875 
As the solid substance /’ and the liquid now have the same com- 
position we have attained the melting point of #. If now we regulate 
the temperature and pressure in such a manner that the solid matter 
F remains in equilibrium with its melt the system proceeds along 
the melting point line #d of fig. 3. Here, it has been assumed that 
the volume v of the solid substance is much smaller than the volume 
V of its melt. If this is not the case, the melting point line FJ starts 
from / towards lower temperatures. In binary systems, VAN DER 
Waars has called the initial point / of the melting point line, the 
minimum melting point of the solid substance /. 
Hence, we have forced the substance /’ to proceed along: 
a. the sublimation line aA 
6. the three phase line AF 
c. the melting point line 4d 
we can, however, consider still other lines. 
In the upper sublimation point A we have solid # + vapour F+ 
infinitely little liquid. We now increase the volume until the solid 
substance /# has been converted totally into vapour, or else we 
remove the solid substance. We then have the system: vapour + 
infinitely little liquid or we may also say, a vapour F which can 
be in equilibrium with a liquid. If the temperature is increased the 
vapour /’ will continue to exist; it is then, however, no longer in 
equilibrium with liquid. In order, to again form an infinitely small 
quantity of liquid, or in other words to again bring the vapour in 
equilibrium with a liquid, it will generally be necessary to increase 
the pressure. 
Hence,. at an increase in temperature, one can always regulate 
the pressure in such a manner that a vapour of the composition # 
is in equilibrium with an infinitely small quantity of liquid which, 
of course, changes its composition with’ the temperature. If pressure 
and temperature are represented in fig. 2, the curve Kf of this figure 
is formed. | 
In the minimum melting point /’ we can start from the system 
solid F+ liquid + infinitely little vapour after we have first 
eliminated the solid substance /’ thereof. If now, we elevate the 
temperature, the pressure may be always regulated in such a manner 
that this liquid of the composition # is in equilibrium with an 
infinitely small quantity of vapour which, of course, changes its 
composition with the temperature. The corresponding P7-line is 
represented in fig. 3 by the curve Fe. 
As, on the line ef, a liquid of the composition Fis in equilibrium 
with vapour we will call this line the evaporation line of #. On 
