216 Dr. "Efc. A. Lehfeldt on the Vapour-Pressure of 



The expression in the above form (i.) is so simple that it is 

 worth while to put it into words, thus : — 



" When a volatile substance is dissolved in a liquid the 

 vapour-pressure of the liquid is altered in the ratio of the 

 molecular fractional amount of solvent (Molenbruch) in the 

 liquid to that in the vapour." 



When the dissolved substance is non-volatile, the latter 

 fraction becomes unity, and we recover the well-known 

 formul a p/ttb = 1 — £• 



It is perhaps worth remarking that the expression con- 

 sidered is a particular case of the thermodynamical relation 

 applicable to any liquid mixtures, obtained by Duhem, 

 Margules, and myself. The latter may be put * : — 



V 



* log I^ _g(l-iQap 

 Blog-^ 



{7) -^ "1 V TV 



whence 



V — K "by _ 1 Bp, 



If we integrate this for a small range from f = and there- 

 fore p = 7r B we get 



(y—K)v _ p— 7r B 



7?(l—»7) ~ 7Tb ' 



or 



P y p — Tr-B 



7T B 7TB 



the formula in question. 



In case of a dilute solution of B in A it is easily seen that 

 equation (i.) becomes 



— = - (11.) 



The experiments made were on four series of mixtures, 

 viz. : — alcohol with benzene and toluene, and cnrbon tetra- 

 chloride with benzene and toluene; so that, taking the end 

 members of each series, we get eight cases of" dilute solution." 

 The second group of liquids, which were chosen as normal in 

 character, give the following results : — 



* L. c. p. 60. 



