Chapter IV: Introduction (2)

[24] It is true that Stahl was acquainted with a fact which directly
disproved his hypothesis. It was already known (from the
experiments of Geber, and more especially of Ray, in 1630) that
metals increase in weight by oxidation, whilst, according to
Stahl's hypothesis, they should decrease in weight, because
phlogiston is separated by oxidation. Stahl speaks on this
point as follows:--'I am well aware that metals, in their
transformation into earths, increase in weight. But not only does
this fact not disprove my theory, but, on the contrary, confirms
it, for phlogiston is lighter than air, and, in combining
with substances, strives to lift them, and so decreases their
weight; consequently, a substance which has lost phlogiston
must be heavier.' This argument, it will be seen, is founded
on a misconception of the properties of gases, regarding them
as having no weight and as not being attracted by the earth,
or else on a confused idea of phlogiston itself, since it was
first defined as imponderable. The conception of imponderable
phlogiston tallies well with the habit and methods of the last
century, when recourse was often had to imponderable fluids for
explaining a large number of phenomena. Heat, light, magnetism,
and electricity were explained as being peculiar imponderable
fluids. In this sense the doctrine of Stahl corresponds entirely
with the spirit of his age. If heat be now regarded as motion
or energy, then phlogiston also should be considered in this
light. In fact, in combustion, of coals for instance, heat and
energy are evolved, and not combined in the coal, although the
oxygen and coal do combine. Consequently, the doctrine of Stahl
contains the essence of a true representation of the evolution
of energy, but naturally this evolution is only a consequence of
the combination occurring between the coal and oxygen. As regards
the history of chemistry prior to Lavoisier, besides Stahl's
work (to which reference has been made above), Priestley's
_Experiments and Observations on Different Kinds of Air_, London,
1790, and also Scheele's _Opuscula Chimica et Physica_, Lips.,
1788-89, 2 vols., must be recommended as the two leading works
of the English and Scandinavian chemists showing the condition
of chemical learning before the propagation of Lavoisier's
views, and containing also many important observations which lie
at the basis of the chemistry of our times. A most interesting
memoir on the history of phlogiston is that of Rodwell, in the
_Philosophical Magazine_, 1868, in which it is shown that the
idea of phlogiston dates very far back, that Basil Valentine
(1394-1415), in the _Cursus Triumphalis Antimonii_, Paracelsus
(1493-1541), in his work, _De Rerum Natura_, Glauber (1604-1668),
and especially John Joachim Becher (1625-1682), in his _Physica
Subterranea_, all referred to phlogiston, but under different
names.

Lavoisier proved by means of the balance that every case of rusting of metals or oxidation, or of combustion, is accompanied by an increase in weight at the expense of the atmosphere. He formed, therefore, the natural opinion that the heavier substance is more complex than the lighter one.[25] Lavoisier's celebrated experiment, made in 1774, gave indubitable support to his opinion, which in many respects was contradictory to Stahl's doctrine. Lavoisier poured four ounces of pure mercury into a glass retort (fig. 3), whose neck was bent as shown in the drawing and dipped into the vessel R S, also full of mercury. The projecting end of the neck was covered with a glass bell-jar P. The weight of all the mercury taken, and the volume of air remaining in the apparatus, namely, that in the upper portion of the retort, and under the bell-jar, were determined before beginning the experiment. It was most important in this experiment to know the volume of air in order to learn what part it played in the oxidation of the mercury, because, according to Stahl, phlogiston is emitted into the air, whilst, according to Lavoisier, the mercury in oxidising absorbs a portion of the air; and consequently it was absolutely necessary to determine whether the amount of air increased or decreased in the oxidation of the metal. It was, therefore, most important to measure the volume of the air in the apparatus both before and after the experiment. For this purpose it was necessary to know the total capacity of the retort, the volume of the mercury poured into it, the volume of the bell-jar above the level of the mercury, and also the temperature and pressure of the air at the time of its measurement. The volume of air contained in the apparatus and isolated from the surrounding atmosphere could be determined from these data. Having arranged his apparatus in this manner, Lavoisier heated the retort holding the mercury for a period of twelve days at a temperature near the boiling point of mercury. The mercury became covered with a quantity of small red scales; that is, it was oxidised or converted into an earth. This substance is the same mercury oxide which has already been mentioned (example 3). After the lapse of twelve days the apparatus was cooled, and it was then seen that the volume of the air in the apparatus had diminished during the time of the experiment. This result was in exact contradiction to Stahl's hypothesis. Out of 50 cubic inches of air originally taken, there only remained 42. Lavoisier's experiment led to other equally important results. The weight of the air taken decreased by as much as the weight of the mercury increased in oxidising; that is, the portion of the air was not destroyed, but only combined with mercury. This portion of the air may be again separated from the mercury oxide and has, as we saw (example 3), properties different from those of air. It is called 'oxygen.' That portion of the air which remained in the apparatus and did not combine with the mercury does not oxidise metals, and cannot support either combustion or respiration, so that a lighted taper is immediately extinguished if it be dipped into the gas which remains in the bell-jar. 'It is extinguished in the residual gas as if it had been plunged into water,' writes Lavoisier in his memoirs. This gas is called 'nitrogen.' Thus air is not a simple substance, but consists of two gases, oxygen and nitrogen, and therefore the opinion that air is an elementary substance is erroneous. The oxygen of the air is absorbed in combustion and the oxidation of metals, and the earths produced by the oxidation of metals are substances composed of oxygen and a metal. By mixing the oxygen with the nitrogen the same air as was originally taken is re-formed. It has also been shown by direct experiment that on reducing an oxide with carbon, the oxygen contained in the oxide is transferred to the carbon, and gives the same gas that is obtained by the combustion of carbon in air. Therefore this gas is a compound of carbon and oxygen, just as the earthy oxides are composed of metals and oxygen.

