( 307 ) 
a \ A 
pr = (n—1) fu — eu’ — uws + B+ rik 
This last equation yields the results we have obtained, in a still 
log 
simpler way than that which we have made use of originally. It 
has moreover the advantage, that the usual signification of « and u, 
as if is established in the theory of a binary system, may be kept 
unchanged. 
Reactions like that of acetaldehyde and paraldehyde, reactions 
which we can bring about at pleasure by means of a eatalyzer and 
in which the composition may be determined experimentally are of 
course of the highest importance for the investigation of the course 
of the molecular transformation. For reactions as that of acetic acid 
the density is the only criterion for the degree of the transformation ; 
and this criterion fails as soon as we work in circumstances in which 
the deviations from the laws for the perfect gases are considerable. 
The experimental investigation will therefore not be able to prove 
the occurrence of a minimum in the number of the double molecules 
in the saturated vapour of acetic acid. At the temperature at 
which the theory predicts that minimum and which lies probably 
between 0.8 7, and 0.9 7, the density of the saturated vapour 
is already so great that it is nearly impossible to deduce reliable 
conclusions concerning the course of the transformation. 
Physics. — “Critical phenomena in partially miscible liquids.” By 
Prof. J. D. van DER WAaLs. 
I have read with great interest the communication of Prof. KupNEN 
under the above title, which occurs in the Proceedings of the previous 
session, and it induces me to draw attention to the following con- 
siderations. 
In my paper of March 25 1899 I started from the thought, 
that the series of plaitpoints, which may occur at different tempe- 
ratures, whether we arrange them to a plaitpoint curve or assign 
a place to them in the 2, v plane, must form one or more continuous 
curves — of course continuous in the mathematical sense. 
When therefore the experiment yielded, e.g. for ethane and ethyl- 
alcohol two separate plaitpoint curves, I have connected them by 
means of a theoretical part. 
If we wish to connect the two pieces of curves found to one 
curve, we may perform this in two simple ways. In thefirst 
place we may connect them in such a way that the curve is con- 
