222 OUTLINES OF PHYSICAL CHEMISTRY
we only consider the attractions to which it is subjected by other molecules of water ; there is, however, another force acting on this molecule, namely, the attraction of the near- est molecule a, and as this is not counterbalanced the mole- cule b is drawn towards the solution.
A continuous oozing of Solvent through the wall towards the interior of the osmotic cell is thus established. The penetrating or entering water is at first attracted by the molecules a nearest the wall ; it does not, however, re- main with them but diffuses through the whole, for the equilibrium requires the equidistance of the dissolved molecules.
This is the qualitative interpretation of the phenomenon. It is not so easy to follow from the quantitative point of view, yet we may try to do so, starting with a theoretical deduction which we have already acquired. We have shown that in a solution the dissolved molecules tend to i separate from each other up .to what might be termed the gaseous distance, that is to say, the molecular attractions (or repulsions) tend towards a final effect whether it is a matter of dissolved or vaporised substance. If we could assume that equal active forces were required for these two modes of disintegration, 1 then the increase in volume of the same quantity of substance would in the two cases stop before the same obstacle. In other words, a pressure capable of limiting the expansion of the vaporised substance would suffice equally well to hinder (in osmotic pressure ex- periments) the increase in volume of the dissolved substance by the entrance of new molecules of solvent. We see at once that this conclusion is only another way of stating van't Hoff's law.
The molecular attraction, which we assume to explain even solution, suffices equally to interpret the phenomena of osmosis. Moreover, by taking this point of view we can
��1 Supposing, of course, that the two processes could be carried out at the same temperature.
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