salts or their hydrates. However also phases with variable com- 
position may occur e.g. a vapour, which contains two or more of 
the components, solutions or mixed crystals. 
Let us take in fig. 1 (ID) the simple case that the phases 1, 2, 
3 and 4 have a constant composition, e.g. that they are salts; we 
take a solution for the phase 5. 
Now we take the bivariant equilibrium 235 and we go in fig. 2 (II) 
from the invariant point towards a point of the region 235. As the 
P and T of the equilibrium 235 have changed now, the solution 5 
will obtain, therefore, also another composition; consequently point 5 
alters its place in fig. 1 (II). 
Hence it is apparent that in each point of the region 235 the 
phase 5 has no more the composition, represented by point 5 in 
fig. 1 (ID, but it has another composition; it appears also that this 
composition changes from point to point. Of course the same is also 
true for other phases with changeable composition. Hence it is 
apparent, therefore, that the composition of the changeable phases 
in fig. 2 (II) changes from point to point, generally so much the 
more in proportion as we remove further from the invariant point. 
Only in the invariant point itself, all phases have the same com- 
position as is expressed in fig. 1 (II). 
These changes in the compositions of the phases may also cause 
radical alterations in the partition of the regions. 
Let us take again the case that in fig. 1 (II) only the phase 5 
has a changeable composition. Now we may imagine that in fig. 1 (ID) 
point 5 takes its place on the line 23 e.g. between 2 and 3; then 
between these phases the reaction 2 + 5275 may occur. 
On further change of P and 7’ point 5 may come now within 
the triangle 234. This involves that the reaction between the phase 
changes in some of the monovariant equilibria. 
Let us take as an example the equilibrium (1) = 2434445; 
as long as the point 5 is situated outside triangle 234, the phase- 
reaction in this equilibrium and the partition of the regions are: 
2+3274+5 | 
ea Ck Mae are | 
935 | 345 
As soon as the point 5 comes however within triangle 234, we find : 
243425 | 
EA EA ok 
234 | 245 | 
‚_ 345 
