376 
heat-effect and the change of volume, which attends the transfor- 
mation (1). If we first consider the denominator, we see that Vs, 
and V., are both negative and differ little. And as further x,— 
—% > Leo dg, We see immediately that the denominator will be 
positive. 
About the numerator we know that it is negative in 6, so that 
8 3 4 d 
it follows from the sign of numerator and denominator, that 7 or 
is negative, and that the three-phase pressure will descend with 
rise of temperature, at least in the neighbourhood of 5. 
With a view to the further discussion it is desirable to examine 
the numerator somewhat more closely. W,,, and Ws,, are the quan- 
tities of heat which are developed when a gr. mol. of S, resp. S, 
evaporates in an infinitely large quantity of the coexisting vapour 
phase. We can divide both quantities into two others, viz. into a 
molecular heat of evaporation and a molecular differential heat of 
mixing e.g. 
W sg = (Ws), = Ws, 9 
The heat of evaporation (Ws), is negative. If now we further 
assume that the formation of FeC is endothermic at 4, so 
C + Fe FC — a Cal, 
which is more probable, the heat of mixing W, , will also be nega- 
wie 
tive, so that Ws, is also negative then. 
For Ws, we may write: 
Weg == (Wade + Was 
The molecular heat of evaporation (Ws,,), is again negative. The 
differential heat of mixing WV, , will consist almost exclusively in 
the heat effect of the conversion: 
eC—Fe-+C-+aCal 
which as has been indicated here, is positive at 6, so that Wag 
can be also positive, and Ws, negative or even positive. So we see 
from this how it is possible here that notwithstanding the fraction 
UG ntt : : . . 
eet 1 the quantity Ws, predominates in equation (I), so that 
Sees es, 
the numerator is negative. 
It is now clear that when on rise of temperature the heat of 
formation of FeC becomes smaller negative in the gas phase, and 
finally passing through zero, assumes a positive value, the negative 
