410 



In case of osmotic* equilibrium the thermodjiiamic potential of the 

 water is equal on both sides of the wall. If water under vapour 

 pressure is brought into contact with the three-phase mixture, un- 

 mixing will take place as appears from the above calculation ; so 

 the thermodynamic potential of the water in the three-phase mixture 

 is greater than the thermodynamic potential of the water under 

 vapour-pressure. 



Reversely if in a vessel water by the side of vapour is made to 

 mix with hexane by the side of vapour, we shall have to conclude 

 to a rise of the thermodynamic potential of the water. 



In order to examine whether the same thing is also the case for 

 hexane, we must be able to calculate the osmotic pressure of the 

 three-phase mixture with respect to a wall permeable to hexane. 

 This, however, is impossible, as the fluid phase contains about 27 

 mol. percent, of water at the critical endpoint, and can, therefore, no 

 longer be considered as dilute solution. It is, however, possible to 

 find out something about the value of the osmotic pressure of the 

 binary equilibria in which dilute solutions of water take part. When, 

 for instance, the P-.r section foi- the critical endpoint (tig. 4) is 

 considered, it appears that a liquid with 2 mol. percent, of water 

 possesses an external pressure of 28 atm., if it coexists with vapour. 

 This solution contains one gram-molecule of water to 49 gram- 

 molecules of hexane. The specific volume of hexane at 222° amounts 

 to 2,69 according to Young. Now at this temperature the hexane 

 is not far from the critical temperature, and we must, accordingly, 

 expect a pretty great compressibility. If the compressibility were 

 zero, the volume of one gram-molecule of water would amount 

 to 49 X 86X2,69 c. c. or about 11,3 1. As now one gram- 

 molecule of gas occupies a volume of 40,6 1. at this temperature 

 under the pressure of one atmosphere, the osmotic pressure of the 

 mixture with respect to a membrane permeable to hexane would 

 amount to about 3,6 atmospheres. The difference in external pressure 

 between the mixture and tlie pui-e hexane under the vapour pressure 

 is, however, three atmospheres. So we conclude that in the experi- 

 ment in which the two-phase mixture (2 mol. " „ of water in the 

 liquid) is brought into contact with hexane under the vapour-pressure 

 by means of a membrane only permeable to hexane, hexane will 

 pass to the two-phase mixture. The compressibility does not affect 

 this conclusion, as the osmotic pressure is still increased in conse- 

 quence of the compressibility, and the qualitative result can, there- 

 fore, not be modified by it. 



If this behaviour continues to exist for greater concentrations, the 



