218 OUTLINES OF PHYSICAL CHEMISTRY
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thus possess all the freedom of movement which is required by the electrolytic phenomena and attributed by Arrhdnius to the free ions. Electrolysis itself and that function which we have termed the coefficient of conduc- tivity (or speed of the ions) are equally well explained by the two hypotheses. These hypotheses differ only in one point : according to one the ions are atomically free, whilst in the other theory the freedom of the ions is caused by the dissociating action of the water, and by the frequent passages which the ions make from one molecule to another. We thus explain the special rdle played by the water, 1 and as we do not assume any great disintegra- tion, we have the advantage of not coming into conflict with thermo-chemical facts.
��The Mechanism of the Osmotic Pbessure {
I. We shall first take up the theory of vartt Hoff 9 as it appears in the interchange of views on the part of this scientist and L. Meyer (' Zeitschrift f. physik. Chem.' and ' Berichte der deut„ chem. Gesell.,' 1890).
In pure water the molecules H 2 must be in equili- brium with the surface tension of the liquid and the ex- ternal pressure. In a solution, on the contrary, the water only occupies a part of the total volume (or, is in a state of expansion) ; it has, therefore, only to bear a part of the supported pressure, the dissolved substance bearing the other part.
In the determination of an osmotic pressure, therefore, the molecular tension of the water is not the same on both sides of the semipermeable wall, but is greater on the outer side (pure water) than on the inner (solution). The result of this is that the solvent filters in from the outside
��1 We shall later have occasion to speak of the dissociating action of other solvents, but we may remark that water is the dissociating liquid par excellence.
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