1206 A 
AF the three-phase and Fd the melting point curve of the compound 
PF these three curves are therefore the same as the homogeneous 
curves of fig. 3 (III). The direction of the melting point curve /’d 
(fig. 4) is determined by: 
dP H—-y 
dT Vr 
From (12) it follows that, in point / of fig. 4, the PT-curve 
ZEZ, must come into contact with the melting point line Fl. The 
further course of this P7-curve in the vicinity of the point / may 
be traced in the following manner : 
We proceed in fig. 1 from F towards 7,, R thus becoming negative. 
rj 
dP 5 
From 41) it now follows that ap remains positive so that the curve 
C 
must be situated like curve FZ, of fig. 4. 
If, in fig. 4 we move from F towards Z, R becomes positive. 
A being small, the denominator of (11) will soon become mil so 
that curve FZ of fig. 4 must have a vertical tangent in the vicinity 
of the point £. If in fig. 1 we move further from # towards Z, then 
dE 
a from (11) will become negative first, and nz/ afterwards, so that 
( 
8) 
a 
curve FZ of fig. 4 must have a horizontal tangent. As En after- 
wards becomes positive, curve FZ is bound to fall at a decreasing 
temperature. 
Proceeding from point 7 we find on curve ZZ, first a pressure- 
and then a temperature maximum, further a point of contact with 
the melting point line FJ of the compound F at the minimum 
melting point of tbe compound and finally a receding branch FZ. 
All this reminds of the P, 7-curves deduced by Van per WAALS 
for solid + liquid + gas in binary systems. 
To some differences, for instance that the 7, 7-curves mentioned 
here do not meet the sublimation line of # in the maximum subli- 
mation point, I will refer later. 
In fig. 4 it has been assumed that curve ZFZ, exhibits a double 
point 4, namely a point of intersection of the branches Zand FZ,. 
In order to perceive the possibility. of a similar double point we take a 
eireumphased boiling point line (fig. 1). On this occurs a point with a 
maximum and another with a minimum temperature. These points 
divide the boiling point line into two branches and in such a man- 
ner that to each point of the one branch appertains a definite point 
of the other branch, namely in that sense that both points indicate 
