Page:Outlines of Physical Chemistry - 1899.djvu/242

 224 OUTLINES OF PHYSICAL CHEMISTRY

��FOUETH PAET

��CHEMICAL MECHANICS

Towards the end of last century the Swedish chemist Bergmann proposed a theory of the mode in which chemi- cal forces act. In few words his doctrine was : — the chemical affinity of a substance manifests itself with an invariable intensity which may be expressed by a number. The reaction ac + b = ab + c goes on to completion, if the affinity of a is greater for b than it is for c.

The ideas promulgated by Berthollet in his * Essai de statique chimique ' (1808) were quite different. According to him, the state of equilibrium resulting from a reaction depends first of all on the active masses of the substances, so that in a given system the determining action of each substance is proportional to the number of equivalents of it present.

If to a solution of one equivalent of sulphuric acid and one equivalent of nitric acid a solution of one equiva- lent of caustic soda were added, each of the acids would take up part of the base and each would be half neutralised. If the proportion of sulphuric acid were doubled, then after the addition of one equivalent of caustic soda there would be four-thirds of an equivalent of sulphuric acid and two- thirds of an equivalent of nitric acid still unsaturated.

Berthollet also foresaw that the normal division as de- termined by the active masses might be more or less modi- fied under the influence of the state of condensation of the

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