771 



Now we will consider some points more in detail. In order to 

 get the conditions of equilibrinm for the system F -\- L -\- G, when 

 F is a binary compound of B and C and when the vapour consists 

 only of ^1 and C. we mnst equate a=^0 and//i=:0. The conditions 

 (J) (II) pass then into: 



Z 



Z, 



dz 



0^ 



0/ 



dZ 



dy 



dZ^ dZ 



dx, dy 



S 



dZ 



dZ, 



Now we put : 



Z =z U -\- RTx log ,v and >^i = U^ -f ET.v^ lo<j x^ 

 Hence the conditions (1) pass into : 



dx 

 dU 



dU 



^) — + RTx 

 dy 



^— + RTx, 

 dy 



l\ + ? = 



+ RT log X z=i ^ -\- RT log .r, 



ÖX dx. 



(1) 



(2) 



(3) 

 (4) 

 (5) 



When we keep the temperature constant, we may deduce from 



(3)— (5) : 



Ixr ^ {y - ii) s -\- RT^ dx + [xs -\- {y — ^) q dy — A dP 



X, 



x,r — ^s -ir — RT 



X 



RT\ 



r -\ diV -f sdi/ 



X J 



dx ^[x,s — ^q dy = {A -^ C)dP. 



(6) 

 (7) 





dx 



dP . (8) 



Here we must equate of course in ^4 and 6' « = and y^ = 0. 

 In order to let the pressure be a maximum or a minimum, dP 

 must be =: 0. From (6) and (7) it follows that then must be satisfied . ■ 



x-?^x,{y-~ii) = (9) 



This means that the point of maximum- or of minimumpressure 

 M {d\y) and the corresponding' vapourpoint J\/^ id\y^) are situated 

 with F on a straight line (fig. 3). 



In order to examine the change of pressure along a saturation- 

 curve under its own vapourpressure in its ends h and n (figs. 2 and 3) 

 we equate in (6) and (7) ,/■ = and ,r, =i U. Then we find : 



OF" 

 [(y-i^) 5 + ^^^1 dx + (y—i^) tdy 



V-v -^{^-y) 



[ 



dy 



^s + '- Rt\ dx — ^tdii = 

 X J 



i' + /i 



dV- 



dP 



dP (10) 

 (11) 



