440 Dr. F. Tinker on Osmotic Pressure. 



From equations [15] and [16] we can derive three 

 possible cases : — 



(i.) If both of the dilute solutions are ideal and have 

 zero heat of dilution, then p l "=p l ! and pi'=P\"- 

 Two ideal solutions of equal concentration are thus 

 in osmotic equilibrium with one another, and there 

 will be no osmotic flow from the one to the other. 



(ii.) If one solution is ideal and the other is not, the 

 system is not in equilibrium, although the concen- 

 trations of the two solutions may be equal. The flow 

 will be from the ideal solution to the non -ideal 

 solution, according as to the heat of dilution of the 

 non-ideal solution is positive or negative. 



(iii.) If both solutions are non-ideal, osmotic equilibrium 

 is possible only when the two heats of dilution are 

 equal. In any other case osmotic flow will take place 

 from the solution having the lesser heat dilution to 

 the solution having the greater. 



By virtue of the relationships which have been established 

 in the foregoing pages between the heat of dilution and the 

 heat of vaporization, either into the membrane or into the 

 vapour phase proper, the above case of osmotic flow for 

 non-ideal solutions can be elaborated in somewhat more 

 detail. 



Thus, since Q = ^A ] =^B 1 (Joe. cit. p. 12), we can write 

 Q"-Q'=A 1 "-A 1 '=-.B 1 "-B,'. 



Hence, we may also state that the osmotic flow will take 

 place from the solution having the lower value of A x ; to 

 that having the higher value of A/. Since A/ is a function 

 of the intrinsic pressure w r hich increases with the latter *, 

 and consequently also with the surface-tension f, we arrive 

 at the further result that the flow will take place from the 

 solution having the lower intrinsic pressure and surface- 

 tension to that having the higher intrinsic pressure and 

 surface-tension. Herein comes the application of the 

 " intrinsic pressure " and surface-tension theory of osmosis 

 which I. Traube has advocated so vigorously on empirical 

 grounds J. The preceding analysis shows that the theory 



* This is an immediate deduction from the Laplace theory of capillarity 

 It is in agreement with experiment also, since liquids which have a 

 high intrinsic pressure have also a large latent heat. {Cf. W. C. McC. 

 Lewis, Trans. Faraday Soc. April 1911.) 



f A liquid which has a high intrinsic pressure and latent heat has 

 also a high surface-tension. 



J Journ. Phys. Chem. xiv. p. 452 (1910) and other papers. Trauhe has 

 correlated surface-tension, vapour pressure, intrinsic pressure, &c\, with 

 their effect on osmotic flow. 



