Causes of Osmotic Pressure. 4U7 



becomes less when a molecule of the solid is surrounded by 

 those of the liquid than by its own sort, the conditions are 

 favourable for the escape of the surface-molecules of solid, 

 that is to say, the solid dissolves in the liquid and its molecules 

 share the relative freedom of those of the liquid. Now the 

 vapour-pressure of most solids at ordinary temperature is so 

 low that it has not been measured, that is to say, that the 

 molecular attraction between their molecules is so great that 

 very few can retain their freedom ; for instance, if common 

 salt were vaporized by heat and then suddenly cooled to 

 ordinary temperature, the molecules would capture one 

 another so thoroughly that almost a vacuum would be created 

 by the precipitation of the solid salt from the space filled 

 with vapour. If, then, we try to picture what occurs at the 

 surface of a solution where the solvent is free to evaporate 

 into a region that ultimately becomes saturated, we must 

 imagine the molecules of solute to be behaving practically 

 as kinetic substitutes for those of the solvent which they 

 displace, and participating with them in chances favourable 

 to escape at the surface, Now, in the vapour the molecules 

 of solvent that approach one another have speed enough also 

 to escape from one another, whereas the strongly attracting 

 molecules of solute freely capture one another (or possibly 

 sometimes the molecules of solvent) with immediate entangle- 

 ment and prompt return to the liquid mass, so that the 

 region of evaporation and condensation of the solute is a very 

 limited one near the free surface. If there are n molecules 

 of solute to Nof solvent, then the difference between a surface 

 of the pure solvent and one of the solution is that for every 

 h + N molecules of solvent liberated at the pure surface, there 

 are N of solvent and n of solute set free at the surface of the 

 solution; the latter capture one another very promptly, just 

 as they would do if present alone, and return to the solution, 

 while the N solvent molecules pass on to the region of vapour. 

 Thus equilibrium at the two surfaces will be maintained in 

 the same manner, namely, when for every N-f-n molecules 

 that leave either, N + n return; but in the case of the pure 

 solvent the vapour has to return N + n molecules for every N 

 returned by the vapour above the solution, that is to say, the 

 vapour- pressure over the solution is to be less than that over 

 the pure solvent in the ratio of N to N + n, which of course is 

 the fundamental experimental law whereby molecular masses 

 are determined from lowering of vapour-pressure*. 



[* An essentially similar explanation of the effect of dissolved sub- 

 stances in lowering' the vapour-pressure of solvents was given by Mr. F. 

 (1. Donnan in this Magazine, vol. xxxiv. pp. 411-414 (189:). — Ed., 

 Phil. Mag.] 



