( 297 ) 
MRT; 
C4 = ’ 
Pk Vk 
which two equations also give immediately the slope of the plait-point 
curve in the 7p-diagram at the critical point of the pure substance. 
Hence we see that the variation of critical plait-point temperature 
and pressure due to the presence of small admixtures is entirely 
determined by the two quantities @ and /. 
From these two equations follows also that for very small values 
of z: 
Ppl — Prk _ (3 
ToT (at), i 6. ey) 
dT 
substance at the critical point. In this way we again find the relation, 
given by VAN DER WAALS Proceedings Nov. ’97, p. 298. 
If therefore we connect in the pZ-diagram the critical points of 
the homogeneous mixtures with the plait-points of these mixtures, 
these connecting lines at the critical point of the pure substance 
are parallel to the vapourpressure curve of the latter. 
dp. . d 
if Ga) is the. for the saturated-vapourpressure curve of the simple 
k 
§ 3. In order to be able to compare these formulae with observations 
2 
on mixtures (=) and Cl a ) are required. 
dz dw OT 
(2) can be determined in two ways. For according to a thesis 
T 
oo : d 
of VAN DER WAALS = at the critical point = le , where 
‘Da coér 
Tcoëz represents the maximum vapourpressure. Prof. VAN DER WAALS 
was so kind as to communicate to me the following proofs for this 
thesis, as develuped many years ago in his lectures. 
1. MAXwWELL’s criterium at a coéxistence pressure independent 
of the volume is given by 
vd 
Peoër (vd — Vv) = 13 p dv, 
Vy 
where v, and vg refer respectively to liquid and vapour. By differen- 
tiating this with regard to 7, we have 
