidity, accompanies the addition of oxygen.
Hence it has been inferred that those atoms of hydrogen
which are exchangeable for metals are combined more
intimately with oxygen than others which are not similarly
exchangeable. On this hypothesis the formule of the three
acids referred to above are represented in the following
manner:H H
Hypophosphorous acid   O = P-OH
R




242             Chemical Philosophy.
H OH
I /
Phosphorous acid         O=P
OH
OH
Phosphoric acid. O=P-OH
OH
But acids are capable of undergoing a variety of modifications under the influence of re-agents, and some of these
are available as evidence towards the establishment of their
basicity. It has been found, for example, that acids which
possess only one basic atom of hydrogen, or are monobasic,
yield only one amide and one chloride. Acetic acid, for
example, gives the following derivatives, the relation of
which to acetic acid is obvious from the formulae:Acetic acid..   HO.C2H30
Acetyl chloride.       C1.C2H3O
Acetamide..   NH,.C2H30.
Dibasic acids give two derivatives of the same kind, and
tribasic acids give three.
We may now resume the case of sulphuric acid adverted
to in Chap. XVI. as a very instructive example.
The simplest formula for sulphuric acid is H2SO4. The
nroblem before us is to prove that this formula, with the
relative weight 98, is the formula of the molecule, and that
the acid is dibasic.
I. The fact of the existence of two classes of sulphates
has been already appealed to. It is clear that the whole
of the hydrogen of the acid is replaceable by metals, and
that it is capable of replacement one half at a time. We
arrive then at such formulae as these:
Na SO4    Na  S04        K}SO4          SO




Sutpihuric Acid.             243
Evidence of this kind is almost conclusive, but it happens
that in the present instance other testimony is abundant.
2. Thus we might show that sulphuric acid is produced
when sulphur trioxide and water, both bodies of known
molecular weight, unite together.
SO3 +  OH2 =  S04H2.
3. Also, when sulphur dioxide is dissolved in water and
the solution exposed to the airSOS  +  OH2  +  0  =  SO4H2.
4. It is also formed by the direct union of hydrogen peroxide and sulphur dioxideS02 +   02H2      SO4H2.
A sulphate being generated in the following strictly
parallel case of combinationPbO2  +  S02  =   S04Pb.
5. Sulphuric acid is generated by the decomposition of
sulphuryl chloride by waterS02C12 +  2HO   =  S04H2  +  2HC1.
According to equations 2, 3, 4, the whole of the acting
materials enter into the composition of the sulphuric acid
which is produced, and therefore there is a strong probability in favour of the molecular weight of sulphuric acid
being the sum of their separate molecular weights. Similar
considerations apply to equation 5. The molecule of sulphuric acid is here shown to be built up of the molecules
of the sulphur oxychloride and water, minus two atoms of
hydrogen and two atoms of chlorine.
6. We now come to some reactions in which various
constituents of the sulphuric acid molecule are replaced.
When the acid is heated with phosphoric chloride, two
successive reactions of the same kind ensue. In the first
R 2




244             Chzenzc/l Philosop/y.
an atom of chlorine is introduced in place of the group
(OH) which is removed.
H2SO4 + PC15 -  HClSO3 + POC13 + HC1.
And this kind of exchange is repeated in the second
reaction:
HClSO3 + PC15 =  C12SO2 + POC13 + HC1.
The relation of the two new chlorides to sulphuric acid
may be exhibited most satisfactorily by writing the formulae
in the following manner:HO            HHO            C1
S02           SO           SO2
HO             C1            C1
Sulphuric acid or   Sulphuryl chlor-    Sulphuryl
Hydrate.      hydrate.      chloride,
7. Two "amides," standing in the same kind of relationship towards sulphuric acid, also exist. Their formulae may
be written thus:HO SO    HO                  NH S
SO2 SO                       S02
HO            NH2            NH2
Sulphuric acid.    Sulphamic acid.  Sulphamide.
The existence of these compounds and the corresponding
chlorides supplies evidence similar to that deducible from a
knowledge of the acid and double sulphates (I). These
facts tend to show that the hydrogen contained in a molecule of sulphuric acid is divisible into two equal parts, and
that these two hydrogen atoms are probably more closely
united with two atoms of oxygen than with other constituents of the molecule.
8. When concentrated sulphuric acid is made to act on
certain hydrocarbons, alcohols, phenols, and other bodies,
compounds are produced which have the properties of
monobasic acids. These compounds are in some degree
analogous to salts; at any rate, sulphuric acid could not
produce derivatives of this kind if it did not contain at
least two atoms of basylous hydrogen.
C,;h   +   H2SO   =   H20  +   C6H5 (SO3H)
Benzene.   Sulphuric acid.   Water.    Benzene-sulphonic acid.




A nrhydrides of Acids.             245
C2H5OH   +  H2SO4 =  H20  +   C2H50(SO3H)
Ethylic alcohol.  Sulphuric acid.   Water.  Ethyl-sulphuric acid.
CH5OH  +  H2SO4  =   H20  +  C6H4(OH)(SO3H)
Phenol.   Sulphuric acid.    Water.    Phenol-sulphonic acid.
All the foregoing facts point to the same conclusion, and
it matters not whether we write the formula of sulphuric
acid:H2SO4 or H}SO, or (HO)2SO2 or H20.SO3,
in the attempt to record one or other of its various modes
of formation or decomposition. These different expressions
represent one and the same molecular weight.
In one respect, which has not yet been referred to, there is
a difference between acids which contain oxygen and those
which do not contain that element. Compounds in which
there is no oxygen cannot by any possibility be made to
yield water. Hence there are no anhydrides corresponding
to the haloid acids. Oxyacids, however, may, in various
ways, be made to furnish water and an oxide, which is called
an anh/ydride. Thus sulphurous acid splits up readily in the
following manner:-m
H2SO3    =      H20      +      SO2
Sulphurous acid.   Water.    Sulphurous anhydride.
Dibasic acids give their anhydrides most readily when
simply heated, whereas monobasic acids generally require
to be treated with some dehydrating agent.  This may,
perhaps, be connected in some way with the fact that a
dibasic acid always contains within itself the elements of
water, whilst a monobasic acid cannot generate water except
by the combined action of two molecules.
Dibasic.
H2CO3    =       H20      +     CO2
Carbonic acid.                   Anhydride.
H2C4H4O4    -       H20      +     C4H403
Succinic acid.                     Anhydride.




246             Chemical Philosophy.
M3onobasic.
HNO,3   -   HO   +   N205.
Nitric acid.           Anhydride.
In the case of some monobasic acids, the anhydride can
only be obtained by a succession of operations. In order
to prepare acetic anhydride, for example, the chloride is
first made from the acid, and this compound is then allowed
to react upon a salt.
C2TO30C1  +   C2H30.ONa =   NaCl  +  (C2H30),O
Aceti(: chloride.    Sodium acetate.  Sodium  Acetic oxide or
chloride.   anhydride.
This difference between monobasic and dibasic acids
is, however, not sufficiently general to be maintained as a
distinction of any practical importance.
-Bases.-The idea implied by the word "base" belongs
to the obsolete dualistic theory of salts.  According to this
theory, every salt was supposed to be made up of two
parts, one of which, consisting of a metallic oxide, formed
the base; whilst the other part, usually the oxide of a nonmetal, or some group consisting of carbon, hydrogen, and
oxygen, was regarded as the acid. Sulphate of soda, for
example, was, according to dualistic phraseology, composed
of the base soda or oxide of sodium, and sulphuric acid or
trioxide of sulphur, and its formula was written NaOSO3,
or, adopting modern atomic weights, NaOSO.
Now, although this idea has been considerably modified,
and we no longer look upon a salt as a double structure of
this kind, the fact remains that those oxides of metals which
are not overcharged with oxygen do agree in the property
of uniting or reacting with acids, so as more or less completely to neutralise them. And this property is shared by
the hydrates corresponding with these oxides. Adopting
the old name, and taking this neutralising faculty as the




