can in nowise be distinguished from the natural one); and immense numbers of substances unknown in nature are being yearly discovered.

§ 10. Since the elements separated from a compound body on its decomposition differ from one another and from the original substance, it follows that when elements are made to reunite so as to form the original body, they, in doing so, must lose their characters in such a way that the resulting body differs from them all. Such union is called chemical union or chemical combination.

Difference between Chemical Union and Mechanical Mixture.

§ 11. Two solid substances, such as sand and sugar, may be mixed mechanically to any degree of intimacy. But

(a) By careful examination the particles of sand may be distinguished lying amongst those of the sugar;

(b) The colour, taste, density, &c. of the mixture are intermediate between those of the sugar and sand; and

(c) The sugar may be completely separated from the sand by dissolving the former out with water.

Again, the two substances water and salt may be mixed, the salt being dissolved in the water. Then

(d) No examination can show the particles of salt separate from those of the water. But

(e) The taste, density, &c. of the mixture is intermediate between those of the salt and of the water; and

(ƒ) The salt can be entirely freed from the water by boiling the latter away.

Finally, alcohol and oil of turpentine may be mixed. Then (g) The taste, density, &c. of the mixture are between those of its constituents; and

(h) The alcohol can be separated from the oil of turpentine by the addition of water.

§ 12. The sugar and sand, the salt and water, and the alcohol and oil of turpentine are said in such cases to be mechanically or, more properly, physically mixed with one another. The marks

of physical mixture are then, briefly, the intermediate nature of the compound between those of its constituents, and the facility with which it may be separated into them*.

§ 13. Further, when bodies are simply physically mixed, there is either no change of temperature at all, or generally an absorption of heat ($$ 98, 99, I.).

§ 14. If mixture takes place at all between two bodies, it may take place in an infinite number of proportions, though not necessarily in all proportions. Thus sand and sugar may be pounded together in all proportions. Alcohol and oil of turpentine may be mixed in all proportions. Salt and water may be mixed in an infinite number of proportions, from the weakest to the strongest brine; but not in all proportions.

§ 15. Elements or compound bodies, when in contact with one another, undergo under some circumstances a very much greater change than that brought about by their physical mixture. Thus, if the solid element carbon be heated in contact with the gaseous element oxygen, or in common air (which contains oxygen), the carbon gradually disappears, and a gas is formed resembling airthat is, colourless and transparent, having no smell, and only a feeble taste. The wasting away of the carbon is known in common language as its burning or combustion. It is accompanied by the liberation of heat and of light. Not only are the constituent elements, carbon and oxygen, of the new substance so intimately blended as not to be distinguishable from one another, but they cannot be separated by any physical means. The new gas has some resemblance to oxygen; but it instantly suffocates animals immersed in it, and extinguishes burning bodies. It has no resemblance whatever to carbon. This and similar changes are instances of chemical union between the elements concerned. The two elements are united chemically; chemical synthesis has been performed; a chemical compound has been produced.

*If the bodies mixed are identical in nature, as when a pint of water is mixed with a quart of water, they cannot by any means be resolved into their original parts; nor could it be ascertained whether this had been effected or not.

§ 16. Similarly, if the two elements sulphur and iron be heated together, they get red-hot, shrink, and a black, porous body results, which resembles neither iron nor sulphur; for it has very few properties of either. It is a chemical compound of the two elements. It can only be melted at a high temperature, is destitute of the metallic lustre, is soft and friable, is scarcely at all attracted by the magnet, and resembles outwardly a black

stone.

§ 17. Bearing in mind the difference in physical peculiarity which is almost always brought about by chemical union, there is little difficulty in admitting that the innumerable varieties of compound matter met with in nature are the result of the chemical union of two or more of the elements already enumerated (§ 6). That such is indeed the case is absolutely proved by the possibility of decomposing all known compound bodies into two or more of the elements already known.

§ 18. Further, if in the first instance of chemical union given (§ 15) the carbon is weighed before it is burnt, and if it is burnt in a great abundance of oxygen, then the weight of the new gas formed is exactly and invariably 3.66 times as much as the weight of the carbon burnt. But if the carbon be greatly in excess of the oxygen employed, the result will be a gas weighing only 2.33 times as heavy as the carbon consumed, and having very different properties from the gas got with excess of oxygen.

§ 19. Thus if 10 lbs. of carbon is burnt in both cases, the gas got in the first case will weigh 366 lbs., in the second only 23.3. Since what is not carbon in the gas is oxygen, it follows that the gases are composed as follows::

That is to say, for the same quantity of carbon the second gas contains exactly half as much oxygen as the first. If, now,

the

second gas be heated with an excess of oxygen, it combines with again as much, and becomes converted into the first gas. And half the oxygen may be extracted from the first gas, whereby it is converted into the second. These two gases have very different properties, and may be easily separated from one another. If a quantity, say, 10 lbs. of carbon, be heated with more than 13.3 and less than 26-6 of oxygen, some of each kind of gas

10 12

10 12

will be formed. But = and == 13.3 16 26.6 32

exactly.

§ 20. Hence it appears that there are two chemical compounds possible between carbon and oxygen, different in nature according to the proportion between the two elements, and that if there be the same quantity of carbon, the oxygen of the one bears a very simple relation to the oxygen of the other; namely,-

§ 21. It is found that whenever oxygen and carbon unite chemically, whether alone or in presence of other elements, the proportion between the carbon and the oxygen is invariably that of 12, or some multiple of 12, to 16, or some multiple of 16.

§ 22. In the second instance of chemical union given (§ 16), according to the quantities of iron or of sulphur present, chemical compounds of iron with sulphur are formed, differing from one another, not of course in the nature of, but in the proportion between the elements. Thus the following bodies are known:

All these bodies contain the iron and sulphur in the proportion of 56 to 32, or some multiple of 56 to some multiple of 32.

§ 23. The main differences between a chemical compound and a mixture are then as follows::

§ 24. In order to explain the reason why elements should unite with one another only in definite proportions, recourse is had to an hypothesis or theory, called the Atomic Theory. It is supposed that every element consists of exceedingly small particles or atoms, of unknown shape, size, distance from one another, and absolute weight, but all in the same element precisely alike. Each of these atoms is indivisible and unalterable. In solids and liquids they are held together by a force called cohesion; in gases this force does not exist. Though the actual weight of a single atom of any one element is unknown, the relative weights of the atoms of two elements may be known, provided we know the relative number of atoms of the elements which unite to form a