1314 
and its correlated pressure Pp is attained at which an infinitesimal 
quantity of liquid Z is formed. The sublimation curve, therefore, 
terminates in a point D of fig. 1 representing the temperature 7’p 
and the pressure Pp which we will call the maximum sublimation 
point of F+’. The liquid L which forms in the point. D will 
as a rule not be represented in an z‚y-representation, by a point of 
the line FF’. As, however, the quantity of this liquid Z is as yet 
but infinitely small, the vapour corresponding with the point D will 
still be represented by a point of the line FF’. 
If the temperature is increased still further, still more liquid is 
formed and the four-phase equilibrium #4 F+ L + G appears. 
As, however, a finite quantity of liquid is now present, 1 and G 
must be in opposition in regard to the line /#”; only incidentally, 
L and G may fall both on this line. 
At a constant Pand 7 one of the following reactions takes place 
between the four phases on increase or withdrawal of heat or on 
a change in volume. 
1 ree See B PSP ae sane ee 
We will call the reaction 1 a congruent reaction, the reactions 
2 and 3 incongruent ones. Which of these reactions takes place 
depends on the situation of the four points in regard to each other. 
As the system F+ F’ +24 G has formed from /'+ F” it is 
evident that in this four-phase equilibrium / and G are always 
present in such proportions that both disappear simultaneously in 
the above reactions. 
Hence by warming the system + #” + G we have arrived on 
the four-phase line # + F” + LG. As on this line the three 
components are present in four phases, this system is a monovari- 
ant one, so that to each temperature appertains a definite vapour 
pressure. 
Hence, the four-phase line in a P, 7-diagram will be represented 
by a curve; a part of this curve is represented in fig. 1 by DS; 
we shall see later that it continues in the points D and S. This 
curve, as we shall see meets the sublimation curve of / + F” in 
its terminal point D. 
We now take a Tand P at which is formed from F+ F’ a 
liquid without vapour, hence the system #-+ /” + L. The liquid 
will then, of course, be represented by a point of the line # /". 
According to the situation of Z in regard to the points Fand Pf”, 
the following reactions may occur at a constant P and 7' on a 
supply, or withdrawal of heat, or on a change in volume. | 
1. If the point Z lies between / and /” the reaction F + F2 L 