Basic Oxides and Hydrates.            247
criterion of the basic function of oxides, we may define a
base as a metallic oxide or hydrate capable of saturating
acids.
It will be noticed that a basic hydrate and basic oxide have
the same relation to each other as an acid and its anhydride, and consequently it would be quite reasonable to
speak of the anhydrous oxide as an anhydride, reserving the
term base for the hydrate. For example:2HN03    -        OH2    =       N205
Acid.           Water.       Acid anhydride.
2KHO   -   OH2  =   K20
Basic hydrate or base.          Basic oxide or basic
anhydride.
H2SO4   -        OH2    =       SO3
Acid.          Water.       Acid anhydride.
Ca(HO)2            OH2    =       CaO
Basic hydrate or base.  Water.  Basic oxide or basic
anhydride.
It must not be forgotten that ammonia and a certain
class of its derivatives also possess the power, in a remarkable degree, of uniting with acids and forming perfectly
neutral compounds, which present all the external characteristics, and many of the chemical properties, of ordinary
salts. Ammonia and the alkaloids are therefore commonly regarded as bases, though it is well to remind
the student that these bodies differ from metallic oxides
or hydrates in their reactions, both with acids and with
anhydrides. Thus with acids, ammonia enters into direct
combination, without the formation of any secondary product, thus:NH3    +      HNO3    =        NH3HNO3
or NH,N03




248              Chemical Philosophy.
Whereas basic oxides or hydrates invariably generate water
in addition to the salt.*
KHO  ~ HNO3 = KNO3 + H20
Base.       Acid.        Salt.       Water.
Again, ammonia combines with acid anhydrides generally,
but the compounds which result are not salts, whilst a basic
oxide under the same circumstances always yields a salt of
the ordinary type. For example:BaO      +     SO3    =       BaS04
Such a reaction is obviously the counterpart of that which
occurs when an acid or hydrogen salt is formed from water
and the anhydride.
H20      +     SO3    =       H2S04
The general property of neutralising acids is associated,
in the case of all those bases which are soluble in water,
with a peculiar soapy taste, and with a power of affecting
vegetable colours. The reds are changed to blue or green,
the blues to green, and the yellows to brown or brownish
red. In litmus the alteration of tint characteristic of alkaline reaction is the result of the conversion of the feeble
litmus acid (red) into a blue salt.  Somewhat similar
changes probably occur in other cases.
Salts.-It has been stated that an acid is a body in which
hydrogen is united with a strongly electro-negative element
or group of elements. When such a body is acted upon by
a metal or metallic oxide, the hydrogen is exchanged for a.metal, or something equivalent to it, and a salt is formed.
We may define a salt, then, as a compound in which a
metallic or other radicle of like nature is combined with
* The demonstration of this fact is very generally neglected by
teachers. An easy experiment by which an appreciable quantity of
water may be collected in a few minutes consists in passing dry hydrochloric acid ovet gently-heated litharge.




Normal and Basic Salts.          249
a radicle of more or less decided electro-negative character.
It has already been explained that electro-negative elements
are those which in the electrolysis of the salt are collected
at the positive electrode, and which by uniting with
hydrogen form acids. In short, the radicles which, in
union with hydrogen, form acids, and with metals form
salts, are always composed either of the non-metallic, or, as
they are called in this book, oxygenic elements, or of certain metals in company with a relatively large quantity of
oxygen.
A considerable number of salts are known in which a
part of the acid radicle of the normal compound is replaced by oxygen or hydroxyl. Such compounds are often
called basic salts. The following are some examples which
serve to show the general character of these compounds:Plumbic chloro hydrate    Pb { H
Plumbic hydrate nitrate   Pb- NO3
NO3
Bismuthic hydrate nitrate  Bi NO3
(OH
Ferric oxysulphate     Fe2 {SO4x
Salts may be formed in a variety of ways, amongst which
are the following:(a) Union of elements,
Na2    +      C12    =      2NaC1.
(b) Union of acid and basic oxide,
CaO    +    CO 2   =       CaCO3.
(c) Action of metal on acid,
Zn  +  H2SO4 =   ZnSO4  +  H2




250             Chlemical Philosophy.
(d) Action of metallic oxide or hydrate on acid,
ZnO   +  H2SO4  =   ZnSO4  +   OH2
NaHO+  HNO3  =   NaNO3 +   OfI02
(e) Double decomposition between two salts.
BaC12  +   Na2HPO4  =   2NaC1  +  BaHPO4
As regards the properties of salts no very general statements can be made. Liquid and solid, soluble and insoluble, crystalline and uncrystallisable bodies are alike included under this denomination, so that no comparison can
now be instituted between the bodies thus named by the
chemist and that substance which was probably the prototype of all salts, namely, common salt (Gr. aXc, Lat. sal, Fr.
sel, Ger. salz).
In the production of salts the potential energy of the reacting materials is more or less exhausted, so that salts are
generally bodies of feeble attractions.  We observe this in
the comparatively small number of compounds into which
entire salts enter, and the facility with which such combinations are broken up.
CHAPTER XXV.
DERIVATIVES OF AMMONIA.
A1mmonizum Theory.-Ammonia is excessively soluble in
water, and the solution has all the properties of a powerful
alkali. Like caustic potash, it restores the blue colour to
litmus, which has been reddened by an acid, it absorbs carbonic acid from the air, it saturates acids forming a class ot
crystallisable neutral salts, which are isomorphous with the
corresponding salts of potassium. Like potash also it precipitates hydrates when added to the solutions of a great
many metallic salts, though in a few cases, e.g., with mer



A inmonium Thzeory.             25 I
curic salts, it gives rise to compounds of a different character. Lastly, solution of ammonia differs from solution
of caustic potash in that when evaporated it leaves no
residue. In consequence of this want of stability, solution
of ammonia is often referred to as the volatile alkali.
Guided by this marked resemblance between the compounds of ammonia and those of potassium, chemists have
been led to regard both series as constituted in the same
manner. Writing down in parallel columns a list of ammoniacal and potash salts, and eliminating from each pair of
formulae the symbols which are common to them both, we
find that the group composed of one atom of nitrogen and
four atoms of hydrogen, NH4, is the residue left over in the
one case, whilst the solitary symbol K remains in the other.
The latter represents the radicle potassium, which, when
isolated, presents all the characteristics of a metal. The
conclusion seems almost inevitable that the radicle NH.
would also exhibit metallic lustre, conductivity, and so
forth, if it could be obtained in a free state. In anticipation of this hypothesis being confirmed, it has been called
ammonium.
Compounds of
Ammonium.                                Potassium.
NH9.H0O    or NH4HO          or AmHO      KHO
NH3.HC1      or NH,4C1      or AmCl       KC1
NH3.HN03  or NH4N03    or AmNO3   KNO3
NH3.H2S04   or NH4HSO4   or AmHSO4  KHSO4
(NH3)2.H2SO4 or (NH4)2SO04  or Am2SO4   K2SO4
NH3H3P04   or (NH4)H2P04 or AmH2PO4  KH2PO4
(NH3)2H3P04 cr (NH4)2HP04 or Am2HPO4 K2HPO4
(NH3)3H3P04 or (NH4)3P04  or Am3PO4    K3P04
A curious phenomenon occurs when a lump of sodium
amalgam is placed in a solution of sal-ammoniac, or when
the same salt, dissolved in water, is submitted to electrolysis, a globule of mercury being employed as the negative
electrode. The mercury swells enormously, becoming so
light as to float in the solution, without at the same time




