607 • 



RÏ' [d^),.^o~ AF • .6- {^ ~SJ • • • ■ (^'^) 

 From tliis relation it follows that the sign of the change of pressure 

 depends on the sign of A V. Now A J^ is almost always positive 

 for the binary equilibrium F -\- L -\- G and negative only between 

 the points F and H (figs. 5 and 6)- Further it follows that the sign 

 of the change of pressure is not fixed by theratio.t\ : .i' (the partition 

 of the third substance between gas and liquid), but by the ratio 

 S:S^; therefore this is by the ratio of the parts which the lines 

 Fa and Fa^ or Fb and i^^i cut off from the ^\-axis. We may consider 

 S and >Si also as the perspective projections of x and a.\ from 

 the point F on the A^-axis. We shall call for that reason S the 

 perspective concentration of the new substance in the liquid, and 

 Si that of the new substance in the vapour. These perspective con- 

 centrations can be as well positive as negative. 



F'rom (14) we can now easily deduce for the addition of a new 

 substance : 



1. The formation of vapour in the binary system F -\- L -{- G 

 takes place with increase of volume {AV^O). 



When in liquid and vapour the new substance has perspective 

 concentrations of opposite s:'gn, the pressure increases. 



When in liquid and vapour the new substance has perspective 

 concentrations of the same sign, the pressure increases, when the 

 perspective concentration (apart from the sign) of the new substance is 

 greater in the vapour than in the liquid, the pressure decreases when 

 the reverse is the case. 



2. The formation of vapour in the binary system F -{- L -{- G 

 takes place with decrease of volume (A ]^ <[()). 



The changes of pressure take place in opposite direction as sub 1. 

 It may be considered with this, that A F <] is the case only between 

 Tjr and 7'//, therefore between the ininimum-meltinir point of i^ and 

 the point of raaximum-tempercxture of the binary system F -{- L -{- G. 



When we take a threephasetriangle in the vicinity of the side ^C, 

 its angle F is either a little greater than 0° {Faa^ and Fbb^ in fig. 2) 

 or a little smaller than J 80° {Fbb, in fig. 3). We will call the three- 

 phasetriangle in the first case acute-angled, in the second case obtuse- 

 angled. We may express the previous rules also in the following way: 



1. The formation of vapour in the binary system F -\- L -\- G 

 takes place with increase of volume (A V ^ 0). 



An obtuse-angled threephasetriangle moves on increase of pressure 

 both its sides solid — liquid and solid — gas towards each other and 

 on decrease of pressure away from each other. 



39* 



