166 J. G. STEPHENS. 



A NEW METHOD OF MEASURING MOLECULAR 



WEIGHTS. 



J. G. Stephens, b.Sc 



John Coutts 7 Research Scholar. 

 (Communicated by Prof. 0. E. Fawsitt.) 



[Read before the Royal Society of N. 8. Wales, November 5, 1919.~\ 



The ordinary boiling-point method for the determination 

 of molecular weights depends upon the fact that the vapour 

 pressure of a solution is lower than that of the pure solvent. 

 This same fact may also be employed as the basis of the 

 following method in which the molecular weight of a 

 material is measured by comparison with that of a standard 

 substance. The theory of this method will be understood 

 on reference to Fig. 1, where each of the side tubes of the 

 vessel represented contains a quantity of some liquid. 

 Suppose that a quantity of a (liquid) solvent containing 

 some known solute is introduced into one side of the 

 apparatus whilst a quantity of the liquid containing some 

 other soluble material is added to the other side. There 

 will thus result two solutions which, having in general 

 different concentrations of dissolved units, will accordingly 

 possess different vapour pressures. A steady distillation 

 will therefore occur from the solution of high to the solu- 

 tion of low vapour pressure, and equilibrium will only be 

 reached when the transfer of liquid from one vessel to the 

 other has rendered the vapour pressure of both solutions 

 equal. 



Now there is a principle stated by Ostwald that if two 

 systems are in equilibrium one way, they must be in equi- 

 librium in every way. It follows, therefore, that since 

 these two solutions are in equilibrium as regards their 



