701 
this the heterogeneous region LG. In the other takes place a disso- 
ciation into solid F and a liquid saturated with F of the saturation 
line of F; we will call this the heterogeneous region FL. 
If we imagine the liquid- and vapour surface 
d a L of the S-surface to be introduced above Fig. 1, it 
\ GO is obvious that the solid substance F can be also 
g in equilibrium with a whole series of ternary 
ae vapours, which equilibria, however, are in the 
: present case all metastable yet. It is also obvious 
that these vapours must form in Fig. 1 a closed 
curve surrounding point F, which curve, however, has been omitted 
from the figure. We will call this curve “vapour saturation curve” 
of the solid substance F. This curve surrounds the heterogeneous 
region FG which, however, is still quite metastable. 
Fig. 1. 
Hence in Fig. 1 we distinguish : 
the saturation line of F. 
the vapour saturation line of F (metastable). 
the vapour- and liquid-line of the heterogeneous region LG. 
the vapour region (G) and the liquid-region (L.) 
and the heterogeneous regions LG, LF and GF, the latter of 
which is metastable. 
The vapour region G can form within as well as without the 
saturation line of F; on further reduction of the pressure there may 
occur also several vapour regions first isolated from each other and 
afterwards amalgamating. Moreover, in the system liquid -+ gas 
there may occur either binary or ternary vapour pressure maxima or 
minima or stationary points so that different cases are to be distin- 
guished. We will first take the case that there occurs neither a binary 
or ternary Maximum or minimum nor a stationary point so that the 
gas region appears in one of the apexes of the component-triangle 
and the liquid region disappears in one of the other apexes. 
In Fig. 1 the gas region is, therefore, formed outside the saturation 
line of F; on reduction of the pressure, the different curves of Fig. 1 
will change in form and position. As a rule, a small change in pres- 
sure only causes an exceedingly small change in the solubility of a 
solid substance; hence, the saturation line of F will alter but little on 
change of pressure within very wide limits. This, however, as we 
will notice presently, becomes different when we get near to the 
melting point of F so that the saturation line of F is only still a 
small curve. 
