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which changes form during a chemical change is degraded : some of it is degraded, but some of it may be raised to a more available form.

In chapters XII. and xIII. we saw that many chemical 269 changes may be justly regarded as proceeding in two directions simultaneously, and that equilibrium results when the velocities of the direct and reverse changes become equal. We saw also that such chemical equilibrium may generally be overthrown by changing the temperature, or sometimes the pressure, of the system, or the relative masses of the interacting substances. The considerations concerning the relations of energy-changes and chemical changes shortly developed in the present chapter may be applied to the conception of chemical equilibrium gained in chaps. XII. and xIII. Suppose that the masses of ferric chloride and potassium sulphocyanide shewn by the formulae Fe, Cl, and KCN S6 (=6KCNS) are mixed in dilute aqueous solution; the system is in a condition in which chemical change can occur; chemical change occurs, and a system is produced the composition of which may be represented by the equation Fe,cl, Aq+6KCNSAq + x Fe Cl. Aq

+ KCNSAq=Fe (CNS),Aq+6KCIAq + « Fe ClAq + x KCNSAq (comp. Chap. XII. par. 238). Some energy is degraded in this change; a portion of this energy appears as heat, a portion is probably employed in effecting some of the physical changes (contraction or expansion of volume &c.) which accompany the chemical change; a portion of the energy degraded in one part of the change is also probably employed in bringing the products of the change into a state in which they can interact to reproduce the original substances. After a very short time the system settles down into a state in which there is equilibrium of energy and of chemical distribution of the interacting substances. A little more potassium sulphocyanide is now added; chemical change again occurs; energy is degraded ; and after a short time equilibrium is established. Potassium sulphocyanide is added little by little until the whole of the ferric chloride originally present has been changed; the addition of more sulphocyanide cannot now cause chemical change; no more energy can be degraded by chemical processes; the system has reached its state of final equilibrium. Each addition of potassium sulphocyanide disturbed the equilibrium of the energies of the system, and this disturbance was attended by chemical change ; but a disturbance of the equilibrium

of the energies of the system would also be produced by raising the temperature of the system ; hence raising the temperature of the system would also alter the distribution of the elements forming the members of the system, i.e. would cause chemical change to occur.

Now consider a chemical change, one of the products of which is a solid under the conditions of the experiment. Suppose aqueous solutions of barium chloride and sodium sulphate to be mixed in the ratio BaCl, : Na Sox Chemical change occurs ; energy is degraded ; there is change from liquid to solid, and more energy is degraded. As one of the products of the chemical change (barium sulphate) is removed from the sphere of action, by precipitation in the solid form, none of the energy degraded in the direct change can be used to bring the products of this change into a state in which they can chemically react to reproduce the original substances; hence the whole, or at any rate nearly the whole, of the energy degraded to the form of heat passes out of the system. The system is, so to speak, rapidly rolling down hill. Chemical change proceeds until the whole of the energy which can be degraded to heat has been degraded. The system is now in its final state of equilibrium. And this final state has been reached without adding an excess of either of the interacting substances.

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We have now gained some fairly clear conceptions regarding chemical change.

Elements and compounds interact to produce other elements and compounds. Numbers are given to the elements expressing the masses of them which combine or interact with unit mass of one element chosen as a standard, and which also interact or combine with each other. These numbers we have called the combining weights of the elements. Numbers are also given to compounds which express the smallest masses of them which chemically interact with each other. These numbers we have called the reacting weights of the compounds. But it is necessary in chemistry to have regard not only to composition but also to properties. Elements are classified in accordance with their properties into metallic or positive, and non-metallic or negative, elements. They are also classified in groups in accordance with the properties and compositions of their oxides, hydrides, haloid and oxyhaloid compounds, &c. This classification of elements carries with it a classification of compounds also. Compounds are classified in accordance with their properties into acids, bases, salts, &c. But with the properties connoted by each of these terms there is associated a certain composition. The term acid, for instance, implies certain common properties, and a certain common composition.

