58 MOLECULAR WEIGHT AND POLYMERISM 



It may be added that recently the boiling-point method 

 lias been applied in a simpler form by Landsberger l and 

 Walker 2 , by passing e. g. ether vapour, generated in a 

 flask, through the ethereal solution of the substance investi- 

 gated, till the thermometer no longer rises, and indicates 

 the boiling point of the solution. 



Molecular y:eight determination by means of the change 

 of solubility ivith temperature*. If no third substance is 

 employed to effect the separation between dissolved body 

 and solvent, and we omit that in the list of methods, it is 

 in the first place difference in state of aggregation which 

 allows of the separation. Thus molecular weight determina- 

 tion by means of vapour pressure and boiling point is 

 possible on account of the separation of solvent as vapour ; 

 determination by means of the absorption coefficient is 

 possible on account of the separation of the dissolved body 

 as gas. Separation of the solvent in the solid form leads 

 to the method of the lowering of the freezing point, and 

 finally the account may be completed with the possibility 

 of separating the dissolved body in the solid form. The 

 method so indicated may now be taken. 



The method is similar to that applied on p. 21 to gas and 

 vapours, involving the well-known formula 



~. 



Here V is the increase of volume due to evaporation of 

 a definite quantity, and g, the heat absorbed, refers to the 

 same. 



With regard .to solutions this equation may be trans- 

 formed, according to Part I, p. 36, into 



d( Q 



dT 2T* CdT~~ 2^' 



1 Berl. Ber. 31. 458. 2 Journ. Chem. Soc. 73. 502. 



3 Verschaffelt, Zeitsc.hr. f. Phys. Chem. 15. 449 ; Van Laar, 1. c. 15. 473 ; 

 17. 545 ; 27. 337 ; Goldschmidt, 1. c. 17. 145 ; 25. 91 ; Noyes, 1. c. 26. 699. 



