by the relation: 
AP We 
fie = —_— 
MEER 
A W is the quantity of heat which must be supplied, A V the change 
in volume occurring when, between the phases in equilibrium at a 
constant Tand P, a reaction takes place in the one or in the other 
direction. 
Let us first consider the sublimation curve a" D. For each of the 
reactions mentioned sub 1-—3 taken in such a direction that vapour 
is formed, AW and AV are positive. 
From (1) it thus follows, as drawn in fig. 1, that, at an elevation 
of temperature, the sublimation curve must proceed towards higher 
pressures. The point D lies as well on the sublimation- as on the 
_four-phase curve. As, however, in this point JD, the quantity of 
liquid of the four-phase equilibrium is still but infinitesimal, 4 W 
and AV are the same for both systems so that the two curves 
(1) 
must meet in D. 
Let us now consider the melting point line Sd". We take each 
of the reactions mentioned sub 1—8 in such a direction that 
liquid is formed so that AW is positive. At the congruent and 
incongruent fusion of /-+ /” AV may, however, be positive as 
well as negative. The melting point line can therefore, proceed from 
S towards the right as well as to the left; in fig. 1 the first case 
has been drawn. The fact that the melting point line and the four- 
phase line meet each other in S follows in the same manner as 
that given above for the meeting of the two curves in D. 
In order to deduce formula (1) for the sublimation or the melting 
point curve, we consider the equilibrium 4’ + 7” + G or F4 HL, 
We represent the composition of /’ by «,9, that of L” by a’, B!, 
that of Z or G by «x, y. We call the volumina of these phases 
v, v and JV, the entropies 4, 7 and H, the thermodynamic poten- 
tials &, 5 and Z. 
- 
As F and fF” are in equilibrium with ZL (G) we have: 
: WZ 0Z 
Z — («—a) ee (y—)) a IP 
. _0Z grs 
Zele) erin So. Mae (5) 
From the condition that the three points /, #” and LZ (G) are 
situated on a straight line, follows: 
(ea) (y= yn) ee} 