But chemistry is not content with finding an answer to the question—What is produced in this process and how much of it is produced ? it seeks to find an answer to this question also—How is it produced? Chemistry therefore examines the conditions and general laws of the interactions of elements and compounds. One substance interacts with another to produce new substances; but the new substances also interact, unless they are prevented by the removal of one or more of them from the sphere of action, and tend to reproduce the original substances. Chemical change results in chemical equilibrium. Each substance taking part in a chemical change, wherein all the substances are free to act and react, probably produces a certain definite and measurable effect on the change, which effect is independent of the interactions of the other substances. In certain classes of changes at any rate it is possible to assign to each of the two primarily interacting substances a definite number, a knowledge of which enables us to predict the amount of change which will occur under defined conditions. Chemical change is accompanied by change of energy; there is a redistribution of the matter which undergoes change and also of the energies of the parts of the changing system.

Although we have thus gained some fairly clear conception 271 of the general character of chemical change, and of the kind of phenomena studied in chemistry, yet we stand greatly in want of a general theory which shall bring the facts together and bind them into a connected whole. There is a theory which to a great extent does this. This theory we must now endeavour to understand.

M. E. C.

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CHAPTER XV.

THE MOLECULAR AND ATOMIC THEORY.

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THE Greek philosophers Leucippus and Democritus (about 440—400 B.C.) were among the first to give definite shape to the conception that “the bodies which we see and handle, which we can set in motion or leave at rest, which we can break in pieces and destroy, are composed of smaller bodies, which we cannot see or handle, which are always in motion, and which can neither be stopped, nor broken in pieces, nor in any way destroyed or deprived of the least of their properties (Clerk Maxwell). This doctrine was developed by Epicurus (340—270 B.C.). In the poem De Rerum Natura, Lucretius gives what purports to be an account of the teaching of Epicurus on the subject. The conception of atoms is fully elucidated in this poem, and on it is based a theory of the physical universe, and to some extent also a theory of things moral and spiritual. Lucretius says that nothing exists except atoms and empty space, that the atoms are of different forms and different weights, and that the number of atoms of each form is infinite; that the atoms are in constant motion, and that all change consists in the separation and combination of atoms. According to Lucretius, every atom is indestructible, and its motion is indestructible likewise. Atoms unite to form different kinds of substances; the properties of the substances so formed depend on the mutual relations of the atoms—it matters much with what others and in what positions the atoms of things are held in union, and what motions they mutually impart and receive."*

Nearly complete as it was in many respects, the Lucretian theory failed as a scientific conception; it did not work. It

* Lucretius, De Rerum Natura, 11. 1007—9 (Munro's translation).

did not admit of accurate applications to the facts of nature. It was not a science-producing theory, but rather a speculation about the possible causes of natural events.

The teachings of the atomists were opposed by the follow- 273 ers of Aristotle, for whom the names of things were as real or more real than the things themselves. As istotelianism prevailed during the middle ages, atomism declined. The atomic theory of the Greek philosophers was revived towards the end of the 16th century by Gassendi. Boyle and Newton were upholders of this theory. Newton's demonstration of the action of the force of gravitation made a science of atomic physics possible; but the great difficulty was, and still is, to form a clear image of the action of gravitation in terms of the atomic conception of the structure of matter.

Not much was done to advance the applications of the 274 atomic theory, after Newton, until in the early years of the present century Dalton made a serious attempt to determine the conditions under which the atoms of elementary bodies unite to form the atoms of compound bodies. Dalton said that it is possible to find the relative weights of the atoms of elements and compounds, and he indicated the method by which this could be done.

Dalton found that the mass of hydrogen which combined 275 with carbon to form a certain compound of these elements was twice as great as the mass of hydrogen which combined with the same quantity of carbon to form another compound of these elements. He found also that a specified mass of carbon combined with a certain mass of oxygen to form one oxide of carbon, and with twice that mass of oxygen to form another oxide of carbon. He noticed similar regularities in the masses of oxygen which combined with a fixed mass of nitrogen. Meanwhile Dalton, led thereto by his physical experiments on the absorption of different gases by water, had been thinking a great deal about the ultimate structure of matter. He pictured to himself a quantity of carbonic acid gas as built up of innumerable minute particles, or atoms, each of which was itself composed of one atom of carbon and two atoms of oxygen; a quantity of nitrous oxide gas as built up of a vast number of atoms, each of which was itself composed of yet smaller portions of matter, viz. of one atom of nitrogen and one atom of oxygen ; and a quantity of hydrogen gas as built up of minute particles, which were single atoms each composed only of hydrogen. Fig. 20, copied from the original in the

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