( 688 ) 
We get into the unstable region when keeping « and y constant, 
and making v increase. For such a point is in consequence 
2 < 0. Differentiating f according to p we get: 
GED oe eGo 05 05 7 or Oe 
op dv Oy? pda? da® dpdy? dady Apderdy 
Oye: } Ov. : : 
and as a, is negative, and > is also negative, we may write: 
U 
Pp 
05 „do dl „do 05 / dv 
(5 2) + ( ge 2 aac) Oe 
dy? de?) dz? dy? dxdy \dedy) 
Here we have to take into account, that ie v. 
fe 
If we have such a point in the limiting surface between the 
unstable and stable phases, one set of variations of v, randy exists 
RAE Ee : 05 
which is of special importance, viz. that for which dp and d= are 
Ow 
equal to zero; so that, for which: 
0 io En Ow 
da — 0 
De he 
and 
BE 
De dx ety oe =— 0 
Then also 
07g 95 
— dy = 0 
Oxdy nl dy? 7 
or a == 10, 
as follows from the condition f= 0. 
For ae set of variations we have: 
eH ae +o +55 — de hatin dy? + 2 ideen en iydo a5 dedy — 0. 
For ia other set the value of the first member of this equation 
is positive. This set of variations of v and « is for a binary mixture 
given by a line on the w-surface, which has the tangent plane in 
a point of the spinodal curve in common with the y-surface at least 
for an element. There it is indicated by an isobar, i.e. by a curve 
p= C. For we have in this can 
dw 
sa Ede +5 ee „dr = 0. 
From this and from the satin of the spinodal curve follows 
ow 
ado do Gp we 0. 
