matter. A complete or relative insolubility is favourable to the predominating formation of a substance. Thus the addition of sulphuric acid to a solution of barium chloride gives rise to the formation of the greatest possible quantity of barium sulphate. The division of the base between the acids takes place normally ; but as the sulphate is preci- pitated it is withdrawn from the system and the equilibrium is disturbed. A renewal of the reaction takes place with formation of a fresh quantity of insoluble salt ; and this goes on until either the sulphuric acid or the barium is completely precipitated.
In an analogous manner, when a volatile substance is formed the equilibrium only becomes stable after the com- plete elimination of the gaseous product.
The ideas of Berthollet were ingenious ; they tended to liken the effects of affinity to those of gravitation, and to bring the laws regulating chemical actions within the sphere of mechanics. 1 These ideas can still be traced in modern works on chemistry, at least in the special cases relating to precipitation and gas evolution. In their general form, however, Berthollet* s theories do not agree with observed facts.
In 1867 the Norwegian chemists Guldberg and Waage improved Berthollet' s system by introducing a new factor. The intensity with which two substances tend to react with each other can be expressed by the product kp q 9 p and q denoting the active masses, that is, the number of equivalents by which the substances are represented in unit volume, and k an affinity coefficient.
In order to understand more clearly what this coeffi- cient means let us see under what conditions two sub-
1 In applying these theories of chemical combination Berthollet came to the conclusion that two elements might unite in any pro- portions whatever and thus form an infinite variety of substances. He knew, for instance, quite a series of mercury oxides. — Proust con- tradicted him on this point, and the result of the extremely courteous discussion between the two scientists is our second law of chemical combination — the law of constant proportions.
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