709 
librium with each other is ni/. The reaction between the three phases 
therefore takes place without a change in volume. We will return 
to this later. 
We may summarise the above as follows: 
The isothermic-isobaric saturation and vapour saturation curves of 
a solid substance F are, at all temperatures and pressures, circum- 
phased and disappear (are formed) in the point F. 
The saturation curves of a solid substance F under their own 
vapour pressure are at a lower temperature circumphased; at a 
definite temperature, one of them passes through the point F after 
which, at higher temperatures, they, become exphased, then they dis- 
appear in a point, if in the reaction between the three phases no 
change of volume occurs. This also applies to the vapour lines 
appertaining to the saturation lines under their own pressure which 
can, however, be already exphased at lower temperatures. 
In fig. 14 are drawn some saturation lines under their own vapour 
pressure with their appertaining vapour lines for different tempera- 
tures. On each of these curves 
occurs a point with a maximum 
and one with a minimum vapour 
pressure which, however, are 
not indicated in the Fig. although 
the arrows indicate the direc- 
tions in which the pressure in- 
creases. These points are situated 
of course, in such a manner, 
that the line which unites two 
Fig. 14. points with a maximum or 
minimum pressure of curves of the same temperature, passes through 
the point F. The saturation line under its own vapour pressure 
disappears in the point M; the appertaining vapour-line in the point 
M, both points lie with F on a straight line. 
In fig. 14 the curves of different temperatures are all united in 
a plane; if, however, we imagine a temperature axis drawn per- 
pendicular to this plane and also the curves in space according 
to their temperatures, two surfaces are formed, namely the 
saturation surface, under its own pressure of F and the appertaining 
vapour surface. The first has its top in M, the second in M,; the 
line MM, is situated horizontally. It is evident that the point F does 
not ‘coincide with the top M of the saturation surface under its own 
vapour pressure but is situated somewhat lower and that the points 
M, M and F lie in a same vertical plane. (Lo be continued). 
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