( 294 ) 
two normal substances, especially, in how far from an empirically 
correct representation of the isothermals (and thus of the w-lines), at 
different temperatures for a simple substance we can find the w-lines 
for mixtures of different composition 2 at one temperature, and 
hence also the unstable part of the w-surface. In Communication 
N°. 595 (in collaboration with Retncanum) this method appeared 
permissible even with entirely arbitrary mixtures of carbon dioxide 
and methylchloride; also when the mixtures under observation differ 
only a little in composition, as when investigating the critical 
phenomena in a normal substance with small impurities, it promised 
to be very useful. For a long time (comp. Communication N°. 68, 
Proceedings March 1901 p. 630) investigations for this work have 
been made at the Leiden Laboratory, in which I have taken part 
by making some measurements. As the first part of them we may 
consider WERSCHAFFELT’s measurements on the conduct of mixtures 
of carbon dioxide and hydrogen, in so far as they concern mixtures 
with a small proportion of the latter substance. They show, as 
VERSCHAFFELT!) has demonstrated in Communication N°. 65 that 
the law of corresponding states indeed is nearly applicable to these 
mixtures, and that by means of the said law, the critical tempera- 
ture and pressure of the homogeneous mixtures can be determined. 
In the following pages we will examine in the first place what 
may be derived from VAN DER WAALS’ investigation of the mixtures, 
by the aid of the law of corresponding states, about the course 
in the pZ-diagram of the plait-point curve of binary mixtures in 
the case that the proportion of one of the components of the 
mixture is very small. Then the formulae obtained are compared 
with VERSCHAFFELT’s observations. Lastly by means of the law of 
corresponding states I have derived an expression for the variation 
of the plait-point volume with the composition. 
§ 2. As a starting point some formulae given by VAN DER WAALS 
in the Proceedings May and June 1895, Arch. Néerl. t. XXX 
p. 266 and 278 are used, which formulae may be also derived using 
the property that in the plait-point: 
90° 
ED, 
(Gt) 2 
where ¢=wy-+ pv, if w represents the free energy. 
1) Arch. d. Se. Néerl. (2) t. V p. 644. Comm. Phys. Lab. Leiden n°. 65. 
