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CHEMICAL AFFINITY. 38. As there is of necessity a much greater number of compound than of simple chemical substances, one of the earliest things which the chemist has to consider is the nature of those compounds, and the means by which they are to be distinguished from their elements, from each other, and from mechanical mixtures. We offer here, accordingly, some preliminary observations on those points. 39. Every chemical compound consists of at least two elements, although, as we have seen already, it may contain many more. For simplicity's sake, however, we shall confine ourselves in the following remarks to a compound of only two elements, and select, in the first place, water as the example. When the gases oxygen and hydrogen are mingled together, they become uniformly diffused through each other, but no chemical combination occurs between them. If, however, a light be applied to the mouth of a vessel filled with the gases mixed in certain proportions, or an electric spark be sent through the mixture, great evolution of heat and light occurs, a sharp report or explosion is heard, and the gases become changed into steam, which, as it cools, condenses into water. The oxygen and hydrogen are now said to have entered into chemical combination, and to have formed water. The smallest conceivable quantity, or minutest particle of water, is supposed to consist of one molecule or atom of hydrogen, and one of oxygen, not fused or run into one larger particle, as two drops of a liquid metal like quicksilver flow into a bigger drop, but consisting of two distinct and unlike particles, Kept near each other by a peculiar constraining force, to which the name has been given of chemicalaffinity. So long as the particles of oxygen and hydrogen are under the influence of this power, the...
