()0G 



When llie binary equilibiiuni F -\- L -\- G is siliuited in an 

 ascending branch of its P, 7'-cnrve, addition of a tliird substance has 

 an opposite influence on tlie jtressure (at constant temperature^ and 

 on the temperature (under constant pressure). When addition of a 

 third- substance e. g. increases the pressure (at constant T) it will 

 decrease the boiling point (under a constant pressure). 



When the binary equilibrium F -\- L -\- (r is situated in an de- 

 sce]iding branch of its F,T curve addition of a third substance has 

 the same influence on the pressure (at constant TJ and on the 

 temperature (under constant P). When addition of a third substance 

 increases for instance the pressure (at constant T) it will also 

 increase the boilingpoint (under constant P). 



These rules are also true when F is instead of a combination one 

 of the components e.g. B or C. 



We will now still examine, in what case the pressure (at constant T) 

 of the binary equilibrium F -\- Tj -\- G is increased or decreased by 

 addition of a third substance. We may express this also in the 

 following way: in what case does the pressure along a saturationcurve 

 under its own vapourpressure from one of its terminatingpoints (A 

 and n in figs. 2, 3, and 4) increase or decrease? 



We take for this formula (8), which mdicates the relation between 

 the change of pressure dP and the quantity f/.i- of the new substance. 



Between the 3 phases of the binar}^ equilibrium F -{- L -\- G a 

 reaction may always take place. We let the reaction take place in 

 such a way that 1 quantity of \'apour occurs. The occurring change 

 of volume we call A T^. The denominator of (8) becomes then 

 (ji — y) A V, so that we may w^rite : 



1 fdP\ 1 fx. i3—v,\ 



RT ' yd.vJ,:=o AF ■ y.v ii-y 



We now take the ternary equilibrium F -\- L -\- G wherein L and 

 G contain still only a little of the third substance. The line solid- 

 liquid (Fa or Fb in figs. 2 — 4) then intersects the A^-axis (side CA 

 of the component triangle) in a point at the distance S from C. 

 The line solid-gas {Fa^ or Fb^ figs. 2 — 4) intersects this A'-axis in a 

 point at the distance 6', from C. We take S and S^ positix'e, when 

 the points of intersection are situated on the right, negative, when 

 they are on the left of C. S and S-^ are fixed by 



Sz=~ S, = -^-^— . . . . (13) 



Substituting /? — y and /? — y^ from (^13) in f 12) we find: 



