75 



salts is ill ecjiiilibriiini with ice: llio satiirjilion- or iceciirve under 

 its own vapourpiessure lias llien, as t'urve ed in fig. '2, one tei-- 

 minatingpoint on CA and one on C B. We find fnrthei': along an 

 icecurve under its own vapourpressnre the pressure is tiic same in 

 ail points and it is equal to the pressure of sublimation of the ice. 



We may deduce the j)revious results also in the following way. 

 As the vapour consists only of C, we equate, in order to find the 

 conditions of equilibrium for the system F -\- L -\- G in (1) (II) 

 .f, zzz and //i = 0. We then find : 



Z-x~ y— =Z,andZ,-f«- + .i - = ^^ .... (1) 



Ox 0'/ O.f 0// 



For the saturationcurve of F under its own vaponrpressure we 

 find : 



(x r + y s) d x + [x s -\- y t) d y = -- Cd P (2) 



{a r + ^s) d X + (« s-^ ^t) d y = — (.4 + C) d P ... (3) 



which relations follow also immediately from 8 (II) and 9 (II). In 

 order that the pressure in a point of this curve should be maximum 

 or minimum, dP must be =0. This can be the case oidy, when 



«i/ = t^'^' (4) 



This means that the liquid is situated in the point of intersection 

 of the curve with the line CF, consequently, that the liquid is a pure 

 solution of F. Consequently we find: along a saturationcurve under 

 its own vapourpressnre of a ternary substance, the pressure is 

 maximum or minimum in the pure solutions. 



In order to examine for which of the two pure solutions the 

 pressure is maximum and for which it is minimum, we add to the 

 first part of (2) still the expressions: 



and to the first term of (3) : 



1 / d;- ös^ / ar a5\ i / a^ a^\ 



2 r d^ + ■ ^^) '^^^ [:% ^%) '-'"' " A% ^'' Wi) ''''' ^' '" 



Now we subtract (2) from (3), after that (2) is multiplied by <? and 

 (3) b}^ X. Substituting further their values for .4 and (', wc find: 



1 



— a{rdx- -f- 2sd.vdy -f tJf) — [{x-a) F, + a V—.vc\dr. . (5) 



Representing the change of volume, when one quantity of vapour 



