874 
the line Kf a vapour of the composition /’ is in equilibrium with 
liquid ; we will, therefore, call Af the condensation line of #. The 
metastable prolongations of /e and Af are represented in the figure 
by Fe’ and Kf’. Hence, in point / three curves coincide namely, 
the melting point line (FJ), the evaporation line (/’e) and the three- 
phase line (AA); in point A’ three curves also meet, namely, the 
sublimation line (aA), the three-phase line (A) and the conden- 
sation line (Af). 
The metastable prolongations of the sublimation line aA and 
the melting point line df intersect in a point S; at this temperature 
Ts and pressure Ps now occurs, in a metastable condition, the 
equilibrium : solid # + liquid # + vapour /’. If now the substance 
F behaved as a simple substance which can only yield a liquid and 
a vapour of the same composition, S would represent the triple 
point of the substance /’; owing to the occurrence of the three-phase 
equilibrium AH LG this triple point is, however, metastable 
here. Through this metastable triple point S now also passes, besides 
the sublimation and the melting point curve of £, the evaporation 
line g’Sg of #. This represents the equilibrium liquid # + vapour 
F occurring in the metastable condition; on this curve g/Sg liquid 
and vapour, therefore, have the composition #£ and not, as on /’ Kf, 
only the vapour, and as in e’Fe only the liquid. We will call the 
curve g/Sg the theoretical evaporation line. 
In order to find what conditions of the substance / are represented 
by the points of the different regions we take this substance in a 
condition answering to a point of the sublimation line aA. We then 
have solid £’ + vapour /. From a consideration of what takes place 
on supply or al withdraw of heat, or on increase or decrease in 
volume we now deduce: to the right and below the line aX occurs 
the vapour region, to the left and above the ine aA is found the 
solid region of /. 
Acting in a similar manner with the points of the other lines, 
we find that four regions may be distinguished, namely, a gas region 
indicated in the figure by an encircled G, a solid region indicated 
by an encireled #, a liquidum region indicated by an encircled L 
and a liquidum-gas region indicated by an encircled + G. Hence 
if the substance fis brought to a temperature and under a pressure 
corresponding with a point of the solid vegion, the substance 4’ is 
solid; if brought to a temperature and under a pressure corresponding 
with a point of the liquidum-gas region, /’ is resolved into liquid 
and gas ete. 
We will also consider fig. 3 just once more in connection with the 
