( 
) 
Q — ~ (Qm)d 4* Q r + ( Qn)B 
{Q>h)a = differential heat of mixing of A 
,, „ ,, „ ,, B 
Qr= mol. heat of transformation. 
It is of importance to point out here that as [Q m ) A and (&»)/? have 
a different sign, the possibility exists that Q has another sign than 
Q r ; this might e.g. occur when Qr was very small, and then we 
should have the special case that e.g. when Q was negative and 
Q positive, the equilibrium shifted from B to A with rise of tem¬ 
perature, whereas the transformation of A into B is endothermic | 
in itself; this, however, will only rarely occur. 
If we drop this question for the present, it is noteworthy that in 
the point S\ immixing occurs, another solid phase S\ appearing 
by the side of S\. Two cases may be distinguished here. 
Generally the newly-formed solid phase S\ will possess another 
form of crystal than S \, but it is possible that the two solid phases 
are isoraorphous, for as is known, also isomorphous substances can 
show partial miscibility; if this latter, the simplest case occursJ 
the heat of transformation will be the sum of a heat of unmixing -! 
a heat of transformation, and a heat of mixing 1 ), another thermal 
quantity being added to this, viz. that which accompanies the change 1 
of crystalline form, when S\ and S\ are not isomorphous. 
If we now follow the inner equilibria above the transition-poutjl 
it is to be expected that the curves S\ q for the solid-, and l 2 k for 
the liquid inner equilibria will have the same direction as S\ S' , j 
as is also assumed in fig. 1. 
Soch, however, has found in his investigation of benzile-orthocdrbonjjM 
acid that the curve of the inner liquid equilibrium meets the melting- ^ 
point curve of the modihcation with the highest melting point viz. | 
By and runs to the ^4-side for higher temperatures. Further he found ‘i 
that at 65° A passes into B, and combining these two facts, he 4 
arrives at the conclusion that the thermal sign of the transformation 1 
A-> B 
must have been reversed between the point of transition and the 
unary melting-point (137°). 
When the pseudo-binary T ,^-figure for this substance agrees with 
fig. 1, which is still an open question, we must of course come to 
the same result also going by this theory, but I will point out here 
that this conclusion is not yet imperative at this moment, because 
though it is not probable, the possibility exists that the mixed crystal 
l > 1 sha]1 discuss this the before-mentioned splitting up more fully later on 
