•7'79 

 Hence follows : 



[ 



dV 

 V, ~ /i. - (I - ,i) { V, - .., ^J 



,//'4-(l —ii).v^iYh, = 0. (22) 



When we assume tluil tlie gas-laws hold for the vupoiir ^', then : 



dV^ ^ RT 



-— =zO and ^=-— (23) 



From (22) now follows: 



[(1 —^)V,-v,-\- ^v] dP = -^ RT . dx, . . . (24) 



1— x^ 



The coefficient of dP repi'esents the change of volume when 1 Mol. 

 F is decomposed into ,i Mol B -\- {1 -^ /i) quantities of G ; this is 

 very nearly (1 — p?) \\. As at the same time Fl\ = R7\ we can 

 write for (24); 



(1 -.v,)dP=Pd.v, (25) 



B>om this follows: 



p=rT <2*'' 



When we call the partial pressures of A and C in the vapour Fa 

 ajid F(, then Pa=vJ' and Pc = (1 — .i^,) i^; I'roni (26) now 



follows : 



Pa=^^^Po and Pr = P, (27) 



l~x, 



This means that in the ternary equilibrium B-\-F-{-G the partial 

 pressure Pc of the substance C is equal to the vapourpressure of 

 the binai-y equilibrium B -\- P" -\- G^. 



Wiien we bear in mind now that in a system the pressure and 

 the composition of the vapour do not change, when we add to this 

 system still a liquid, which is in equilibrium with all phases of this 

 system, then follows: 



In the ternary equilibria B -\- F -\- G and B -{- F -^ P^^ -{- G^, {\\q 

 partialpressure of the substance C in the vapour is equal to the 

 vapourpressure of the binary equilibrium B -\- F -\- Go- 



The first equilibrium (viz. B -\- F -\- G) exists at a whole series 

 of pressures; both the others occur under a definite pressure only. 



The binary equilibrium Na^SO . 10 H^O -|- Na^SO^ + watervapour 

 has at 25° a vapourpressure of 18.1 ra.m. when we add alcohol, 

 then, when the gas laws hold iji the vapour, in the equilibrium 

 Na.SO, . 1011,0 + Na.SO, + G and Na.SO, . 1 OH,0 + Na,S(), + L + G 

 the partialpressure of the watervapour will also be ecpial lo 18.1 m.ni. 



Now we will put the question, whether we can also deduce some- 



