74 A MODERN ZOEOASTRIAK.
—carbon, hydrogen, and oxygen—united in the same proportions. This applies to a number of substances, and is called Isomerism, or formation of different wholes from the same parts.
The principle of polarity, therefore, aided by the subsidiary conditions of quantivalence, atomicity, and isomerism, gives the clue to the construction of the inorganic world out of some seventy elementary substances. Of the substances thus formed, whether of molecules or of combinations of molecules, some are stable and some unstable. As a rule the simpler combinations are the most stable, and instability increases with complexity. Thus the diamond, which is merely a crystal of pure carbon, is very hard and indestructible; while dynamite, or nitro-glycerine, which is a very complex compound, explodes at a touch.
The stability of a substance depends partly on the stable structure of its component elements, and partly on their mutual affinity being strong enough to keep them together in presence of the attractions of other outside elements, which, in the case of most natural substances at the surface of the earth, consist principally of air and water. Thus, the rocks, earths, metallic oxides, water, carbonic dioxide, and nitrogen are extremely stable, and resist decomposition, or chemical union with other substances, with great energy. With regard to all substances this law holds good, that the tendency is to fall back from a less stable to a more stable condition, and that such a falling back is always attended with an evolution of heat; while, on the other hand, heat is always absorbed and disappears whenever the elements of a more stable substance are made to enter into a fess stable condition. Thus, when wood