[25] An Englishman, named Mayow, who lived a whole century before
Lavoisier (in 1666), understood certain phenomena of oxidation
in their true aspect, but was not able to develop his views
with clearness, or support them by conclusive experiments; he
cannot therefore be considered, like Lavoisier, as the founder of
contemporary chemical learning. Science is a universal heritage,
and therefore it is only just to give the highest honour in
science, not to those who first enunciate a certain truth, but to
those who are first able to convince others of its authenticity
and establish it for the general welfare. But scientific
discoveries are rarely made all at once; as a rule, the first
teachers do not succeed in convincing others of the truth they
have discovered; with time, however, a true herald comes forward,
possessing every means for making the truth apparent to all, but
it must not be forgotten that such are entirely indebted to the
labours and mass of data accumulated by many others. Such was
Lavoisier, and such are all the great founders of science. They
are the enunciators of all past and present learning, and their
names will always be revered by posterity.

The many examples of the formation and decomposition of substances which are met with convince us that the majority of substances with which we have to deal are compounds made up of several other substances. By heating chalk (or else copper carbonate, as in the second example) we obtain lime and the same carbonic acid gas which is produced by the combustion of carbon. On bringing lime into contact with this gas and water, at the ordinary temperature, we again obtain the compound, carbonate of lime, or chalk. Therefore chalk is a compound. So also are those substances from which it may be built up. Carbonic anhydride is formed by the combination of carbon and oxygen; and lime is produced by the oxidation of a certain metal called 'calcium.' By resolving substances in this manner into their component parts, we arrive at last at such as are indivisible into two or more substances by any means whatever, and which cannot be formed from other substances. All we can do is to make such substances combine together to act on other substances. Substances which cannot be formed from or decomposed into others are termed _simple substances_ (elements). Thus all homogeneous substances may be classified into simple and compound substances. This view was introduced and established as a scientific fact during the lifetime of Lavoisier. The number of these elements is very small in comparison with the number of compound substances which are formed by them. At the present time, only seventy elements are known with certainty to exist. Some of them are very rarely met with in nature, or are found in very small quantities, whilst the existence of others is still doubtful. The number of elements with whose compounds we commonly deal in everyday life is very small. Elements cannot be transmuted into one another--at least up to the present not a single case of such a transformation has been met with; it may therefore be said that, as yet, it is impossible to transmute one metal into another. And as yet, notwithstanding the number of attempts which have been made in this direction, no fact has been discovered which could in any way support the idea of the complexity of such well-known elements[26] as oxygen, iron, sulphur, &c. Therefore, from its very conception, an element is not susceptible to reactions of decomposition.[27]

[26] Many of the ancient philosophers assumed the existence of one
elementary form of matter. This idea still appears in our times,
in the constant efforts which are made to reduce the number of
the elements; to prove, for instance, that bromine contains
chlorine or that chlorine contains oxygen. Many methods, founded
both on experiment and theory, have been tried to prove the
compound nature of the elements. All labour in this direction has
as yet been in vain, and the assurance that elementary matter
is not so homogeneous (single) as the mind would desire in its
first transport of rapid generalisation is strengthened from year
to year. All our knowledge shows that iron and other elements
remain, even at such a high temperature as there exists in the
sun, as different substances, and are not converted into one
common material. Admitting, even mentally, the possibility of one
elementary form of matter, a method must be imagined by which
it could give rise to the various elements, as also the _modus
operandi_ of their formation from one material. If it be said
that this diversitude only takes place at low temperatures, as is
observed with isomerides, then there would be reason to expect,
if not the transition of the various elements into one particular
and more stable form, at least the mutual transformation of some
into others. But nothing of the kind has as yet been observed,
and the alchemist's hope to manufacture (as Berthollet puts it)
elements has no theoretical or practical foundation.

[27] The weakest point in the idea of elements is the negative
character of the determinative signs given them by Lavoisier, and
from that time ruling in chemistry. They do _not_ decompose, they
do _not_ change into one another. But it must be remarked that
elements form the limiting horizon of our knowledge of matter,
and it is always difficult to determine a positive side on the
borderland of what is known. Besides, there is no doubt (from the
results of spectrum analysis) that the elements are distributed
as far as the most distant stars, and that they support the
highest attainable temperatures without decomposing.

The quantity, therefore, of each element remains constant in all chemical changes: a fact which may be deduced as a consequence of the law of the indestructibility of matter, and of the conception of elements themselves. Thus the equation expressing the law of the indestructibility of matter acquires a new and still more important signification. If we know the quantities of the elements which occur in the re-acting substances, and if from these substances there proceed, by means of chemical changes, a series of new compound substances, then the latter will together contain the same quantity of each of the elements as there originally existed in the re-acting substances. The essence of chemical change is embraced in the study of how, and with what substances, each element is combined before and after change.

