( 913 ) 
unstability are always further apart for the real mixture than those 
for the mixture taken as homogeneous, except for the case w, = u’, 
in which case they coincide. So long, therefore, as not a single 
mixture has been found with a maximum plaitpoint temperature, it 
is unnecessary to examine the different cases for the course of the 
isobars, with which a maximum 7), might be accompanied. 
In § 10 we have mentioned a number of points in which the 
supposition that 6 is a quadratic function of w is distinguished from 
the supposition of a linear function. After the foregoing it will, 
however, be easy to apply in the given figures for the course of 
the isobars the comparatively slight modifications which would be 
the consequence of this second supposition. 
Physics. — “On the phenomena of condensation for mixtures of 
carbonic acid and urethane, in connection with double retrograde 
condensation.” By Prof. Pu. Konxstamm and J. Cnr. Reepers. 
(Communicated by Prof. J. D. van per WAALS). 
In these Proceedings of March 27 1909, p. 816 van per Waats 
showed that under certain circumstances the phenomenon of double 
retrograde condensation must occur. We have set ourselves the task 
to test this result of the theory by the experiment. For this purpose 
we chose carbonic acid and urethane as a first system. For according 
to Bicnner') a solution of +°/,*) urethane splits up into two layers 
at 30°.5*), and according to p. 29 loc. cit. the succession of the 
phases is GL,L,‘). So the three-phase-pressure will certainly exist 
1) Thesis for the doctorate, Amsterdam 1905, p. 115. 
2) Percentage by weight according to an oral communication from Dr. BiicHNer. 
3) This temperature depends, of course, on the degree of filling of the sealed tube 
used, and the pressure in the tube which depends on this; it does not imply by 
any means, that a mixture of 4 percentages by weight of urethane could not split 
up below 307.5. Compare the table of three-phase-pressures at the end of this 
communication. 
4) It is not quite clear on what ground BücHNeR arrives at this conclusion; it 
does not follow from his experiments, as il seems to us. But it is undoubtedly 
plausible to assume that the gas phase consists here of almost pure carbonic acid, 
which will really appear to be the case. We may point out in passing that the 
existence of a lower critical point of mixing to which BitcHNER concludes on 
p. 113, is proved neither by BiicHNER’s experiments nor by ours. As this point 
is not to be reached directly (we too always found crystallisation of the urethane), 
this would only be possible by an accurate investigation of the change of x) and 
x3 with the temperature. 
