O12 
such a composition that we can form from both the solid substance 
i’; the three figurating points are, therefore, situated on a straight 
Fig. 3. 
line. Besides. and G are always present in quantities equivalent 
to the reaction Z + Gl; L and G are. consequently, present 
in such amounts that from both we can form F without any £ or 
(7 remaining. 
As a rule, the three-phase equilibrium /#’'+ 1+ G ean exist, 
at the temperature 7” with a whole series of pressures, namely, with 
the pressures occurring on the saturation line under its own vapour 
pressure of the solid substance /’ at the temperature 7”. As in this 
particular case a phase reaction is possible between the three phases 
or in other words, as the points /’, L, and G lie ona straight line, 
the three-phase equilibrium exists here only at a definite pressure, 
namely, the maximum or minimum pressure which occur at the 
temperature 7” on the saturation line of / under its own vapour 
pressure. In this particular case it is,the minimum pressure, as will 
appear later. 
At a furtner increase of temperature more of the substance F 
keeps on melting and / and G alter their composition ; we will 
regulate the volume in such a manner that there is but an infinitely 
small amount of vapour which, of course, does not affect the pres- 
sure. If we represent the pressure and temperature graphically, a 
curve is formed such as curve K F of fig. 3. 
Finally we now arrive at a temperature and correlated pressure 
at which all solid / has fused; as particularly at the last moments, 
we have taken care that but infinitely little vapour is present, the 
liquid now has the composition 4’; the vapour has quite a different 
composition D. 