In order to be able to express various chemical changes by equations, it has been agreed to represent each element by the first or some two letters of its (Latin) name. Thus, for example, oxygen is represented by the letter O; nitrogen by N; mercury (hydrargyrum) by Hg; iron (ferrum) by Fe; and so on for all the elements, as is seen in the tables on page 24. A compound substance is represented by placing the symbols representing the elements of which it is made up side by side. For example, red mercury oxide is represented by HgO, which shows that it is composed of oxygen and mercury. Besides this, the symbol of every element corresponds with a certain relative quantity of it by weight, called its 'combining' weight, or the weight of an atom; so that the chemical formula of a compound substance not only designates the nature of the elements of which it is composed, but also their quantitative proportion. Every chemical process may be expressed by an equation composed of the formulæ corresponding with those substances which take part in it and are produced by it. The amount by weight of the elements in every chemical equation must be equal on both sides of the equation, since no element is either formed or destroyed in a chemical change.

On pages 24, 25, and 26 a list of the elements, with their symbols and combining or atomic weights, is given, and we shall see afterwards on what basis the atomic weights of elements are determined. At present we will only point out that a compound containing the elements A and B is designated by the formula A_n_ B_m_, where _m_ and _n_ are the coefficients or multiples in which the combining weights of the elements enter into the composition of the substance. If we represent the combining weight of the substance A by _a_ and that of the substance B by _b_, then the composition of the substance A_n_ B_m_ will be expressed thus: it contains _na_ parts by weight of the substance A and _mb_ parts by weight of the substance B, and consequently 100 parts of our compound contain _na_ 100/_na_ + _mb_ percentage parts by weight of the substance A and _mb_ 100/_na_ + _mb_ of the substance B. It is evident that as a formula shows the relative amounts of all the elements contained in a compound, the actual weights of the elements contained in a given weight of a compound may be calculated from its formula. For example, the formula NaCl of table salt shows (as Na = 23 and Cl = 35·5) that 58·5 lbs. of salt contain 23 lbs. of sodium and 35·5 lbs. of chlorine, and that 100 parts of it contain 39·3 per cent. of sodium and 60·7 per cent. of chlorine.

What has been said above clearly limits the province of chemical changes, because from substances of a given kind there can be obtained only such as contain the same elements. Even with this limitation, however, the number of possible combinations is infinitely great. Only a comparatively small number of compounds have yet been described or subjected to research, and any one working in this direction may easily discover new compounds which had not before been obtained. It often happens, however, that such newly-discovered compounds were foreseen by chemistry, whose object is the apprehension of that uniformity which rules over the multitude of compound substances, and whose aim is the comprehension of those laws which govern their formation and properties. The conception of elements having been established, the next objects of chemistry were: the determination of the properties of compound substances on the basis of the determination of the quantity and kind of elements of which they are composed; the investigation of the elements themselves; the determination of what compound substances can be formed from each element and the properties which these compounds show; and the apprehension of the nature of the connection between the elements in different compounds. An element thus serves as the starting point, and is taken as the primary conception on which all other substances are built up.

When we state that a certain element enters into the composition of a given compound (when we say, for instance, that mercury oxide contains oxygen) we do not mean that it contains oxygen as a gaseous substance, but only desire to express those transformations which mercury oxide is capable of making; that is, we wish to say that it is possible to obtain oxygen from mercury oxide, and that it can give up oxygen to various other substances; in a word, we desire only to express those transformations of which mercury oxide is capable. Or, more concisely, it may be said that the _composition_ of a compound is the expression of those transformations of which it is capable. It is useful in this sense to make a clear distinction between the conception of an element as a _separate_ homogeneous substance, and as a _material_ but invisible _part_ of a compound. Mercury oxide does not contain two simple bodies, a gas and a metal, but two elements, mercury and oxygen, which, when free, are a gas and a metal. Neither mercury as a metal nor oxygen as a gas is contained in mercury oxide; it only contains the substance of these elements, just as steam only contains the substance of ice, but not ice itself, or as corn contains the substance of the seed, but not the seed itself. The existence of an element may be recognised without knowing it in the uncombined state, but only from an investigation of its combinations, and from the knowledge that it gives, under all possible conditions, substances which are unlike other known combinations of substances. Fluorine is an example of this kind. It was for a long time unknown in a free state, and nevertheless was recognised as an element because its combinations with other elements were known, and their difference from all other similar compound substances was determined. In order to grasp the difference between the conception of the visible form of an element as we know it in the free state, and of the intrinsic element (or 'radicle,' as Lavoisier called it) contained in the visible form, it should be remarked that compound substances also combine together forming yet more complex compounds, and that they evolve heat in the process of combination. The original compound may often be extracted from these new compounds by exactly the same methods as elements are extracted from their corresponding combinations. Besides, many elements exist under various visible forms whilst the intrinsic element contained in these various forms is something which is not subject to change. Thus carbon appears as charcoal, graphite, and diamond, but yet the element carbon alone, contained in each, is one and the same. Carbonic anhydride contains carbon, and not charcoal, or graphite, or the diamond.

Elements alone, although not all of them, have the peculiar lustre, opacity, malleability, and the great heat and electrical conductivity which are proper to metals and their mutual combinations. But elements are far from all being _metals_. Those which do not possess the physical properties of metals are called _non-metals_ (or _metalloids_). It is, however, impossible to draw a strict line of demarcation between metals and non-metals, there being many intermediary substances. Thus graphite, from which pencils are manufactured, is an element with the lustre and other properties of a metal; but charcoal and the diamond, which are composed of the same substance as graphite, do not show any metallic properties. Both classes of elements are clearly distinguished in definite examples, but in particular cases the distinction is not clear and cannot serve as a basis for the exact division of the elements into two groups.

