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 less stable condition. Thus, when wood burns, there is a falling back from a substance unstable, on account of its affinity for the oxygen in the air, into the stable products, carbonic dioxide and water, and the heat evolved is the effect of this fall. As the tendency of all changes is towards stability we arrive at the following law, which is one of the most recent generalisations of modern chemistry: In all cases of chemical change the tendency is to those products whose formation will determine the greatest evolution of heat. This, however, does not imply that the tendency may not be overcome and unstable products formed, for just as a weight may be lifted against the force of gravity, so may the chemical tendency be overcome by a sufficient energy acting against it. Heat is the principal means of supplying this energy, and by increasing it sufficiently not only are molecules drawn apart and most solids converted into fluids and finally into gases, but there is reason to believe that at extremely high temperatures, such as may prevail in the sun, all matter would be resolved into isolated or dissociated atoms. Thus, water at a temperature of 1,200° is resolved into a mixture of oxygen and hydrogen atoms no longer chemically united into water-molecules; and iodine-vapour, which below 700° degrees consists of molecules of two atoms, above that temperature consists of single atoms only.