Page:Outlines of Physical Chemistry - 1899.djvu/238

 220 OUTLINES OF PHYSICAL CHEMISTRY

substance would exercise if it were in the gaseous condition (van't Hoff's law).

The interpretation which has just been given leaves much to be desired from the quantitative point of view (Nernst). We must not forget that the dissolved mole- cules are under an enormous pressure ( p + ~ K and that

consequently their external pressure, i.e. the osmotic pres- sure, could not be predicted from the general equation of a gas but could be from van der WaaWs equation. But according to this latter we should require to find pres- This forms a serious objection, and the problem seems not yet to be definitely solved.

II. As a sequel to van't Hoff's interpretation I venture to bring forward another which does not require the kinetic theory of liquids and which is founded only on some ideas already old (perhaps even somewhat anti- quated ?). If we assume that, in an aqueous solution, the dissolved substance is in the liquid state, and that the attraction attains its greatest value when acting between molecules of different natures, we may reason as follows.

In fig. 51 a and a' represent two dissolved molecules, and r is the radius of their spheres of molecular attraction.

A molecule b of solvent will be attracted both by a and a', and the resultant will cause b to approach the line joining a and a'.

By the united action of many molecules of solvent a force will be established which will separate the two dissolved mole- cules until the distance between them is 2r. The spheres

1 In the last edition (1898) of NernsVs « Theoretische Chemie' this criticism is omitted.

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