The conception of elements forms the basis of chemical knowledge, and in giving a list of them at the very beginning of our work, we wish to tabulate our present knowledge on the subject. Altogether about seventy elements are now authentically known, but many of them are so rarely met with in nature, and have been obtained in such small quantities, that we possess but a very insufficient knowledge of them. The substances most widely distributed in nature contain a very small number of elements. These elements have been more completely studied than the others, because a greater number of investigators have been able to carry on experiments and observations on them. The elements most widely distributed in nature are:--

Hydrogen, H = 1. In water, and in animal and vegetable
organisms.
Carbon, C = 12. In organisms, coal, limestones.
Nitrogen, N = 14. In air and in organisms.
Oxygen, O = 16. In air, water, earth. It forms the greater
part of the mass of the earth.
Sodium, Na = 23. In common salt and in many minerals.
Magnesium, Mg = 24. In sea-water and in many minerals.
Aluminium, Al = 27. In minerals and clay.
Silicon, Si = 28. In sand, minerals, and clay.
Phosphorus, P = 31. In bones, ashes of plants, and soil.
Sulphur, S = 32. In pyrites, gypsum, and in sea-water.
Chlorine, Cl = 35·5. In common salt, and in the salts of sea-water.
Potassium, K = 39. In minerals, ashes of plants, and in nitre.
Calcium, Ca = 40. In limestones, gypsum, and in organisms.
Iron, Fe = 56. In the earth, iron ores, and in organisms.

Besides these, the following elements, although not very largely distributed in nature, are all more or less well known from their applications to the requirements of everyday life or the arts, either in a free state or in their compounds:--

Lithium, Li = 7. In medicine (Li_{2}CO_{3}), and in photography
(LiBr).
Boron, B = 11. As borax, B_{4}Na_{2}O_{7}, and as boric
anhydride, B_{2}O_{3}.
Fluorine, F = 19. As fluor spar, CaF_{2}, and as hydrofluoric
acid, HF.
Chromium, Cr = 52. As chromic anhydride, CrO_{3}, and potassium
dichromate, K_{2}Cr_{2}O_{7}.
Manganese, Mn = 55. As manganese peroxide, MnO_{2}, and potassium
permanganate, MnKO_{4}.
Cobalt, Co = 59·5 In smalt and blue glass.
Nickel, Ni = 59·5 For electro-plating other metals.
Copper, Cu = 63. The well-known red metal.
Zinc, Zn = 65. Used for the plates of batteries, roofing, &c.
Arsenic, As = 75. White arsenic (poison), As_{2}O_{3}.
Bromine, Br = 80. A brown volatile liquid; sodium bromide, NaBr.
Strontium, Sr = 87. In coloured fires (SrN_{2}O_{6}).
Silver, Ag = 109. The well-known white metal.
Cadmium, Cd = 112. In alloys. Yellow paint (CdS).
Tin, Sn = 119. The well-known metal.
Antimony, Sb = 120. In alloys such as type metal.
Iodine, I = 127. In medicine and photography; free, and as KI.
Barium, Ba = 137. "Permanent white," and as an adulterant in
white lead, and in heavy spar, BaSO_{4}.
Platinum, Pt = 196.}
Gold, Au = 197.}
Mercury, Hg = 200.} Well-known metals.
Lead, Pb = 207.}
Bismuth, Bi = 209. In medicine and fusible alloys.
Uranium, U = 239. In green fluorescent glass.

The compounds of the following metals and semi-metals have fewer applications, but are well known, and are somewhat frequently met with in nature, although in small quantities:--

Beryllium, Be = 9. Palladium, Pd = 107.
Titanium, Ti = 48. Cerium, Ce = 140.
Vanadium, V = 51. Tungsten, W = 184.
Selenium, Se = 79. Osmium, Os = 192.
Zirconium, Zr = 91. Iridium, Ir = 193.
Molybdenum, Mo = 96. Thallium, Tl = 204.

The following rare metals are still more seldom met with in nature, but have been studied somewhat fully:--

Scandium, Sc = 44. Germanium, Ge = 72.
Gallium, Ga = 70. Rubidium, Rb = 86.
Yttrium, Y = 89. Cæsium, Cs = 133.
Niobium, Nb = 94. Lanthanum, La = 138.
Ruthenium, Ru = 102. Didymium, Di = 142.
Rhodium, Rh = 103. Ytterbium, Yb = 173.
Indium, In = 114. Tantalum, Ta = 183.
Tellurium, Te = 125. Thorium, Th = 232.

Besides these 66 elements there have been discovered:--Erbium, Terbium, Samarium, Thullium, Holmium, Mosandrium, Phillipium, and several others. But their properties and combinations, owing to their extreme rarity, are very little known, and even their existence as independent substances[28] is doubtful.

[28] Possibly some of their compounds are compounds of other
already-known elements. Pure and incontestably independent
compounds of these substances are unknown, and some of them have
not even been separated, but are only supposed to exist from the
results of spectroscopic researches. There can be no mention
of such contestable and doubtful elements in a short general
handbook of chemistry.

