OSMOTIC PRESSURE 37 



few direct measurements that have been carried out will 

 be mentioned. 



First, as to the semipermeable membrane. The peculiar 

 property of allowing one substance to pass, and not the 

 other, seems to depend not so much on sieve-like action 

 as on the ability to dissolve or absorb, or loosely combine 

 with one substance. Such a selective action occurs with 

 gases, e.g. palladium 1 allows the passage of hydrogen, 

 which is absorbed by it. Nernst 2 has described a semi- 

 permeable membrane for liquids, based upon the same 

 principle, and consisting of an animal membrane, moistened 

 with water, placed between (moist) ether on one side, and 

 (moist) ether, in which benzene was dissolved, on the other. 

 The ether, being slightly soluble in water, passes through 

 the membrane towards the side containing benzene, whilst 

 the benzene, insoluble in water, fails to pass through. 

 Quite recently the property of caoutchouc, noticed by 

 Graham, of transmitting certain gases, such as sulphur 

 dioxide and carbon dioxide, has been studied with regard 

 to liquids 3 : whilst e. g. methyl and ethyl alcohol cannot 

 permeate the caoutchouc, ether, carbon disulphide, chloro- 

 form, benzene, &c., can, so that if one of these liquids be 

 placed on one side of the membrane, and methyl alcohol 

 on the other, ether, &c., passes through. Moreover, it has 

 been shown that the rate at which the transmission takes 

 place is in agreement with the rate at which the liquid 

 in question is absorbed by caoutchouc. Finally, Tammann 

 discovered in the zeoliths a material permeable only for 

 water not for substances dissolved in it; these zeoliths 

 are known to be hydrated silicates, which, according to 

 Mallard, possess the peculiar property of absorbing and 

 giving up water, without losing their crystalline form. 



Whilst those membranes are of importance for the expla- 

 nation of semipermeability, an entirely different material 



1 Ramsay, Zeif.schr. /. Phys. Chem. 15. 518; Hoitsema, 1. c. 17. i. 



2 Neriist, 1. c 6. 38. 



3 Raoult, Comptes Rendus, 121. 187 ; Fusin, 1. c. 121. 794. 



