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XX. Note on the Vapour-Pressure of Solutions of Volatile 

 Substances. By R. A. Lehfeldt, D.Sc* 



THE change of vapour-pressure of a solvent due to the 

 solution in it of a small quantity of volatile material has 

 been calculated on the basis of Raoult's rule for the correT 

 sponding case of a non-volatile dissolved body, first by Planck, 

 and later, more correctly, by Nernst ; the formulae obtained^ 

 and one experiment in support of them, are given in Ostwald's 

 Lehrbuch (2. ii. 588). Further examples may be obtained 

 from the measurements of vapour-pressures I have carried out 

 on certain mixtures of organic liquids f ; those measurements 

 were made primarily with the object of following out the be-; 

 haviour of liquids in solutions not dilute ; but in the extreme 

 cases the results obtained will serve as a test of the formulae 

 applying to dilute solutions. 



Adopting the notation used in the memoir referred to, we 

 have for a mixture of the two liquids A and B : — 



vta 7tb, vapour-pressure of A and B respectively, 

 p, ,, mixture 



£, ratio of number of mols. of A to mols. of B in liquid, 

 V, » „ » vapour. 



Then the relation as expressed by Nernst becomes 

 ir-B—p p 



7T B 7Tb 



for the case in which £ is small, L e. in which a small quantity 

 of A is dissolved in B ; 7r B is therefore the vapour-pressure 

 of the solvent. This equation may more conveniently be 

 transformed into 



i-J=5 CD 



7T B 1 — 7] v / 



When £, and consequently rj, is very small, this becomes 



^b 



the form in which Planck first gave the result, but which is 

 less exact than the other when moderate concentrations are 

 considered. 



* Communicated by the Physical Society read May 12, 1899 

 t Phil. Mag. [5] vol. xlvi. p. 46. 