It has been incontestably proved from observations on the spectra of the heavenly bodies that many of the commoner elements (such as H, Na, Mg, Fe) occur on the far distant stars. This fact confirms the belief that those forms of matter which appear on the earth as elements are widely distributed over the entire universe. But we do not yet know why, in nature, the mass of some elements should be greater than that of others.[28 bis]

[28 bis] Clark in America made an approximate calculation of the amount
of the different elements contained in the earth's crust (to a
depth of 15 kilometres), and found that the chief mass (over 50
per cent.) is composed of oxygen; then comes silicon, &c.; while
the amount of hydrogen is less than 1 per cent., carbon scarcely
0·25 per cent., nitrogen even less than 0·03 per cent. The
relative masses of such metals as Cu, Ni, Au is minute. Judging
from the density (see Chapter VIII.) of the earth, a large
proportion of its mass must be composed of iron.

The capacity of each element to combine with one or another element, and to form compounds with them which are in a greater or less degree prone to give new and yet more complex substances, forms the fundamental character of each element. Thus sulphur easily combines with the metals, oxygen, chlorine, or carbon, whilst gold and silver enter into combinations with difficulty, and form unstable compounds, which are easily decomposed by heat. The cause or force which induces the elements to enter into chemical change must be considered, as also the cause which holds different substances in combination--that is, which endues the substances formed with their particular degree of stability. This cause or force is called _affinity_ (_affinitas_, _affinité_, _Verwandtschaft_), or chemical affinity.[29] Since this force must be regarded as exclusively an attractive force, like gravity, many writers (for instance, Bergmann at the end of the last, and Berthollet at the beginning of this, century) supposed affinity to be essentially similar to the universal force of gravity, from which it only differs in that the latter acts at observable distances whilst affinity only evinces itself at the smallest possible distances. But chemical affinity cannot be entirely identified with the universal attraction of gravity, which acts at appreciable distances and is dependent only on mass and distance, and not on the quality of the material on which it acts, whilst it is by the quality of matter that affinity is most forcibly influenced. Neither can it be entirely identified with cohesion, which gives to homogeneous solid substances their crystalline form, elasticity, hardness, ductility, and other properties, and to liquids their surface tension, drop formation, capillarity, and other properties, because affinity acts between the component parts of a substance and cohesion on a substance in its homogeneity, although both act at imperceptible distances (by contact) and have much in common. Chemical force, which makes one substance penetrate into another, cannot be entirely identified with even those attracting forces which make different substances adhere to each other, or hold together (as when two plane-polished surfaces of solid substances are brought into close contact), or which cause liquids to soak into solids, or adhere to their surfaces, or gases and vapours to condense on the surfaces of solids. These forces must not be confounded with chemical forces, which cause one substance to penetrate into the substance of another and to form a new substance, which is never the case with cohesion. But it is evident that the forces which determine cohesion form a connecting-link between mechanical and chemical forces, because they only act by intimate contact. For a long time, and especially during the first half of this century, chemical attraction and chemical forces were identified with electrical forces. There is certainly an intimate relation between them, for electricity is evolved in chemical reactions, and has also a powerful influence on chemical processes--for instance, compounds are decomposed by the action of an electrical current. And the exactly similar relation which exists between chemical phenomena and the phenomena of heat (heat being developed by chemical phenomena, and heat being able to decompose compounds) only proves the unity of the forces of nature, the capability of one force to produce and to be transformed into others. For this reason the identification of chemical force with electricity will not bear experimental proof.[30] As of all the (molecular) phenomena of nature which act on substances at immeasurably small distances, the phenomena of heat are at present the best (comparatively) known, having been reduced to the simplest fundamental principles of mechanics (of energy, equilibrium, and movement), which, since Newton, have been subjected to strict mathematical analysis, it is quite natural that an effort, which has been particularly pronounced during recent years, should have been made to bring chemical phenomena into strict correlation with the already investigated phenomena of heat, without, however, aiming at any identification of chemical with heat phenomena. The true nature of chemical force is still a secret to us, just as is the nature of the universal force of gravity, and yet without knowing what gravity really is, by applying mechanical conceptions, astronomical phenomena have been subjected not only to exact generalisation but to the detailed prediction of a number of particular facts; and so, also, although the true nature of chemical affinity may be unknown, there is reason to hope for considerable progress in chemical science by applying the laws of mechanics to chemical phenomena by means of the mechanical theory of heat. As yet this portion of chemistry has been but little worked at, and therefore, while forming a current problem of the science, it is treated more fully in that particular field which is termed either 'theoretical' or 'physical' chemistry, or, more correctly, _chemical mechanics_. As this province of chemistry requires a knowledge not only of the various homogeneous substances which have yet been obtained and of the chemical transformations which they undergo, but also of the phenomena (of heat and other kinds) by which these transformations are accompanied, it is only possible to enter on the study of chemical mechanics after an acquaintance with the fundamental chemical conceptions and substances which form the subject of this book.[31]

[29] This word, first introduced, if I mistake not, into chemistry
by Glauber, is based on the idea of the ancient philosophers that
combination can only take place when the substances combining
have something in common--a medium. As is generally the case,
another idea evolved itself in antiquity, and has lived until
now, side by side with the first, to which it is exactly
contradictory; this considers union as dependent on contrast, on
polar difference, on an effort to fill up a want.

