706 
Although this is the case generally, it no longer holds good if 
we take a temperature elose to the melting point of F. The satu- 
ration line of F then surrounds a comparatively small region which 
on change of pressure, can rapidly extend, cr possibly contract. 
The saturation line of F will then move more rapidly than the liquid- 
line of the region LG. We will now distinguish two cases in one 
of which the substance expands on melting whilst in the other case 
it contracts. 
F expands on melting. An inerease in pressure (at constant T) 
will cause a solidification of the molten F, a decrease in pressure 
a fusion of solid F. On decrease in pressure, the isothermic satu- 
ration line of F will consequently contract rapidly and disappear 
in the point F. We now start from Fig. 1 and assume that, on 
lowering the pressure, the saturation line of F contracts at first 
rather slowly and then more rapidly; its movement is more rapid 
than that of the liquid line of the heterogeneous region LG. 
If now the movement of the saturation line of / is slower than 
that of the liquid line, Fig 1 may be converted into Fig. 2 and then 
into Fig. 3 from which are then formed either the Figs. 4, 5 and 6 
or the Figs. 8, 9 and 10. If however, after the isotherms have 
assumed a form as in Fig. 3, the movement of the saturation line 
becomes more rapid than that of the liquidline, then, after the appear- 
ance of the isotherms of Fig. 3, those of Fig. 2 and 1 reappear. 
On reduction of the pressure we then geta series of isotherms such as: 
fig. | — fig. 2 — fig. 3 — fig. 2a — fig. la 
in which the figures occurring after fig. 3 are indicated by 2a and 
da. Fig. 2 and 2a resemble each other with this great difference, 
however, that in fig. 2a the saturation line of F is much smaller 
and that the liquid and vapour lines of the heterogeneous region lie 
more adjacent to F than in fig. 2. The same applies to fig. ta in 
regard to fig. 1. Between fig. 2 and 2a there is also still this diffe- 
rence that Fig. 2 applies to the maximum and Fig. 2a to the minimum 
pressure of the system F+1L-+G. We will therefore assume that 
in fig. 2a the letters M, and M of fig. 2 have been replaced by 
m, and m. 
From the previous considerations it now 
F follows at once that the saturation line of F 
under its own vapour pressure must be situated 
as in fig. 12; contrary to this same curve in 
fig. 7 and 11 it does not surround the point 
M, F which represents the solid phase with which 
Fig. 12. its solutions are saturated. We will, therefore 
