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differs less, might offer a greater chance to the realisation of the 
said phenomenon. Such systems, however, are pretty rare. For 
“normal” substances such a behaviour will probably seldom occur. 
The system ether-water, on the other hand, which as appears from 
KveNEN’s observations, possesses three-phase equilibria which extend 
to the critical neighbourhood of the ether, satisfies the requirement 
that the vapour under three-phase pressure contains an appreciable 
quantity of the least volatile component, in casu the water, the 
vapour tension of the water amounting to about 14 atms. at the 
critical temperature of ether (critical pressure 36 atms.). That this 
system differs from those used by Rerprrs in this that the three- 
phase pressure lies higher than the pressure of saturation of the two 
components at the same temperature need not interfere with the 
appearance of the phenomenon. Therefore I carried out some expe- 
riments about a year ago with a view of examining whether double 
retrograde condensation can be observed in the system ether-water. 
However, this system too appeared unsuitable for the observation. 
It is true that the critical point of the upper layer lies at a concen- 
tration of about 350 mol. °/, water, and that it is therefore not so 
one-sided as for the mentioned systems of carbonic acid, but an 
altogether different difficulty prevents the observation, viz. the invi- 
sibitity of the lower layer for comparatively small quantities. So 
after some futile attempts I discontinued the observations with this 
system. Hence a direct observation of the phenomenon in question 
has not succeeded as yet, and will, it seems to me, be always 
attended with great experimental difficulties. 
2. Of late attention has been drawn to the system ether-water 
in consequence of an investigation by Prof. Van per Waats. In his 
17th contribution to the theory of binary mixtures VaN per Waars 
discusses this system fully as an example of that series of systems 
for which under three-phase pressure the concentration of the vapour 
phase lies between that of the two coexisting liquids. As far as the 
system ether-water is concerned, this investigation led to a number 
of conclusions, some of which could be experimentally tested through 
the investigations mentioned in § 1, as was already stated by Prof. 
Van DER Waars in the cited paper. As it was, however, of impor- 
tance to examine this system more closely with a view to the 
remaining conclusions, I have taken up again the interrupted in- 
vestigation. In the following pages I intend to discuss the results 
obtained * for so far as they are necessary as a test of the above- 
mentioned conclusions. 