[30] Especially conclusive are those cases of so-called metalepsis
(Dumas, Laurent). Chlorine, in combining with hydrogen, forms
a very stable substance called 'hydrochloric acid,' which is
split up by the action of an electrical current into chlorine
and hydrogen, the chlorine appearing at the positive and
the hydrogen at the negative pole. Hence electro-chemists
considered hydrogen to be an electro-positive and chlorine
an electro-negative element, and that they are held together
in virtue of their opposite electrical charges. It appears,
however, from metalepsis, that chlorine can replace hydrogen
(and, inversely, hydrogen can replace chlorine) in its compounds
without in any way changing the grouping of the other elements,
or altering their chief chemical properties. For instance, acetic
acid in which hydrogen has been replaced by chlorine is still
capable of forming salts. It must be observed, whilst considering
this subject, that the explanation suggesting electricity as the
origin of chemical phenomena is unsound, since it attempts to
explain one class of phenomena whose nature is almost unknown by
another class which is no better known. It is most instructive
to remark that together with the electrical theory of chemical
attraction there arose and survives a view which explains the
galvanic current as being a transference of chemical action
through the circuit--_i.e._, regards the origin of electricity as
being a chemical one. It is evident that the connection is very
intimate, although both phenomena are independent and represent
different forms of molecular (atomic) motion, whose real nature
is not yet understood. Nevertheless, the connection between
the phenomena of both categories is not only in itself very
instructive, but it extends the applicability of the general idea
of the unity of the forces of nature, conviction of the truth of
which has held so important a place in the science of the last
ten years.

[31] I consider that in an elementary text-book of chemistry, like the
present, it is only possible and advisable to mention, in
reference to chemical mechanics, a few general ideas and some
particular examples referring more especially to gases, whose
mechanical theory must be regarded as the most complete. The
molecular mechanics of liquids and solids is as yet in embryo,
and contains much that is disputable; for this reason, chemical
mechanics has made less progress in relation to these substances.
It may not be superfluous here to remark, with respect to the
conception of chemical affinity, that up to the present time
gravity, electricity, and heat have all been applied to its
elucidation. Efforts have also been made to introduce the
luminiferous ether into theoretical chemistry, and should that
connection between the phenomena of light and electricity
which was established by Maxwell be worked out more in detail,
doubtless these efforts to elucidate all or a great deal by
the aid of luminiferous ether will again appear in theoretical
chemistry. An independent chemical mechanics of the material
particles of matter, and of their internal (atomic) changes,
would, in my opinion, arise as the result of these efforts.
Two hundred years ago Newton laid the foundation of a truly
scientific theoretical mechanics of external visible motion, and
on this foundation erected the edifice of celestial mechanics.
One hundred years ago Lavoisier arrived at the first fundamental
law of the internal mechanics of invisible particles of matter.
This subject is far from having been developed into a harmonious
whole, because it is much more difficult, and, although many
details have been completely investigated, it does not possess
any starting points. Newton only came after Copernicus and
Kepler, who had discovered empirically the exterior simplicity
of celestial phenomena. Lavoisier and Dalton may, in respect to
the chemical mechanics of the molecular world, be compared to
Copernicus and Kepler. But a Newton has not yet appeared in the
molecular world; when he does, I think that he will find the
fundamental laws of the mechanics of the invisible motions of
matter more easily and more quickly in the chemical structure
of matter than in physical phenomena (of electricity, heat,
and light); for these latter are accomplished by particles of
matter already arranged, whilst it is now clear that the problem
of chemical mechanics mainly lies in the apprehension of those
motions which are invisibly accomplished by the smallest atoms of
matter.

As the chemical changes to which substances are liable proceed from internal forces proper to these substances, as chemical phenomena certainly consist of motions of material parts (from the laws of the indestructibility of matter and of elements), and as the investigation of mechanical and physical phenomena proves the law of the _indestructibility of forces_, or the conservation of energy--that is, the possibility of the transformation of one kind of motion into another (of visible or mechanical into invisible or physical)--we are inevitably obliged to acknowledge the presence in substances (and especially in the elements of which all others are composed) of a store of _chemical energy_ or invisible motion inducing them to enter into combinations. If heat be evolved in a reaction, it means that a portion of chemical energy is transformed into heat;[32] if heat be absorbed in a reaction,[33] that it is partly transformed (rendered latent) into chemical energy. The store of force or energy going to the formation of new compounds may, after several combinations, accomplished with an absorption of heat, at last diminish to such a degree that indifferent compounds will be obtained, although these sometimes, by combining with energetic elements or compounds, give more complex compounds, which may be capable of entering into chemical combination. Among elements, gold, platinum, and nitrogen have but little energy, whilst potassium, oxygen, and chlorine have a very marked degree of energy. When dissimilar substances enter into combination they often form substances of diminished energy. Thus sulphur and potassium when heated easily burn in air, but when combined together their compound is neither inflammable nor burns in air like its component parts. Part of the energy of the potassium and of the sulphur was evolved in their combination in the form of heat. Just as in the passage of substances from one physical state into another a portion of their store of heat is absorbed or evolved, so in combinations or decompositions and in every chemical process, there occurs a change in the store of chemical energy, and at the same time an evolution or absorption of heat.[34]

[32] The theory of heat gave the idea of a store of internal motion
or energy, and therefore with it, it became necessary to
acknowledge chemical energy, but there is no foundation whatever
for identifying heat energy with chemical energy. It may be
supposed, but not positively affirmed, that heat motion is
proper to molecules and chemical motion to atoms, but that as
molecules are made up of atoms, the motion of the one passes to
the other, and that for this reason heat strongly influences
reaction and appears or disappears (is absorbed) in reactions.
These relations, which are apparent and hardly subject to doubt
on general lines, still present much that is doubtful in detail,
because all forms of molecular and atomic motion are able to pass
into each other.