252             C&hemical Philosophy.
losing altogether its metallic aspect. In a few minutes. however, the spongy mass again contracts, hydrogen and ammonia make their escape, and the mercury recovers its
original volume and lustre. The body thus produced was
long regarded as a true compound of mercury with the
hypothetical metallic radicle, and as such received the name
ammonium amalgam. It has been shown, however, that
the mercury whilst in this condition yields to pressure in
the same way as froth, and that the phenomenon is really
due, not to the formation of an amalgam, but to the
mechanical distension of the mercury by gas.
The radicle ammonium has, in fact, never been isolated,*
and there is no great probability of its ever being obtained.
The ammonium theory finds a sufficient basis in the isomorphism of the ammonia and potash salts, and the general
resemblance of the hydrate and its derivatives (to be mentioned presently) to the fixed alkalis, and the theory itself:should be regarded merely as a summary and memorandum
of these relations.
Amines.-The hydrogen of ammonia admits of substitution in three successive stages, and the compounds thus
fornmed are believed to be constituted in the same manner
as ammonia itself, or, using the ordinary expression, they
belong to the ammonia type,
AH
NHI
the replacing radicles being severally combined with the
nitrogen in the same manner as the hydrogen atoms of the
type.
When positive radicles, consisting of metals or hydrocarbons, are substituted for the hydrogen in the ammonia
molecule, the resulting compounds retain the alkalinity and
general basic character of ammonia. Such compounds are
* Notwithstanding statements to the contrary.




Basic Derivatives of Ammonia.             253
dalled amines. They generally combine with acids, but are
unaffected by treatment with alkalies. There are several
classes of amines resulting from the more or less extensive
replacement of the hydrogen or the coalescence of two or
more molecules of ammonia. Their constitution will be
understood after inspection of the following table, in which
the symbol R represents a positive radicle, the valency of
which is indicated by dashes:Monamines.         Diamines.           Triamines.
Primary.           Primary.            Primary.
RI           (R"                  R/(R
N  H               N  H2              N3 H3
H                  H2                 Ha
Secondary.         Secondary.         Secondary.
N  R'I             R/
N2R" R    N  R/2 N3 R/' N3 R'3, etc.
H              It2       H2      Ha       Ha
Tertiary.          Tertiary.           Tertiary.
RI       ~~R"/        R// -R/   R//a  R/a[
N{R'      N{ R"N, R"N  (R' 1N3 NR"'N23 {    3  /3
ih;2            3:/,
RI     R//   R/2   R 2    R"' ~R, R/3 R/3
etc.
Monamines are capable of saturating only one molecule
of a monobasic acid, but diamines unite with two, and
triamines with three molecules of such acids. The diamines
and triamines also yield intermediate compounds. Representing the monamines, diamines, and triamines respectively
by the symbols Am, Am' and Am3, the general formula of
the chlorides may be written in the following manner:
Am.HC1         Am'. HC         Am'. HC1.
Am'. 2HC1       Am3. 21C1.
Am3.3 HC1.
The tertiary amines unite with iodides and other salts
of alcohol radicles in the same manner as with acids, fur



254            Chemical Philosopy.
nishing compounds which may be formulated either as
ammonia compounds or as derived from the hypothetical
ammonium  radicle.  Thus triethyl-amine combines with
ethyl iodide, forming a crystalline compound, which may
be represented either as N(C,H5)3.C2HI, or, more con,
sistently and conveniently, as tetrethyl-ammonium iodide
N(C2H1)J. This compound is obviously the analogue ol
ammonium iodide. When acted upon by silver oxide and.
water, it exchanges iodine for hydroxyl, and gives rise to a
hydrate of tetrethyl-ammonium, N(C2H5),HO, a compound
which may be obtained in the solid state, and which in its
causticity and alkalinity closely resembles potassic hydrate.
The existence of this compound, and others of similar
nature, furnishes strong evidence in favour of the ammonium
theory. Like their hypothetical prototype, however, the
substituted ammonium radicles N(CH,),, etc., are not
known in the free state.
A question has at different times been raised regarding
the constitution of ammonium compounds. It seems, however, to be now definitely admitted that they contain pentad
nitrogen, ammonium chloride being represented graphically by the formula:
H   H
1\ /
N
Cl.
and other ammonium compounds in a corresponding
manner. Experiment has recently been brought to bear
on the subject in the following manner:
Methyl-tsiethyl ammonium chloride may be represented
either as an ammonium compound,
C2H5 C21H5
N
/I.\
C2H5C1CCHa




Constitution of A mmonium Salts.      25 5
or, as formed on the type of sal-ammoniac, regarded as
ammonia hydrochloride NH3.HC1. If the latter view is
correct, there ought to be two isomeric compounds having
the formulae
N(C2H,),.CH3Cl and N1(C,H5)2(CH3).C2HCl.
It is found, however, that the result of bringing together
triethylamine and methyl chloride, is a body identical in
every respect with the product obtained by combining
methyl-diethylamine with ethyl chloride.
According to the ammonium formula, which these results
tend to support, each radicle is in the same position with
respect to the nitrogen, and no isomerism is possible.
Concerning the formation of salammoniac, however, it
has been justly remarked, that if we adopt the formula
N'(H4C1'), it is difficult to explain why, in the combination
of hydrochloric acid and ammonia, the chlorine should be
represented as leaving the hydrogen, for which it has great
attraction, in order to link itself with nitrogen, for which it
appears to have little attraction.
H    H   H
H   /
I + N-H  =            N-H
C1  \                / \
H        C1      H
The truth is probably this, that it is the hydrogen of the
ammonia, and not the nitrogen, which attracts the chlorine.
But this is only one of many cases which show how incompetent are all existing systems of formulae to represent
adequately the essential nature of bodies and the chemical
changes they undergo.
Phosphines, Arsines, Stibines.-By replacing the hydrogen
of phosphoretted, arsenetted, and antimonetted hydrogen
by hydrocarbon groups, extensive series of bases are obtained, which have received the above names, and, save




256             C,2emical Philosophy.
for their extreme oxidability, agree in general characters
with the compound ammonias.
A4mides.-When the hydrogen of ammonia is replaced by
oxidised or other negative radicles, compounds called
amides are formed.  These bodies are, generally speaking,
neutral, though some which contain the radicles of weak
acids are still capable of entering into combination with
acids, and a few are even very decidedly basic; urea or
carbamide, CO (NH2)2 for example.
On the other hand, some of the amides derived from
polybasic acids are themselves acid bodies. This has been
already referred to (Acids, p. 244).
The general formule of amides correspond with those of
amines, and they may be classified into primary, secondary,
and tertiary monamides, diamides, and triamides, and, substituting negative R or R for R in the formule given on
p. 253, they may be tabulated in the same way as the amines.
One slight addition is necessary.  Monamines containing
bivalent radicles are not known, but monamides in which
two atoms of hydrogen are replaced by one bivalent acid
radicle, are known under the name imides.
Cyanic acid or Carbimide.    Succinimide.
N i(co)               N  (CH402)"
Tertiary derivatives of ammonia, called nitriles or cyanides,
are known in which the three atoms of hydrogen are replaced
by trivalent hydrocarbon groups. We have for example,
Formonitrile    Acetonitrile     Benzonitrile
N                (C2H)3)"'      N(C715) "'
or Hydrogen cyanide.  or Methyl cyanide.   or Phenyl cyanide.
These bodies are not basic, and are decomposed, like all
amides, by boiling with alkalis. Ammonia is evolved, and
a salt formed corresponding with the acid from the ammonium salt of which they were derived by dehydration. Notwithstanding, therefore, that the replacing radicle contains




