702 
The influence of a change in pressure on a 
a d L liquid and vapour region is, however, great in 
comparison with that on the saturation line of F. 
6 When the pressure decreases, the gas region 
L extends and the liquid region contracts; the 
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heterogeneous region LG shifts, therefore, in Fig. 1 
Fig. 2. towards the heterogeneous region FL. Hence, on 
reduction of pressure, there will oecur a pressure as represented in 
Fig. 2 where the liquidline and the saturation line of F will meet 
in a point M so that the equilibrium F + Ly + Gm, appears. The 
three points F, M, and M, as follows readily from the indicatrix theorem, 
are situated on a straight line. With the aid of the two sheets of 
the $-surface it is also easy to see that the non-drawn vapour saturation 
line of F in Fig. 2, must meet the vapour line in M,. 
On further decrease of the pressure, the 
saturation line of F and the liquidline intersect 
each other in two points; these intersecting 
points are represented in Fig. 3 by a and b; 
in the solid substance the letter F is omitted. 
From a contemplation of the liquid sheet 
and vapour sheet of the ¢-surface it follows at 
onee that with each intersecting point of the 
saturation line of F and the liquidline is 
Fig. 3. conjugated an intersecting point of the vapour 
saturation line of F and the vapour line. As the saturation line of 
F and the liquidline intersect each other in the points a and b, the 
vapour saturation line of F and the vapour line must intersect each 
other in the two points a, and b,. The curve a, b, is the vapour 
saturation line of F, the vapour line consists of the two parts e, b, 
and a,d, which are, of course, connected with a metastable branch 
not drawn in the figure. 
At the pressure contemplated here, two three-phase equilibria 
F + L + G therefore occur, namely: 
F ~~ Li: — Epe and F “= La —— Gp, 
On further reduction of the pressure, the heterogeneous region 
LG shifts more and more in such a direction that the vapour region 
becomes larger and the liquid-region smaller. At a certain pressure, 
the liquidline will pass through the point F, after which this gets 
situated within the heterogeneous region. We then obtain an isotherm 
as in Fig. 4 which does not differ essentially from that of Fig. 3. 
