703 
On further decrease of the pressure, the points a and b and con- 
sequently a, and b, will at a definite pressure coincide ; here we 
first assume that F then still lies within the heterogeneous region 
LG. We then obtain an isotherm like in Fig. 5 in which we must 
zi 
a\ 
. 
rh 
AE > 
Fig. 6. 
imagine m to be formed by the coincidence of a and b, and m, 
by the coincidence of a, and b,. It is obvious that m,, F and m 
must lie on a straight line and that the non-drawn and metastable 
saturation line of F must meet the curve de in m and that the 
vapour saturation line of F must meet the curve d, e‚ in m,. 
On further decrease of pressure the saturation and vapour satu- 
ration curves of F arrive quite within the heterogeneous region LG; 
F cannot, therefore, occur any longer in the solid condition but 
splits into vapour + liquid; the compositon of the vapour is now 
represented by a point of the vapour line d,e,, that of the liquid 
by a point of the liqnidline de. Both points lie with F on a 
straight line. 
On reducing the pressure still further we obtain, when the gas 
region has extended itself over the point F, isotherms like those in 
Fig. 6. Tbe vapour saturation line of F has now disappeared, the 
saturation line of F can, however, still exist but then represents 
only metastable solutions and has, therefore, been omitted from 
the figure. 
From the foregoing views, it now follows at once that the liquid 
as well as the vapour of the system F + L-+ G trace a closed 
curve, like in Fig. 7 and that on each of these lines occurs a 
point of maximum and of minimum pressure. As the points of 
the curves of Fig. 7 all appertain to a same temperature but to 
different pressures, we may call Fig. 7 an isothermic — polybaric 
diagram. 