Substituted A mides.                 257
no oxygen, these bodies should be ranked with the amides.
The isocyanides (pp. I57 and I92) are basic, and are therefore called carbamznes.
The residual hydrogen of primary and secondary amides
may be replaced by positive or hydrocarbon radicles.
Compounds intermediate between amines and amides then
result.  The following are examples:C2H5( C2H5                       C2H5
N  H             N  C2H5 N           C2H5
HN               H               (CC2H5
Ethylamine.      Diethylamine.     Triethylamine.
( C2H30           C2H30           (C2H3O
NH              N  C,2H3O         N  C2H3O
t I  H                           ( C2H30
Acetainide.      Diacetamide.      Triacetamide.
C2H30            C2H30            (C2H3O
N  C2 H5        N  C2H30          N    1C2H5
CH,   C2H2O
C2H5            lC2H -             H
Diethylacetamide.   Ethyldiacetamide.    Ethylacetamide.
CHAPTER XXVI.
CARBON  COMPOUNDS.
BODIES having the same chemical functions, and constituting a series in which, between any two contiguous terms,
there is a constant difference of CH2, are said to be homologous, and the series is a homologous series. Relations of
a like nature have not hitherto been observed among the
compounds of any element but carbon.* This peculiarity
* And probably silicon. Unless, indeed, we choose to regard such a
pair of compounds as tetrachloride and sesquichloride of manganese,
MnCl4, Mn2Cl6, as the two first terms of a series Mn- Cl:,.+2, the
higher members of which are unknown.
S




258           Chemical Philosophy.
seems to be connected with a special faculty with which
carbon atoms appear to be endowed, the power, namely, of
uniting with one another without the intervention of any
other element. Thus a very large number of different compounds containing carbon and hydrogen, either alone or
associated with oxygen or nitrogen, have already been produced, and a still larger number of possible combinations
of carbon is indicated by theory.
Notwithstanding that, by reason of their composition
being unknown or their reactions imperfectly studied, a
considerable number of carbon compounds remain unclassified, it has been found that nearly all well-known compounds
range themselves naturally in one or other of the following
classes:I. I-IHYDROCARBONS AND HALOID DERIVATIVES.
Some hundreds of compounds consisting only of carbon
and hydrogen have been described, but no such compound
is known of which two volumes of the vapour contain a
larger quantity of hydrogen than is represented by the
general formula CH2,+2. This fact is explained by the
following hypothesis. A single atom of carbon has four
units of atomicity, each of which is capable of being
saturated by one atom of hydrogen. We thus arrive at the
formula,
H
I
H-C-H
H
which represents marsh gas. This body is absolutely saturated, and nothing will induce it to take up additional elements of any kind, except on condition of removing one
or more of its hydrogen atoms. If, now, we imagine another
atom of carbon similarly loaded with hydrogen presented to
it, the only mode in which they can unite together is by




Constitutionz of Hydrocarbons.        2 59
the severance of an atom of hydrogen from each group, as
represented in the following diagram:H                     H...........................................
H-C- [1 H                 H-C —H
H'AH
or
H H
H —C —C —H
H H
The same process may be repeated ad infzilzunm. The
number of hydrogen atoms in the first term of the series
being 4, in the second 2 x 4-2, in the third 3 x 4 -4, in
the nth term the number will be 4n- (2n11- 2), or 2n+ 2.
The most important series of hydrocarbons at present
known are represented by the following general formulhe:Paraffins, or Marsh Gas Series,    CnH2,n+2
Olefines, or Olefiant Gas Series,   CnH2n
Acetylene Series,               CnH2n-2
Terpene Series,                 C.H2n_4
The lowest known term of the terpene
series contains CI0.
Benzene series,                   CnH2n_6
The lowest term of this series contains C6, it is called
benzene, and is represented by the following graphic
formula:
H  H
C-C
HC   CH
C-C
H  H
S 2




26o            Chemical Philosopay.
It gives rise to a large number of substitution derivatives,
among which occur many isomerides. The differences
observed among these compounds are attributed to the
assumption of different relative positions by the replacing
radicles.
Hydrocarbons differ very much in their physical properties, and in the nature of the reactions to which they lend
themselves. This, of course, depends upon the series to
which they belong, but taking any one series, such as the
paraffins, it is found that a regular gradation may be traced
in the physical properties of the several members of the
series, passing from the lowest to the highest terms.
Thus the first three members of the paraffin series are
gaseous, methane having indeed resisted all attempts to
liquefy it; the succeeding members are liquid, but
become less and less volatile,* and at the same time more
dense and viscid, till among the highest members of the
series we come to crystalline solid bodies, which are volatilisable only at such high temperatures that they cannot be
distilled without extensive decomposition. In proportion
as we ascend the series the percentage of hydrogen rapidly
decreases, and this, together with the diminished mobility of
the body, may perhaps account for the comparative indifference to chemical reagents exhibited by the higher members.
A corresponding seriation of properties, both physical
and chemical, is observed in other groups of bodies related
to one another in a similar manner.
2. ALCOHOLS.
These bodies may be regarded as hydrates of hydrocarbon radicles, considering that in their most prominent and characteristic reactions, they resemble water
* See Chapter IV., p. 23.




Reactions of A icohols.            26 I
and the hydrates of the metals. They are either liquids
or solids, and they are obtained in a variety of processes; but the ethylic, or common alcohol of fermentation
was the first studied, and is the best known of the class.
Taking it as the representative of alcohols in general, we
may record the following as its characteristic reactions:
(a). By the action of an alkali metal it loses hydrogen.
C2HI5HO   +   K   = C2HKO   +   H.
Alcohol.             Potassic ethylate.
(b) By the action of acids or acid anhydrides, it is converted into salts, called compound ethers, eg.:
C2H5HO   +   HC1  =   CH5C1  +   HO.
Alcohol.               Ethyl chloride.
C2H5HO   +  H2SO4=  C2Hs5HS04 +   H20.
Alcohol.              Ethyl-hydrogen
sulphate.
2C2CH5HO  -+  (C2H30)20O      2C2HC2H3O2 +  H20O.
Alcohol.    Acetic anhydride.    Ethyl acetate.
(c) The alcohol is reproduced from these salts by the
action of alkalies:
C2H5C1  +   KHO   =   C2H5I- O   +   KCI1.
Alcohols are divisible into several classes, according to
the number of hydroxyl groups in the molecule.  Monohydric alcohols yield one saline derivative or compound
ether, dihydric alcohols give two, trihydric alcohols give
three such compounds, and so on.
But they are also divisible into several other groups,
which are characterised by their behaviour when submitted
to oxidation.
Prinacy alcohols, when oxidised, lose hydrogen, and
yield bodies which are called aldehyds; thus common
alcohol loses two atoms of hydrogen:
C2H60   +   0   =   C2H.IO    +   OH2.
Alcohol.              A!dehyd.




262             Chemiicacl  PZilosopjy.
Secondary alcohols, under the influence of similar re&
agents, yield ketones; for example:
C3H80  +   O =      C3HO60  +   OH2.
Isopropylic alcohol.    Acetone.
Tertiary alcohols, when oxidised, break up into a mixture
of acids, each containing a smaller number of carbon atoms
than the alcohol.
These differences are accounted for by the following
hypothesis. The generating hydrocarbon is supposed to
be marsh gas or methane, and by the replacement of one
of its hydrogen atoms by hydroxyl, an alcohol called
methylic alcohol is formed.
Methane.    Methylic hydrate or alcohol.
ci H
H               H
By replacing another atom of hydrogen by a hydrocarbon
group, one of the higher homologues of methylic alcohol is
produced. Now, so long as the replacement is effected
in such a manner as to leave two of the hydrogen atoms of
the marsh gas type undisturbed, the product is a primary
alcohol, which by the substitution of an atom of oxygen for
these two atoms of hydrogen, is capable of giving rise to an
aldehyd.
Primary alcohols.          Aldehyds.
(CU3    (C(CH3) 112CHa C                C(CH3)H2
C11 H-                  etc. C 0       C  0      etc.
(011     (OHI                1H         H
Ethylic.    Propylic.      Acetic.       Propionic.
If, however, the second step in the process of replacement affects one of these typical hydrogen atoms, then a
secondary alcohol is produced, and this body cannot take
in oxygen in the same manner as the primary alcohols, but




