1342 
The line G,, Gu represents the internal equilibrium in the 
gas phase. At G, this line meets the vapour line aG of the pseudo- 
binary system, from which follows that when arrived at G, we 
compress the gas further, and no metastable states occur, the mixed 
crystal phase S,, will deposit, which just as the gas phase Gy, in 
case of behaviour as a unary substance, will be in internal equilibrium. 
When all the gas has been removed by pressure, a further com- 
pression will Jead to an increase of pressure, and now the internal 
equilibrium in the solid phase can be indicated by the line Sy,S’a,- 
If, however, when we have arrived at G,. the solid phase Sy, 
fails to appear on further compression, then the metastable prolon- 
gation of the vapour line bG will be reached at G’,, which means 
that when no further retardations make their appearance, on further 
compression the metastable solid phase 5,, 
case Of unary behaviour, will likewise be in internal equilibrium. 
When the vapour has quite disappeared, further compression 
will give rise to an increase of pressure, and the internal equilibrium 
in the metastable substance will be displaced along the line 5,,5/q,- 
will appear, which in 
At the temperature chosen here the unary solid phase Sy, remains 
stable under all pressures, and S,, metastable. so that we may say 
that our system presents monotropy at this temperature. 
Just as it happens that a system is monotropic under the vapour 
pressure, and enantiotropic under higher pressure, a system that 
shows monotropy at lower pressure, can be enantiotropic at higher 
temperatures. 
In the case under consideration, where the phenomenon of enan- 
tiotropy has been assumed under the vapour pressure, this is actually 
the case. If we namely choose f, as second temperature (see Fig. 1), 
we get a section as is indicated in Fig. 3. 
Noteworthy is the change in situation of the unary system with 
respect to the pseudo binary system on increase of temperature. 
The line for the internal equilibrium in the vapour has shifted to 
the right with respect to the pseudo binary figure, which causes the 
meeting with the pseudo binary figure, which first took place on 
the vapour branch aG, to take place on the vapour branch bG. At 
an intermediate temperature the intermediate position must of course 
be reached, in which the just-mentioned meeting took place exactly 
in the point G, G, and G therefore coinciding. 
Hence at this temperature coexistence will be found in the unary 
system between two solid phases and a vapour phase, all three in 
internal equilibrium, i.e. this temperature will be that of the transition 
point under the vapour pressure. 
