of Miscible Liquids by Distillation. 635 



all proportions, V a and Y b be the masses in the vapour at 

 any moment and L rt and L b the masses in the liquid in 

 contact with and in equilibrium with the vapour, and if E be 

 a mass evaporated and a mass condensed, the criteria for 

 equilibrium are 



BL« 



BL & ~ Y b 

 BE 



B Lfi L& 

 30 



and 



If: the mass L& be conserved, as is the case in discon- 

 tinuous distillation, 



= 0. 



.dE+-^.dC = JL b 



These three equations show that 



— { l+ ^r l -i)} dL - ....... -,(i) 



where ^ --^=a, the proportion o£ liquid which is A, 



W + J-J6 



and where k=- ^ r " * T • 



Equation (1) signifies that the separation of unit mass of A 



involves the evaporation of a mass J 1 ■{ Tl rr 1- of the 



I «(* — 1) J 



liquid. If Q be its latent heat of evaporation, the quantity 



of heat required is 



q{ 1+ «-(^I)} «) 