Secondary and Tertiary Alcohzols.       263
it may lose two atoms of hydrogen, as represented by the
following formula:
Secondary alcohol.          Ketone.
C                       CH3
C C                     c CH3
H               ~~~0" 
Propylic.        Acetone or dimethyl-ketone.
Lastly, the tertiary alcohols are generated in the same
manner by the replacement of the last atom of hydrogen
originally belonging to the type, thus:
Tertiary alcohols.
(CH3                    (C(CH3)HS
CH3                      CH3       etc.
OH                      \OH
Butylic.                   Amylic.
It is obvious from this that, although a primary alcohol
may contain one or two atoms of carbon, no secondary
alcohol containing less than three atoms, and no tertiary
alcohol containing less than four atoms of carbon in the
molecule can exist.
Another class of alcohols, called phenols, is also known.
These agree to some extent with secondary alcohols, inasmuch as they yield no aldehyd by oxidation, but furnish
compounds containing the same number of carbon atoms as
themselves. These substances, however, are not ketones.
They are called quinones.
No phenol is known to contain fewer than six carbon
atoms in the molecule. They are regarded as derived from
benzene or some analogous hydrocarbon. The best known
phenol is the body which usually goes by that name among
chemists, or, in common parlance, carbolic acid, C6H5OH.
In accordance with the formula, it might also be called
phenyl hydrate, or phenyl alcohol. It differs from ordinary
alcohols in yielding a great number of substitution derivatives, most of which exhibit well-marked acid properties.




264             ClCemical  PhilosopAy.
Thus the trinitrophenol is commonly called picric, or
trinitrophenic acid. The following are the formule of its
chloro and nitro derivatives:
Monochlorophenol.    C6H4C1.OH
Dichlorophenol           C6H3C12. O H
Trichlorophenol          C6,H2C13. OH
Pentachlorophenol.   C6C15. OH
Mononitrophenol.   C6H4(N02)' OH
Dinitrophenol.   C6H3(NO2),.OH
Trinitrophenol      ~   C6H2(NO2)3.OH
Several isomeric modifications of the mono and di derivatives are known.
3. ETHERS.
These bodies have the same relation to alcohols as the
metallic oxides to their hydrates. They are, however,
not easily converted into alcohols by the direct action of
water, neither are they obtained from alcohols so readily
as the metallic oxides from hydrates.  The oxides corresponding with the dihydric alcohols seem to undergo this
transformation more readily than those belonging to the
monohydric alcohols.
The following are some examples of simple and mixed
ethers:
CH3' 0         CH3}           C      o5
CH,3           C2H5           C2H5
Methylic oxide or ether.  Methyl-ethylic   Ethylic oxide (common
oxide or ether.   ether).
(C2H4)/O                 (CH5)2 "'0,
Ethene oxide or ether.     Glyceryl oxide or ether.
The known ethers are volatile liquids of aromatic odour.
4. ALDEHYDS AND KETONES.
These two classes of compounds are closely related, both
as regards their mode of formation, and properties and
composition.
Aldehyds are formed by the oxidation of primary alcohols,
ketones by the oxidation of secondary alcohols.  Thus




A ldeayds and Ketones.            265
common aldehyd results from the removal of two atoms of
hydrogen from common (ethylic) alcohol:C2H60  --  H   =   C2H40
Acetone, the best known of the ketones, stands in the
same relation towards isopropylic alcohol:C,H8O  -   H, =   C3H60
It has already been shown that no secondary alcohol
can contain less than three atoms of carbon, and, consequently, there can be no ketone, properly so called, containing less carbon than acetone, C3H60.
The formation of this body by the action of zinc methyl
(see organo-metaneZic comnpounds) on carbonyl chloride shows
that it contains two methyl groups united with the carbonyl
radicle:
CO C1       Zn        CH3  ZnC1,  +   CO   CH3
+   Zn CH                           Cf13
Carbonyl chloride.  Zinc methyl.   Zinc chloride.  Acetone.
It thus appears that the aldehyds of the monocarbon and
dicarbon series may be considered to behomologous with the
higher aldehyds on the one hand, and with the ketones
on the other. The following formulae indicate the relationship:Formic aldehyd             H       (gaseous)
or ketone.              H
Acetic aldehyd         co          (B.P. 2H
or ketone.             jCH3
Aldehyds.                 Ketones.
Propionic.             Dimethyl (Acetone).
COIC H  (B.P.48o.5)      CO CH3 (B.P. 566)
Butyric.               Methyl-ethyl.
c{H7 (B.P. 750)          CO)   CH3 (B.P. 8 o)
CO Hl7                     [ C2H5




265             Clzeiical Philosopphy.
Valeric.                      Diethyl.
CO{ H(B.P. IO2)             ciC2H  (B.P. )ooo)
CX9,                        C2H5
Aethyl-propyl.
G { COH{ (B.P. IoI0)
M|ethyl-isopropyl.
CO ICH(CH3)2(B.P. 93~ )
From the existence of such intimate relations between
these two classes of compounds, it is not extraordinary that
in many of their properties there should be considerable
resemblance between them.
(a) By the action of nascent hydrogen, generated by
the action of sodium amalgam in the presence of water,
aldehyds and ketones are converted into the corresponding
alcohols, e.g.:C2H40   +   H2  =   CH;O
Acetic aldehyd.        Ethyl alcohol.
C3H60  +   H2  =   C3H80
Acetone.           Isopropyl alcohol;
(b) All aldehyds and many ketones combine with the
acid sulphite of sodium, forming crystallisable compounds,
which yield up the aldehyd or ketone again by treatment
with an acid or an alkaline carbonate.
(c) By boiling with hydrocyanic acid and hydrochloric
acid, aldehyds and ketones are converted into acids containing in the molecule one more atom of carbon than
themselves. For example, common aldehyd is converted
into one form of lactic acid.
H-                                HC
CO t CH3  +   HlCN   +   21120  +   UCI
Aldehyd.  Hydrocyanic acid.
CHO
CHidee lactic acid.  Ammoium chloride.
Ethidene lactic acid.  Ammonium chloride.




Carbonz Acids.                267
Acetone is converted by a similar process into a homologue of lactic acid,
CO CH3 + HCN  +  H0 + HC1
Acetone.    Hydrocyanic
acid.
I CH,
/C/H      +   NH4C1
%CO.OH
Acetonic or      Ammonium
oxybutyric acid.  chloride.
Notwithstanding these various points of resemblance,
ketones are sharply distinguished from  aldehyds by the
action of agents of oxidation.
An aldehyd is always converted by oxidation into an
acid containing the same number of atoms of carbon.
A ketone, on the contrary, is much more difficult to
oxidise, and is then broken up into a mixture of acids,
each containing a smaller number of carbon atoms.  This
oxidation proceeds according to a definite law.
5. ACIDS.
Every carbon acid may be considered to have been
derived from an alcohol by the substitution of an atom of
oxygen for two atoms of hydrogen.
Monohydric alcohols yield monobasic acids. For example, ethylic alcohol yields acetic acid:C2H5.OH.  -   H2  +   0         C2H30.OH
Ethyl alcohol.                    Acetic acid.
Dihydric alcohols may yield either monobasic or dibasic
acids. Ethene alcohol or glycol, for example, may be converted into monobasic glycollic or dibasic oxalic acid, thus:
C2H4(OH)2  -   H2  +   0  =   C2H20(OH)2
Ethene alcohol.                   Glycollic acid.
C2H4(OH)2       2112  +   02       C202(HO)
Ethene alcohol.                     Oxalic acid,




