MOLECULAR MIXTURES 239 



and, of course, increasing the height of the column. 

 An equilibrium position is finally reached when, for a 

 further increase, the work done by the solutions would 

 be less than the corresponding increase in gravitational 

 potential energy of the column of liquid. The osmotic 

 pressure is then taken as equal to that of the column 

 of liquid. 



In a paper before the Swedish Royal Academy in 

 1885, Van't Hoff advanced the explanation of osmosis 

 as due to the kinetics of the molecules themselves. He 

 found confirmation of this theory not only in the 

 experimental facts that osmotic pressure does vary 

 directly as the absolute temperature and as the concen- 

 tration of the solution, in other words in the same 

 way as gaseous pressure, but also in previous experi- 

 ments on the effect of dissolved substances in lowering 

 the freezing point and in raising the boiling point of 

 solutions. 



As to the last of these phenomena, we have seen from 

 the isothermals of page 231 that for any pressure there 

 is a boiling-point tem- 

 perature, namely that at 

 which the pressure of the 

 saturated vapor of the 

 liquid equals the external 

 pressure. The relation of 

 boiling-point and pres- 

 sure is illustrated for FIG 29 

 water in Fig. 29. Let 



us now see how this temperature is altered by mixing 

 with the liquid some other substance. The case of 

 immediate interest to us in connection with osmosis 



p 



