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(S,. = GH L+Ss [Curve(S,) or q8 fig. 8 and gf fig. 7] 
(Ss) = GH LAS, [Curve (Sz) or gd tig. 8 and qd fig. 7] 
When S, and S; are not volatile, then G consists of watervapour 
only. If they are volatile, then G contains also S. The more 
S is contained in G, the more the point G shifts towards the right 
in the concentration-diagram of fig. 8. As long as the four phases 
with respect to one another are situated, however, as in fig. 8, the 
P,7-diagramtype remains the same. 
As it appears from the change in volume at the reaction Sue Se 
which is generally small, the (J/)-curve proceeds general fairly 
parallel to the P-axis: it terminates towards lower pressures in the 
Fig. 7. Fig. 8. 
triplepoint : 5, + S;-+ vapour 5. It may proceed from this triple- 
point as well towards higher as towards lower temperatures; in 
figs. 7 and 8 we have assumed that it goes towards higher 7. 
The position of the curves q3 and qd with respect to one another 
infig.S follows from fig. 7; for this we have to draw a horizontal 
line, which intersects the stable part of the one and the metastable 
part of the other curve. 
As the concentration-diagrams of figs. 1 and 8 belong to the same 
type, this must also be the case with the P,7-diagrams of both 
figures. We see that this is really the case. 
Now we shall discuss a binary system, in which occurs a P,7- 
diagram of the type of fig. 2. For this we take the system: water 
+ salt S, in which a hydrate H occurs in the two modifications 
H, and Hs [fig. 9). 
When we represent the solutions of the equilibrium G + £ + H, 