268            CChemical Philosophy.
Trihydric alcohols may yield theoretically either monobasic, dibasic, or tribasic acids. Very few of these derivatives, however, are actually known. Thus glycerine, which is
a trihydric alcohol, yields, upon oxidation, only one acid
containing the same number of carbon atoms.  This
glyceric acid is monobasic.
C,H5(OH)3  -  H2 +  0  -   CH30(OH),3
Glycerine.                     Glyceric acid.
A large number of acids are known, the alcohols corresponding to which have not yet been discovered.
Acids, however, may be generated by an entirely different
process from alcohols containing one atom less carbon.
This process is interesting, because it serves to indicate the
general constitution of carbon acids.
Starting from methyl alcohol, CH3'OH, for example, we
may convert this body into the corresponding cyanide
CH3'CN.  When boiled with caustic potash this compound undergoes decomposition, ammonia is evolved, and
the potassium salt of acetic acid is generated.
CH3,CN' KIO  + H,O = CH,3CO(OK) + NH3
Methyl cyanide.  Potash.   Water.   Potasium acetate.   Ammonia.
Or leaving the potassium out of sight, the reaction may
be represented as follows:CH3'CN + 2H20 = CH3'CO(OH) + NH3
From this it appears that the acid is generated by the
substitution of O'"+(OH)' for the N"' of the cyanide.
Acetic acid then consists of a methyl group, CH3, combined with a carboxyl group, CO OH.  Now methylic
alcohol is monohydric, and therefore is capable of yielding
one cyanide and no more. Consequently, from methylic
alcohol only one derivative containing the carboxyl group
can be formed, and that derivative is a monobasic acid. In
a similar manner it is found that, by decomposing the
cyanide of a bivalent alcohol radicle bv boiling alkali, a




Constituztion of Carbon Acids.        269
bibasic acid is produced, whilst the cyanide of a trivalent
radicle yields by the same process a tribasic acid.  Setting
down examples of compounds generated in this way, we
find that they all contain the group CO.OH, once, twice, or
three times in the molecule, according to the saturating
power of the acid.
Monobasic          Dibasic            Tribasic
acetic acid.     succinic acid.*   tricarballylic acid.
c.co ( Co() c O cH(O H)CO(OH)
This         I CO(OcH)
This carboxyl group then is contained in a very large
number of carbon acids, and its presence seems to be
intimately connected with the development of the acid
character in these compounds. We have seen (Phenols,
p. 263) that the halogens and the nitroxyl group (NO2)' when
substituted for hydrogen in certain oxidised bodies also constitute acidifying agents, but the presence of these radicles in
varying quantities does not affect the basicity of the resulting acid; whereas it seems to have been established that
an acid which contains the carboxyl, CO(OH), group n
times is n basic.
Carbon acids, when submitted to the action of the halogens,
yield a great many substitution derivatives, such as the following chlorinated compounds, which are obtained from
acetic acid:
Monochloracetic acid CH2C1lCOOH
Dichloracetic acid   CHC12ICOOH
Trichloracetic acid CC13'COOH
But here the process of substitution stops, the hydrogen
of the carboxyl being replaceable only by metallic or positive radicles, and not by negative bodies.
The metallic salts, amides, anhydrides, and chlorides
derived from a carbon acid, correspond in number, mode of
* See molecular weights, Chapter XVI. p. 126.




270             Chemical PhilosopkA.
formation, and general properties with the same derivatives
of mineral acids.*
6. BASIC DERIVATIVES OF AMMONIA.
The constitution and general properties of these bodies
have already been disoussed.  (Amines, p. 252.)
The  "alkaloids," quinine, morphine, strychnine, and
many others which are obtained from various plants, and
constitute important medicinal agents, are referable to this
type. They are for the most part tertiary amines.
7. COMPOUND ETHERS OR ETHEREAL SALTS.
The most general method for the formation of these compounds consists in heating together an acid and an alcohol.
A double decomposition ensues; water is formed, together
with the ethereal salt; so that the reaction is exactly
parallel with the change which occurs when an ordinary
acid and basic oxide are brought into contact. The following equations, for example, are strictly comparable:KHO + HN03 = KNO3 + H20
Basic oxide.  Acid.  Salt.    Water.
C2H5.HO + HC2H302 = C2H5.C2H302 + H20
Alcohol.     Acid.       Ethereal Salt.  Water.
Compound ethers also imitate metallic salts in their
general reactions. Thus they are decomposed by acids, by
alkalies, and by water in precisely the same manner. They
differ from metallic salts, however, in many physical characters, being for the most part volatile liquids of aromatic
odour, and very slightly soluble in water, though generally
miscible with alcohol in all proportions. Comparatively
few are crystallisable. Among the most interesting examples
* Pp. 242 to 246.




Organo-Metallic Cornpounds.            271
of crystallisable ethereal salts are the constituents of the
natural fats:Palmitin              C5H3I1.CO.OK
or glyceric palmitate  C15H31.CO.O-(C3H5)
and
Stearine or          C17H35.CO'.O
glyceric stearate.C,7H35.CO. O-(C3H,)
8. ORGANO-METALLIC COMPOUNDS.
These are compounds of metals or metalloids with
hydrocarbon radicles. They are volatile, very oxidisable,
and generally heavy liquids. The zinc-ethyl compound was
first obtained. It is a colourless liquid, boiling at i I80, and.
spontaneously inflammable when thrown into the air. It is
instantly decomposed by water, and when submitted to the
action of successive small quantities of oxygen, it yields two
oxidised compounds, thus:Zinc ethide.    Zinc ethyl-ethylate.  Zinc ethylate.
Zn  C2H5       Zn n  C25         ZnOC2H5
C2 H5       OC2H,         ZOC2H,
Iodine removes from it half and then the whole of the
ethyl, according to these equations:Zn(C2H5)2 + I2  =   Zn(C2H5)I  +   C2H5I
Zinc ethiodide.  Ethyl iodide.
Zn(C2H5)I + I2  =       ZnI2      +      C2H,51
Zinc iodide.   Ethyl iodide.
The existence of these compounds furnishes conclusive
testimony of the diadic character of the zinc atom, and in
other cases similar evidence, of great value in settling questions relating to atomicity and atomic weight, has been
obtained by the study of these compounds.
The following are the formulae of some of the most
interesting organo-metallic bodies. It will be noticed that
they are constituted in the same way as the chlorides and




272             Chemical Philosophfy.
oxides of the same hydrocarbon radicles, the special
peculiarities of the organo-metallic bodies being due to the
unoxidised condition of the metals they contain.
The vapour densities of all these compounds are normal,
so that each formula represents two volumes of vapour.
Ethyl chloride                C  C
or chlorine ethide
Ethyl oxide..             C2H5
or oxygen ethide..         C2H5,,,(C2H5
Boron ethide                  B C2H5
(CH,5
C2H5
Silicon ethide..        Si C2H5
C2H5
Trimethyl arsine.                 CH3
or arsenious trimethide           CH3
CH3
Diarsenious tetramrethide.     As(CH3)2
(kakodyl)                    (CH3)
As(CH3)2
Trimethyl stibine             (/  CH3
or antimonious trimethide. SbCH
Triethyl bismuthine,, C2H5
or bismuthous triethide        i  CH5
C2H5
Zinc ethide..         Zn 
C2H5
Mercuric ethide              HgUC2H5
Stannous ethide.    SC2H




Exercises on Section F.                 273
I C2H5
"v CH11
Stannic ethide                     Sn C2Hu
C2H5
C2H5
C2H5
EXERCISES ON SECTION V.
I. Write down the formulae of all the possible silver salts of the
following acids:HC103,  H,2SO4,  H3PO4,  I14SiO4, H2SO3, H3PO0.
2. Write down the formulae for all the chlorides and amides theoretically derivable from the following acids:HNO3, H2SO4, H4P207, HC7H,02 (benzoic acid, monobasic).
3. Write the constitutional formulae of Nordhausen sulphuric acid,
H2S207, of permanganic acid, H2Mn208, and of chromic acid, H2CrO4,
assuming sulphur, manganese, and chromium hexad.
4. Write down the formulae for the possible anhydrides derived from
orthosulphuric acid, S(OH)6, by successive stages of dehydration.
5. Give the formulae for the anhydrides corresponding with sulphuric
acid, HSO4, phosphoric acid, H3PO4, pyrophosphoric acid, H4P207,
nitric acid, HNO3, nitrous acid, HNO2, hyponitrous acid, HN O,
theiosulphuric acid, H2S203, chloric acid, HC103, perchloric acid,
HC104, and acetic acid, HC2HO02, pointing out those which are actually
known.
6. Give concise definitions of the terms acid, base, salt, with examples
of their characteristic properties drawn from your own experience.
7. How would you explain the terms acid, and neutral salt? Give
examples.
S. A number of salts having the general formula M'tIS02 were
called by the discoverer hydrosulphites. What name should they bear
according to the rules laid down in Chapter XII.? Write the graphic
formulae for these bodies, and mention compounds which are similarly
constituted.




