( 449 ) 
1897. pag. 212 and Arch. Néerl. Série IT Tome II pag. 71), viz. 
BURDA vgl 2 = y) + weg + y ogy} + 
a, 
+} pe MRT bog (0 — by) — “4 de + By HC. 
In this equation we must think v as being eliminated by means 
of the equation of state, and therefore determined by p, 7,2 and y. 
I have previously pointed out l.c. pag. 69, that such a ¢-surface 
can generally consist of three sheets. These three sheets might be 
distinguished by calling them: liquid sheet, vapour sheet and sheet 
for the unstable state. 
Strictly it is not necessary from an experimental point of view 
to know all these sheets and the way in which they cohere, for 
only the conditions represented by the lowest sheet are stable. The 
others are unstable or metastable, and can therefore not be realized 
or only as phenomena of retardation. But it has been proved even 
for a simple substance that for the laws of coexistence the knowledge 
of unstable conditions is required. ‘Think e.g. of the criterium of 
MAXWELL for the determination of the pressure of coexistence. For 
a binary mixture the knowledge of the plait on the w-surface is 
necessary for the deduction of the critical phenomena. In all these 
cases the connection of what may be realized on one side of a 
certain limit and that which may be realized on the other side of 
another limit can only be fully grasped when also the conditions 
that cannot be realized are known — so when we assume continuity. 
So we can only make the full use of the function ¢ for a binary 
mixture, when we know the connection of the three before mentioned 
sheets and the shape of the ¢-surface also for the metastable and 
unstable phases. For the w-surface of a binary mixture the unstable 
and metastable part appeared to form a plait in the surface, which 
was for the rest convex-convex (seen from below). A plane section 
through this plait brought a convex part of a curve in connection 
with a convex part lying on the other side of the plait through a 
curve whose course was continuous, and which did not show any 
complication except two points of inflection. That this will not 
generally be the case for the ¢-sheets, and that much greater com- 
plications may be expected there, might already be anticipated by 
the shape of ¢ of a simple substance, as occurs in fig. (1) p. 4 of 
