42 MOLECULAR WEIGHT AND POLYMERISM 



solution is semipermeable, and the piston exercises a 

 pressure exactly equal to the osmotic pressure then 

 a thermo-dynamic relation may be deduced which involves 

 the possibility of measuring the molecular weight. 



Viewed from this point of view, the indirect methods for 

 determination of molecular weights may be divided into 

 two groups, according as the cyclic process underlying them 

 can be carried out at constant temperature or not; in 

 the first case the theoretical argument is simpler, and can 

 often be simply demonstrated without the aid of thermo- 

 dynamics. 



(a) The cyclic process can be carried out at constant 

 temperature. Molecular weight determination by vapour 

 pressure measurement. The common character of methods 

 based on cyclic processes which can be carried out at 

 constant temperature is that they can be performed at any 

 temperature, whether the separation be as vapour or by 

 shaking with a third substance. We have, however, to 

 distinguish as distinct methods the separation of solvent 

 and of dissolved substance, and so we arrive at the following 

 resume' : 



1 . Separation as vapour. 



(a) The dissolved substance separates out : phenomena 



of absorption. 

 (/3) The solvent separates out : lowering of vapour 



pressure. 



2. Separation by a solvent. 



(a) The dissolved substance separates out : ratio of 



partition. 



(/3) The solvent separates out : lowering of solubility. 

 With regard to the first of these methods, based upon the 

 phenomena of absorption, we may refer to p. 28, and only 

 note that when Henry's law of absorption holds (propor- 

 tionality between pressure of the gas and concentration in 

 solution) the absorbed gas in question must exist with 

 unchanged molecular weight in solution. 



We must now deal with the second method, based on the 



