S43 



liquidciirve of a I'cgioii IJt , the doKod ciii-vc siliialed in (lu' vicinily 

 is the vapoiircurve. On decrease of pressure tliis region L(i shifts 

 along BC from B to C and along BA from B to A. 



In the case sub 3" we may imagine e. g. that on decrease of 

 pressure the region LG arises in C and disappears in A. On decrease 

 of pressure this region shifts with its liquidcurve ahead along BC 

 from C to B and along 6V1 from C to A. When the one extremity 

 is in B, the other is still somewhere on CA-. in fig. 1 Bu is the 

 liquidcurve and Bu^ is the vapourcurve of a similar region. On 

 further decrease of pressure this region shifts towards point A ; as 

 soon as the one extremity has left point B, it obtains of course again 

 a certain breadth. 



Let us at first consider the equilibrium B -{• L -{- (i of the binary 

 system BC. If we represent the quantity of C in the liquid by y 

 and in the vapour by y^ we ha\e : 



T — = ^ (1) 



wherein : 



^ y y 



Under a lower P and at a lower T ^W n and A T^y are positive. 

 Under a lower P and at a lower T the P7^-curve will therefore 

 consist of an ascendant part. The question is whether this curve 

 will show in its subsequent course a point of maximum pressure 

 and a point of maximum temperature. 



We shall call, as B is the solid substance, C the solvent, y, <^y 

 means then, that the concentration of the solvent is smaller in the 

 vapour than in the liquid; or also (as 1 — y^ ^ 1 — y) that the con- 

 centration of the solid substance in the vapour is greater than in 

 the liquid. We shall express this in the following way : the solvent 

 is less volatile than the solid substance, y^^y means then: tlie 

 solvent is more volatile than the solid substance. We now distinguish 

 two cases. 



.Vi <C y- As ^\ — "*.' ^s greater than H—ij and l\ — v > V — v, 

 A W,/ and A Vi, can never become zero or negative. Therefore the 

 P, T-Quvve consists only of a part ascending with the temperature, 

 without a point of maximum pressure or of maximum temjierature ; 

 it has its highest P and T in the minimum mellingpoint of the 

 solid substance. 



Vi'^y- When y^-.y is greater than 1, this quotient can yet be 

 smaller than {H\ — i]) : [H — i]), so that in the P, 7'-curve the point 

 of maximum pressure and of maximum temperature are wanting. 



