864 
> 
moves, however, downwards; as V is smaller than v the correlated 
melting pressure will fall on increase of the melting temperature of #. 
As a small change in the melting point usually causes a very 
great change in pressure both surfaces of figs. 6 and 7 will generally 
move much more rapidly than the vapour and liquidum side of 
fig. 5. 
V>v We now suppose the saturation line of fig. 6 to be 
introduced also in fig. 5; to begin with we assume the point /” of 
the saturation surface to be far below the liquidum side. All points 
of the section of both surfaces now represent liquids saturated with 
solid # and in equilibrium with vapour, consequently the system 
F4 L+G. As the points of tne section all appertain to the same 
temperature, this section is therefore tie previously recorded satu- 
ration line of the solid substance /’ under its own vapour pressure. 
If we project this section on the component triangle we obtain a 
curve surrounding point # like the drawn curves in fig. 7 (I) or 
fig. 11 (1). It is also evident that the pressure must increase in the 
direction of the arrows of these figures. We now again imagine in 
fig. 5 the section of liquidum surface and saturation surface ; with each 
point of this section corresponds a definite point of the vapour surface. 
On the vapour surface is situated, therefore, a curve indicating the 
vapours in equilibrium with the solutions saturated with /’; this 
curve is the vapour line appertaining to the saturaton line under 
its own vapour pressure. If this curve is prciected on the component 
triangle we obtain a curve surrounding point / such as the dotted 
curve of Hes. „LN or: da): 
If the temperature is increased the liquidum, gas, and saturation 
surfaces of /’ move upwards; as the latter surface, however, moves 
more rapidly than the first, there occurs a temperature where £ 
falls on the liquidum surface so that the solid substance /’ is in 
equilibrium with a liquid of the same composition and with a vapour. 
Like van per Waars in the binary systems, we may call this tem- 
perature the minimum melting point of /. 
As the plane of contact imtreduced in F at the saturation surface 
is horizontal, the saturation surface must intersect the liquidum surface. 
We notice that this section proceeds from /” towards the direc- 
tion of the vapour surface. If we project this curve on the com- 
ponent triangle we obtain the curve afb of figs. 1 or 2. The curves 
de or de’ of these figures are the sections of the plane of contact 
in F at the saturation surface with the liquidum side; they are 
consequently the liquidum ‘lines of the heterogeneous region LG 
at this minimum melting point of the substance /’. 
