Page:Outlines of Physical Chemistry - 1899.djvu/237

 until the partial tension of the water in the solution has become equal to the complete tension of the pure water outside. The cell thus becomes the seat of an increase of pressure which can be measured on the manometer, and which is solely due to the partial tension of the dissolved substance.

This interpretation recalls the laws of mixture and diffusion of gases. 1 It depends, furthermore, on the kinetic theory of the liquid state, 2 and attributes to the dissolved (and consequently liquid) molecules the kinetic energy which they would have in the gaseous state. For otherwise we could not understand that the osmotic pressure, caused solely by the partial tension of the dissolved substance, is equal to the pressure which this

1 The reader has only to think what the result of the following experiment would be. A cell with rigid walls permeable to hydrogen only is filled with a mixture of hydrogen and nitrogen, and is im- mersed in an atmosphere of pure hydrogen. We suppose, of course, that the gaseous mixture and the pure hydrogen are under the same pressure. — The result would be that hydrogen would penetrate into the cell and there cause an increase of pressure equal to the initial partial pressure of the nitrogen.

2 It will not be out of place to give here some brief indications about this theory. The liquid and the gaseous states are connected by a perfect transition (the critical state), and are governed by the same equation (that of van der Wools). There is, therefore, no qualitative difference between these two states. A liquid is simply a very concentrated gas ; only a small fraction (p) of its enormous

internal pressure ( p + — j becomes externally manifest, whilst the

rest (^ j is equalised by molecular attraction and notably by the

surface tension. — When we compress a gaseous mass we diminish the distance between its molecules and thus favour molecular attrac- tion. By applying a sufficiently high pressure we may even make this distance so small that the mutual attraction of the molecules outweighs the kinetic energy of translation — that is to say, that the liquid state becomes realised. The possibility of this transformation depends not only on the pressure applied, but also on the tem- perature, for this conditions the kinetic energy of the molecules

(~ )• This energy is, moreover, independent of the degree of condensation of a substance.

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