274                Chemical Philosophy.
9. Complete the following equations, adding the name to each
formula:UNO3 + PbO 
HNO3 + NH3 =
H2SO4 + KHO
H2SO4 + PC15 -
(NH4)sSO4 - 2H20
(NH4)HSO4 - H0O
(NtI4)2CO3 - H20 
(NH4)2CO3 - 2HTO =
C2H3OC1 + OH2
Ca(HO)2 - I-20 =
CaCI2 + NaCO3 -
CaCO3 + 2NH4C1 
NH3 + C2H5I =
NH2C2H5 + C21I5I =
NH(C2HI5)2 + C2H5I =
N(C2H5)3 + C2HI =
N(C2H5)4I + AgHO 
Io. Give reasons in favour of each of these formaule for saIammoniac:NH3IHC1
NH4CI
II. To which class of compounds would you refer the following
substances:Caustic potash, KHO; lime, CaO; nitrous oxide, N20; nitrogen
trioxide, N203; ferric oxide, Fe2O3; chromium trioxide, CrO3; wood
spirit, CH3-OH; glycerine, C3H5(OH)3; urea, CO(NH,)2.
12. Accepting the formula HC1 for hydrochloric acid, what facts
determine the choice of the formula H2C204 for oxalic acid?
13. How far do the following compounds agree with definitions of
the terms acid, base, salt? Give sufficient reasons in each case.
Chlorides of hydrogen, potassium, aluminium, phosphorus, sulphur;
oxides of hydrogen, potassium, carbon; hydrates of potassium,
aluminium, arsenic, silicon.
I4. Define the term hydrate.  Classify the following compounds:C12. IoH20-KHO-H3A103-C2H50 H.
15. Write the constitutional formula of urea, and of the compound
with which it is isomeric.
I6. How many litres of marsh gas would be equal in weight to 25
litres of ethene?




Exercises on Section V.                  275
17. Benzoate of silver has the formula C7H5AgO2. Calculate its
percentage composition, also the weight of silver contained in'5736
gram of this salt.
I8. From the formulae HBr, H2S, CC14, NH4C1, H2SO4, state the
quantivalence of the symbols Br, S, C, N, and S, giving your reasons
in each case,
9r. Name and classify the following compounds:CH4,            CHC13,           C2H6
5HCI,          C2 5  0,         C2H5  Zn,
C2H5Ct,         C2H5  CH5
N, C~s }Zn,
CC2H            CH N,            C2H3 
1-12            (C2Hs).~          C2H3O
20. Assign a formula to a body containing C26 6, H2'2, and 07'I
per cent.
2i. How would you prove that the formula of marsh gas is CH4?
Also, how could you show that ethylene contains twice as much
carbon?
22. Give reasons for representing oxalic acid as C2H204, not CH02;
acetic acid as C2H402, not CH20; ethylene as C2H4, not CH2.
23. By what experiments could you prove that the molecule of benzene is correctly represented by the formula C6H6 not C3Ha?
24. A body having the composition C5H 20 is an alcohol. Express
by equations the probable action upon it of acetic acid, oxalic acid, and
oxidising agents.
25. The compound C4H80 is either an aldehyd or a ketone. How
would you recognise its true character?  Write out the constitutional
formula of the aldehyd and ketone of this composition, and indicate
the constitution of any metameric modification that may exist.
26. Dissect the crude formula C7H1402, showing the nature of the
several metameric compounds which it may represent.
27. In respect of what properties may you regard alcohols as water
molecules in which hydrogen has been replaced by hydrocarbon radicles?
Illustrate by reference to common alcohol, C2H5OH.
28. The ethylic series of monohydric alcohols are represented by the
general formulaCnH2n + 10H.
Write out the formula for the corresponding aldehyds, acids, and acetic
derivatives (acetates).
29. How many acids could be derived from the alcoholCH,-OH
CH2'OH,
and what would be their formulae?




276                Chemical Philosophy.
30. Carbolic acid is sometimes called phenylic alcohol. In what
respects do its alcoholic properties differ from those of the alcohols of
the ethylic series, CnH2n + 20?
3I. Complete these equations:CH'sCO(OH  +* PC15
CH3OH + HC1CHa
CHa3}O + 2HC1 
2CH3'CO(OH) + Ag2O =
CH3OIt + C2H50 SO,
CH3'CO(OCH3) + NaHO =
CH3'CO}O + H20, 
CH3'CO
32.. In what respect does prussic acid resemble and how does it
differ from hydrochloric and hydrobromic acids?
33.'3339 gram of a compound gave'7896 gram of CO2 and'2924
gram  H20.  The vapour density (air = i) was found to be 6'59.
Calculate a rational formula.
34.'1483 gram of ethylated acetone gave'3799 gram of CO2 and
1575 gram of water.  Its vapour density (air = I) was 2-95I.  Calculate the formula of the compound.
35. The analysis of barium diacetotartrate gave the following
results:I.'2065 gram of substance gave'1I297 gram of BaSO4.
II.'I377 gram of substance gave'1307 of CO2 and'0300 of H20.
Calculate the formula of the salt and of the corresponding acid.
36. The following results were obtained in the analysis of the copper
salt of ethyl-benzoic acid:-'22I8 gram gave'4827 gram of CO2, Ioo004
gram of water, and'o489 gram of CuO. Calculate the formula of the
acid.




INDEX.
ABS                     ATO                          DUA
A BSOLUTE  tempe-   Atomicity of tin and allies,   Classification of elements,
ratures, 37            202                        I7I
Absorption spectra, 44     Atomic theory, Dalton's,   - of reactions, 69
Acids, 239, 267              65                        Cobalt, 226
- basicity of, 240         Atomic weights, IIc         Cohesion, 5, 85
- molecular weight of,   Atoms, 4                     Colloids, I8
127, 242                 Avogadro's law, 37         Combustion, 97
Adhesion, 85                                          - heat produced by, Ior
Affinity, 93                                          Composition expressed in
Alcohols, 260       "          ARIUM, 2r4               percentages, 63, I20
- primary, 26I             1I - peroxide, 215          Compound ethers, 270
- secondary and tertiary,   Barometric pressure, 33    Conditions of chemical
262                      Bases, 246                   change, 85
Aldehyds, 264             — molecular weight of,   Constitutional  formulae,
Alkaloids, 270               127                         I44
Allotropic  modifications   Basic salts, 249           Constitution of matter, 3
of elements, 54, I58     Basigenic elements, 172    - of hydrocarbons, 258
Aluminium, 226             Benzene, formula of, 259    - of carbon acids, 269
Amides, 256                Bismuth, 193, 203          Copper, 232
- substituted, 257         Boiling points, 2I          Critical point, 26
Amines, 252                - relation to molecular   Cryohydrates, i2
Ammonia, 294, 250            weight, 22               Crystallisation water, 13
Ammonium theory, 250       - determination of, 24     Crystalloids, i8
- amalgam, 252             Boron, I74, 189            Cupric salts, 233
- compounds, 254           Boyle's law, 32            Cuprous salts, 233
Anhydrides of acids, 245    Bromine, r73              Cyanides, I57, I92
- of bases, 247
Antimony, I93, 203
Apatite, 207                  ADMIUM, 2i6
Aqua-regia, 234,          Cxesium, 212            ALTON'S   atomic
Arseisic, 193, 203         Calcium, 2I4                     theory, 65
Arsenious acid, 206        Calomel vapour, density    Decomposition, 53
Arsenious oxide, vapour      of, I34, 220             Definite proportions, 6o
density of, 205          Carbamines, I57, 257       Deliquescent bodies, I4
Atomicity of carbon and   Carbon, I59, 1s0            Dialysis liquid, I9
silicon, I9g             - compounds, 257           - gaseous, 31
- of elements, I41         - acids, 267               Diamond, I59
- of halogens, I79         Chemical  combination,   Diffusion of liquids, I7
- of iron group, 230        laws of, 59               - of gases, 27
- of nitrogen and phos-   Chemistry, province of, 4   Dimorphism, I53
phorus, i95              Chloranilines, 176         Dissociation, 130
— of oxygen  and sul-   Chlorine, I73                 Distillation, fractional, 24
phtur, x86               Chromium, 226              Dualistic theory, 246




