THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



NO V EMBER 1895. 



XXXV. On the Properties of a Mixture of Liquids. By 

 R. A. Lehfeldt, B.A., B.Sc, Demonstrator at Firth 

 College, Sheffield*. 



(i.) Deduction of a Thermodynamic Relation. 



WHEN a mixture of two liquids is allowed to evaporate, 

 interesting questions arise on the relation between the 

 composition of the liquid and the composition of the vapour 

 rising from it ; and the relations of each to the pressure of 

 the vapour. There is, unfortunately, very little experimental 

 evidence on either of these points, particularly on the com- 

 position of the vapour. It is possible, however, by thermo- 

 dynamic reasoning, to obtain a relation between the quantities 

 above mentioned, and so make experimental data on one 

 point supply the lack of information on another. 



When equilibrium exists between two bodies, the thermo- 

 dynamic potential of the whole must be unchanged by any 

 small variation of the system ; and if the two bodies are a 

 liquid and its vapour, this condition may be applied to the 

 change involved in the conversion of a small amount of the 

 one into the other. Now suppose a mixture of two liquids A 

 and B contained in a vessel in the upper part of which are two 

 cylinders, with pistons ; and let the lower end of the cylin- 

 ders be separated from the vessel by two semipermeable 

 membranes, of which one lets only the vapour of A pass ; the 

 other only vapour of B. Then by allowing the first piston to 



* Communicated by the Author. 

 Phil Mag. S. 5. Vol. 40. No. 246. Nov. 1895. 2 F 