[33] The reactions which take place (at the ordinary or at a high
temperature) directly between substances may be clearly divided
into exothermal, which are accompanied by an evolution of heat,
and endothermal, which are accompanied by an absorption of heat.
It is evident that the latter require a source of heat. They are
determined either by the directly surrounding medium (as in the
formation of carbon bisulphide from charcoal and sulphur, or in
decompositions which take place at high temperatures), or else by
a secondary reaction proceeding simultaneously, or by some other
form of energy (light, electricity). So, for instance, hydrogen
sulphide is decomposed by iodine in the presence of water at the
expense of the heat which is evolved by the solution in water
of the hydrogen iodide produced. This is the reason why this
reaction, as exothermal, only takes place in the presence of
water; otherwise it would be accompanied by a cooling effect. As
in the combination of dissimilar substances, the bonds existing
between the molecules and atoms of the homogeneous substances
have to be broken asunder, whilst in reactions of rearrangement
the formation of any one substance proceeds simultaneously with
the formation of another, and, as in reactions, a series of
physical and mechanical changes take place, it is impossible to
separate the heat directly depending on a given reaction from
the total sum of the observed heat effect. For this reason,
thermochemical data are very complex, and cannot by themselves
give the key to many chemical problems, as it was at first
supposed they might. They ought to form a part of chemical
mechanics, but alone they do not constitute it.

[34] As chemical reactions are effected by heating, so the heat
absorbed by substances before decomposition or change of
state, and called 'specific heat,' goes in many cases to the
preparation, if it may be so expressed, of reaction, even when
the limit of the temperature of reaction is not attained. The
molecules of a substance A, which is not able to react on a
substance B below a temperature _t_, by being heated from a
somewhat lower temperature to _t_, undergoes that change which
had to be arrived at for the formation of A B.

For the comprehension of chemical phenomena as mechanical processes--_i.e._, the study of the _modus operandi_ of chemical phenomena--it is most important to consider: (1) the facts gathered from stoïchiometry, or that part of chemistry which treats of the quantitative relation, by weight or volume, of the reacting substances; (2) the distinction between the different forms and classes of chemical reactions; (3) the study of the changes in properties produced by alteration in composition; (4) the study of the phenomena which accompany chemical transformation; (5) a generalisation of the conditions under which reactions occur. As regards stoïchiometry, this branch of chemistry has been worked out most thoroughly, and comprises laws (of Dalton, Avogadro-Gerhardt, and others) which bear so deeply on all parts of chemistry that at the present time the chief problem of chemical research consists in the application of general stoïchiometrical laws to concrete examples, _i.e._, the quantitative (volumetric or gravimetric) composition of substances. All other branches of chemistry are clearly subordinate to this most important portion of chemical knowledge. Even the very signification of reactions of combination, decomposition, and rearrangement, acquired, as we shall see, a particular and new character under the influence of the progress of exact ideas concerning the quantitative relations of substances entering into chemical changes. Furthermore, in this sense there arose a new--and, till then, unknown--division of compound substances into _definite_ and _indefinite_ compounds. Even at the beginning of this century, Berthollet had not made this distinction. But Prout showed that a number of compounds contain the substances of which they are they break up, in exact definite proportions by weight, which are unalterable under any conditions. Thus, for example, red mercury oxide always contains sixteen parts by weight of oxygen for every 200 parts by weight of mercury, which is expressed by the formula HgO. But in an alloy of copper and silver one or the other metal may be added at will, and in an aqueous solution of sugar, the relative proportion of the sugar and water may be altered and nevertheless a homogeneous whole with the sum of the independent properties will be obtained--_i.e._, in these cases there was indefinite chemical combination. Although in nature and chemical practice the formation of indefinite compounds (such as alloys and solutions) plays as essential a part as the formation of definite chemical compounds, yet, as the stoïchiometrical laws at present apply chiefly to the latter, all facts concerning indefinite compounds suffer from inexactitude, and it is only during recent years that the attention of chemists has been directed to this province of chemistry.

In chemical mechanics it is, from a qualitative point of view, very important to clearly distinguish at the very beginning between _reversible_ and _non-reversible reactions_. Substances capable of reacting on each other at a certain temperature produce substances which at the same temperature either can or cannot give back the original substances. For example, salt dissolves in water at the ordinary temperature, and the solution so obtained is capable of breaking up at the same temperature, leaving salt and separating the water by evaporation. Carbon bisulphide is formed from sulphur and carbon at about the same temperature at which it can be resolved into sulphur and carbon. Iron, at a certain temperature, separates hydrogen from water, forming iron oxide, which, in contact with hydrogen at the same temperature, is able to produce iron and water. It is evident that if two substances, A and B, give two others C and D, and the reaction be reversible, then C and D will form A and B, and, consequently, by taking a definite mass of A and B, or a corresponding mass of C and D, we shall obtain, in each case, all four substances--that is to say, there will be a state of _chemical equilibrium_ between the reacting substances. By increasing the mass of one of the substances we obtain a new condition of equilibrium, so that reversible reactions present a means of studying the _influence of mass_ on the _modus operandi_ of chemical changes. Many of those reactions which occur with very complicated compounds or mixtures may serve as examples of non-reversible reactions. Thus many of the compound substances of animal and vegetable organisms are broken up by heat, but cannot be re-formed from their products of decomposition at any temperature. Gunpowder, as a mixture of sulphur, nitre, and carbon, on being exploded, forms gases from which the original substances cannot be re-formed at any temperature. In order to obtain them, recourse must be had to an indirect method _of combination at the moment of separation_. If A does not under any circumstances combine directly with B, it does not follow that it cannot give a compound A B. For A can often combine with C and B with D, and if C has a great affinity for D, then the reaction of A C or B D produces not only C D, but also A B. As on the formation of C D, the substances A and B (previously in A C and B D) are left in a peculiar state of separation, it is supposed that their mutual combination occurs because they meet together in this _nascent state_ at the moment of separation (_in statu nascendi_). Thus chlorine does not directly combine with charcoal, graphite, or diamond; there are, nevertheless, compounds of chlorine with carbon, and many of them are distinguished by their stability. They are obtained in the action of chlorine on hydrocarbons, as the separation products from the direct action of chlorine on hydrogen. Chlorine takes up the hydrogen, and the carbon liberated at the moment of its separation, enters into combination with another portion of the chlorine, so that in the end the chlorine is combined with both the hydrogen and the carbon.[35]

