Callendar is to trace the effect of possible combination of molecules of solute with molecules of the solvent.
If each molecule of the solute combines with a certain number of molecules of the solvent in such a way as to render them inactive for evaporation, we get a lowering of vapour pressure.
The solubilities of solids may be expressed in terms of the mass of solute which will dissolve in loo grammes of water.
The vapour pressure of the solution of a non-volatile solute is less than the vapour pressure of the pure solvent.
There must, then, be a relation between the rate of change of the concentration and the osmotic pressure gradient, and thus we may consider the osmotic pressure gradient as a force driving the solute through a viscous medium.