278                                 lndex.
EFF                        IND                          PIE
cFFLORESCENT 3,              U       I N 222        M etine, 207
compounds, 03, I3I    1   Iodine, -73, 276        Mixtures, 72, 78
Electrolysis, 89            Iridium, 235                 Molecular   compounds,
Elements defined, 54        Iron, 226                     c8I
- distribution of, in na-   Isocyanides, 257, 392       - weights, I2i
ture, 55                  Isomerism, I53              - theory, 7
- symbols and  atomic   Isomorphismsi.6                 Molecules, 3,weights, 57                                          - of elements, 123
- classification of, I7I                                 Molybdenum, 208
Energy of isomerides, I6I    17ETONES, 264              -- pentachboride,.48
- of salts, 250              X                           Multiple proportions, 63
Equations, 65
Equivalents, iog
Ethereal salts, 270         T  AWS of Avogadso, 37    NAMES of acids and
Ethers, 264                       - Boyle, 32            N     salts, 82
Ethyl formate, I56          - chemical combination,   - alcohols, 84
Exercises, 50, Io04, x6.2,    59                        - aromatic    hydrocar237, 273                  - definite proportions, 6o    bons, 84
- Dulong and Petit, II4    - binary compounds, 80o
-Gay - Lussac  and          - carbon compounds, 83'ILAME,  luminosity           Charles, 34               - elemens, So'
1    of, 47                 -- isomorphism, II7         - nitrogenous bases, 84
-spectra, 40                - -multiples, 6I             Nascent state, I23
-temperature of, too        - reciprocals, 6I            Natural fats, 271
Fluorine, J73, 178          - volumes, 62               Nickel, 226
Fluosilicates, etc., 202    Lead, 222                   Nitric oxide and peroxide,
Formula, explained, 58      - peroxide, 224               I47
- calculation of, I20, I65,   Liquefaction of gases, 25   - acid, I94
i68                       Liquids, 5                  Nitrogen, I73, I93
Fractional distillation, 24,   - diffusion of, 17       - trioxide, 094
75                        Lithium, 2I2                - pentoxide, I94
Fraunhofer's lines, 45                                  - chloride (note), I94
Fusion of solids, 9                                     Nomenclature, 78
AGNESIUM, 226          Non-metallic  elements,
Manganese, 226         I7I, I73
G ASES, 5                  Mass, 9s                     Non-metals, class i, 274
-  diffusion of, 27    Matter, constitution o.f, 3   - class 2, E8i
expansion of, by heat,   - intermediate states of, 8   - class 3, I89
35                        Melting point and mole-   - class. 4, I90
- liquefaction of, 25         cular weight, 9           - class 5, 192
- occlusion of, 3I          Mercuric compounds, 22I
- passage through metals   Mercury, 2i8
and membranes, 3I         Metalloids, I72, 198  C CL U S I ON   of
-pressure of, 30            - class I, i99                     gases, 30
- solution of, 4            - class 2, 20oI              lefines, I55
Gay-Lussac's law of ga-   - class 3, 202                Organo-metallic    comseous expansion, 34       - class 4, 203                pounds, 27I
- of volumes, 62            - class 5, 207              Ortho-phosphoric  acid,
Gold, 234                   - class 6, 208                197
Graham's law  of diffu-   Metals, 172                   Ortho-eulphates, i89
sion, 28                  - class I, zxI              Osmium, 235
Graphic formulae, 144       - class 2, 24               Oxides of metals, I84
Graphite, I59, 173          - class 3, 2r6              Oxyacids, 183
- class 4, 21.8             Oxygen, I73, t82
- class 5, 222              - atomicity of, I87
HALOGENS, x74              - class 6, 226               Oxygenic elements, 27x
1    - atomicity of, I79    - class 7, 234              Ozone, I55
Heat generated  in che-   -general characters ot,
mical actions, 87           2I0
Homologous series, 257      Metals of the alkalis, 2r 2     ALLADIUM, 235
Hydrocarbons, 258           - alkaline earths, 204       1    Palladium-hydrogeHydrogen salts, I99         Metamerism, 156               nium, I99
Hypophosphorous  acid,   Metaphosphoric acid, I97   Percentages, 63, I20
i66f                      Methyl acetate, I56         Phenols, 265




dIdex.                                 279
PHL                       SIL                        ZIR
Phlogiston, 98             Silver, 2T8                Tungsten, 208
P;losphine, I94            Sodium, 212                - pentachloride, I48
- oxide, 196               Solids; 4, 5               Types, 135
Phosphines, 255            Solution of solids, eo, Ii
Phosphorus, 159, I73, 193,   Specific heat, z"        T  N SATU RATE 
195                      Spectra, 39                      compounds, 146
- trioxide, I94            - of compounds, 43          ranium, 2o8
- pentoxide, 194           - of solids, 39entachoride48
Phosphorous acid, 206      - of vapours, 3948
Physical isomerides, 153    - thermal and chemical,
Platinum, 235               46'TALENCY  and atoPolymerism, 154            Spectmroscopic analysis, 43    V   micity, I39
Potassium, 212-            Stibines, 255              - common salt radicles
Propionic acid, r56        Strontium, 2I4               (table), I52
Pyromorphite, 207          Substitution compounds,   - compound radicles, 143
Pyrophosphoric acid, 197     7I, 128                  Vanadium, I93, 203
Pyrosulphuric acid, I89    Sulphur, 153, I55, I58,   Vanadite, 207
173, 184                 Vapour density, II2
ATIONAL formul,   - atomicity of, i86  -  and molecular weight,
X  ATIO.NAL formule, - tetrachloride, etc., i88        I2I
ReactIons,37   clasifict Sulphuric acids, i89   - calomel, 220
Reactiof, 69 ns, clasficaton   Synthesis of molecules,   - of arsenious oxide, 205
Reciprocal proportions, 66  125                      Vapours, liquefaction of,
Rhodium, 235                                            24
Rubidium, 212                                         Volatile liquids, 2t
Ruhidlium, 212             TANTALUM, 207              Volumes, law of, 62
Ruthenium, 235                  Tellurium, 2oi
Temperatures  absolute,      TATER of crystalliCALTS, 248                   37                        VY   sation, I3, 187. - formation of, 249    Terpenes, 154, 259           Weights and measures 2
Saturating power of acids   Thallium, 222
and bases, I27           Thermolysis, I31
Selenion, I73, I84         Tin, 202                    7INC, 2e6
Silicon, I73, I90          Titaniulr, 20a2            L    ZI'rconium, 202