871 
geneous region LG contracts so as to disappear in a point outside 
the saturation line of /. 
After the contact of the liquidum and saturation lines two points 
of intersection appear; if now no further contact takes place, these 
points finally coincide in a point of contact so that the saturation 
line under its own vapour pressure exhibits but one maximum or 
minimum. 
If, however, after the appearance of the first two points of inter- 
section a second point of contact occurs we obtain four points of 
intersection of which, at first two, and afterwards the other two 
coincide in a point of contact, so that in all four of these points 
are formed. The saturation line under its own vapour pressure then 
exhibits two maxima and two minima. 
By way of a transition case it might happen that the second 
point of contact, which appears after the formation of the two tirst 
points of intersection, coincided with one of these points so that a 
point of the second order was formed. On further change of pressure 
two points of intersection then again occurred, which finally coincided 
in a new point of contact. The saturation line under its own vapour 
pressure then represents the transition form between that with ene 
maximum and one minimum and that with two maxima and two 
minima. 
After what has been stated it will surely be unnecessary to con- 
sider the case where, in the system liquid-vapour, a vapour pressure 
maximum or a stationary point occurs; we will refer to this and to 
a few peculiar boiling point lines perhaps later. 
_ We will now just consider what happens if we take the compound 
F only and apply heat. If we imagine /” placed in a vacuum at a 
low temperature a portion of this compound / will evaporate and 
there is formed the equilibrium: solid # + vapour 4. On increase 
of temperature the vapour pressure of / is raised; in a P,7-diagram 
we thus obtain a curve such as aA of fig. 3, namely the sublima- 
tion curve of the substance 4. At a detinite temperature 7% and a 
pressure Pr: an infinitely small quantity of liquid is now formed; 
this, of course, has not the composition / but another composition 
K. As only an infinitely small amount of liquid has formed as yet, 
the vapour still has the composition 4. The point A’ is, therefore 
the terminal point of the sublimation line, called by vaN Der Waars 
in his binary systems the upper sublimation point of the compound. 
If we increase the temperature, say, to 7”, more of the compound 
melts; there is then formed the three-phase equilibrium # + LG 
in which neither £ nor G have the composition /. £ and G have 
: 57 