[35] It is possible to imagine that the cause of a great many of such
reactions is, that substances taken in a separate state, for
instance, charcoal, present a complex molecule composed of
separate atoms of carbon which are fastened together (united, as
is usually said) by a considerable affinity; for atoms of the
same kind, just like atoms of different kinds, possess a mutual
affinity. The affinity of chlorine for carbon, although unable
to break this bond asunder, may be sufficient to form a stable
compound with atoms of carbon, which are already separate. Such
a view of the subject presents a hypothesis which, although
dominant at the present time, is without sufficiently firm
foundation. It is evident, however, that not only does chemical
reaction itself consist of motions, but that in the compound
formed (in the molecules) the elements (atoms) forming it are in
harmonious stable motion (like the planets in the solar system),
and this motion will affect the stability and capacity for
reaction, and therefore the mechanical side of chemical action
must be exceedingly complex. Just as there are solid, physically
constant non-volatile substances like rock, gold, charcoal,
&c., so are there stable and chemically constant bodies; while
corresponding to physically volatile substances there are bodies
like camphor, which are chemically unstable and variable.

As regards those phenomena which accompany chemical action, the most important circumstance in reference to chemical mechanics is that not only do chemical processes produce a mechanical displacement (a motion of particles), heat, light, electrical potential and current; but that all these agents are themselves capable of changing and governing chemical transformations. This reciprocity or reversibility naturally depends on the fact that all the phenomena of nature are only different kinds and forms of visible and invisible (molecular) motions. First sound, and then light, was shown to consist of vibratory motions, as the laws of physics have proved and developed beyond a doubt. The connection between heat and mechanical motion and work has ceased to be a supposition, but has become an accepted fact, and the mechanical equivalent of heat (425 kilogrammetres of mechanical work correspond with one kilogram unit of heat or Calorie) gives a mechanical measure for thermal phenomena. Although the mechanical theory of electrical phenomena cannot be considered so fully developed as the theory of heat, both statical and dynamical electricity are produced by mechanical means (in common electrical machines or in Gramme or other dynamos), and conversely, a current (in electric motors) can produce mechanical motion. Thus by connecting a current with the poles of a Gramme dynamo it may be made to revolve, and, conversely, by rotating it an electrical current is produced, which demonstrates the reversibility of electricity into mechanical motion. Accordingly chemical mechanics must look for the fundamental lines of its advancement in the correlation of chemical with physical and mechanical phenomena. But this subject, owing to its complexity and comparative novelty, has not yet been expressed by a harmonious theory, or even by a satisfactory hypothesis, and therefore we shall avoid lingering over it.

A chemical change in a certain direction is accomplished not only by reason of the difference of masses, the composition of the substances concerned, the distribution of their parts, and their affinity or chemical energy, but also by reason of the _conditions_ under which the substances occur. In order that a certain chemical reaction may take place between substances which are capable of reacting on each other, it is often necessary to have recourse to conditions which are sometimes very different from those in which the substances usually occur in nature. For example, not only is the presence of air (oxygen) necessary for the combustion of charcoal, but the latter must also be heated to redness. The red-hot portion of the charcoal burns--_i.e._ combines with the oxygen of the atmosphere--and in so doing evolves heat, which raises the temperature of the adjacent parts of charcoal, so that they burn. Just as the combustion of charcoal is dependent on its being heated to redness, so also every chemical reaction only takes place under certain physical, mechanical, or other conditions. The following are the chief conditions which exert an influence on the progress of chemical reactions.

(_a_) _Temperature._--Chemical reactions of combination only take place within certain definite limits of temperature, and cannot be accomplished outside these limits. We may cite as examples not only that the combustion of charcoal begins at a red heat, but also that chlorine and salt only combine with water at a temperature below 0°. These compounds cannot be formed at a higher temperature, for they are then wholly or partially broken up into their component parts. A certain rise in temperature is necessary to start combustion. In certain cases the effect of this rise may be explained as causing one of the reacting bodies to change from a solid into a liquid or gaseous form. The transference into a fluid form facilitates the progress of the reaction, because it aids the intimate contact of the particles reacting on each other. Another reason, and to this must be ascribed the chief influence of heat in exciting chemical action, is that the physical cohesion, or the internal chemical union, of homogeneous particles is thereby weakened, and in this way the separation of the particles of the original substances, and their transference into new compounds, is rendered easier. When a reaction absorbs heat--as in decomposition--the reason why heat is necessary is self-evident.