Distillation of Binary Mixtures. 



525 



equilibrium o£ the piston under a constant load be stable ? 

 There must of course be some position of stable equilibrium 

 for a given load and temperature ; but this might, for all 

 that appears, correspond to complete evaporation of the liquid 

 or to complete condensation of the vapour. 



The following argument, however, suffices to show that 

 Konowaiow's principle is a necessary consequence of the 

 second law of Thermodynamics. Suppose that the cylinder 

 in which are contained the given liquid and vapour com- 

 municates by a lateral channel (fig. 2) with a large reservoir 



Fig:. 2. 



filled with liquid of similar composition, and that all are main- 

 tained at the prescribed temperature. As a first operation 

 close the tap between the vessels, and then let the piston rise 

 a little. The motion is supposed to be so slow that equili- 

 brium prevails throughout. The result of the expansion 

 may be that the compositions of the liquid and of the vapour 

 undergo a change. Now open the tap, and allow diffusion to 

 take place, if necessary, until equilibrium is again established. 

 On account of the large quantity of liquid in the reservoir 

 the pressure is sensibly restored to its original value and 

 remains undisturbed as the piston is slowly pushed back to 

 its first position. During this cycle of operations work 

 cannot be gained ; and thus is excluded the possibility of a 

 rise of pressure during the expansion. It follows that a fall 

 of pressure cannot accompany compression. 



Upon the basis of this principle Konowalow proceeds as 

 follows : — Suppose that at a particular composition-ratio the 

 pressure of vapour increases as the liquid becomes richer 

 in a specified component. In this case the expansion of the 

 mass cannot enrich the liquid ; for if this result occurred the 

 pressure would rise, which we have proved it cannot do. 



