OSMOSIS. 



265 



In practice such membranes are formed in the interstices of an 

 indifferent supporting structure, such as the pores of a porous battery 

 pot (preferably previously soaked in gelatin), by placing one of the mem- 

 branogens inside the pot, which is then lowered into a solution of the 

 other, so that the precipitate is formed within the structure of the 

 earthenware where the two solutions come into contact. It is only by 

 such an artifice that the membrane can be sufficiently supported to 

 enable it to withstand the high pressure produced by the osmosis under 

 the conditions. 1 



If, now, a battery pot with such a membrane in its pores be filled 

 with a solution of sugar in water, hermetically sealed, and placed in 

 a vessel of water, the water molecules will diffuse in either direction 

 through the membrane, which is permeable to them ; the sugar molecules, 

 on the other hand, cannot pass out, for to them the membrane is imper- 

 meable. As a result of the presence of the sugar on the inner side of 

 the membrane, in unit time, more water enters the pot than passes out, 

 and the pressure rises until it is sufficient to bring about the condition 

 of equality in the number of water molecules entering and leaving the pot. 



This pressure is called the osmotic pressure of the solution of sugar, 

 under the conditions of concentration and temperature. That this 

 pressure is comparable to that of a gas was first clearly pointed out 

 by van 't Hoff. 2 



Thus the osmotic pressure of a dilute solution at constant tem- 

 perature is proportional to its concentration (i.e. density of a gas in 

 the law of Boyle). This is illustrated by the following table from 

 Pfeffer: 3 



Cane Sugar Solutions at 13'5 C. to 16-1 C. 



Again, at constant concentration of a dilute solution, the osmotic 

 pressure is proportional to the absolute temperature (law of Charles). 

 Thus, again, taking Pfeffer's data 



1 per Cent. Cane Sugar Solution. 



1 For details of manufacture see Adie, Journ. Chem. Soc., London, 1891, vol. lix. p. 344. 



2 Arch, netrl. d. sc. exactes, etc., 1885, Bd. xx. S. 239; Ztschr. f. physikal. Chem. 

 Leipzig, 1887, Bd. i. S. 479. 



* Loc. cit., p. 85. 



