638 Prof. J. P. Kuenen on the 



criticiil point, or, in other words, the critical temperature was 

 raised by pressure. It is probably not wholly superfluous in 

 this connexion to emphasize the fact that what is usually 

 called critical temperature or mixing temperature for the 

 liquids is in reahty this temperature under the pressure of 

 the saturated vapour, i. e. in the presence of the vapour phase ; 

 at other pressures the critical temperature as well as the 

 composition and the density of the critical phase must be 

 different. 



In the investigation of the influence of pressure in cases 

 where an upper critical temperature occurs, such as that of 

 hexane and methyl-alcohol, we were anticipated by van der 

 Lee *, who investigated mixtures of phenol and water ; he 

 found that with these mixtures, just above the temperature at 

 which the two liquid layers mix^ increase of pressure over and 

 above the vapour-pressure brings the two layers back, i. e. the 

 critical temperature was again raised by pressure. 



As was pointed out in our previous paper f, mixtures often 

 possess a maximum vapour-pressure (minimum boiling-point) 

 beyond the critical point of the liquid layers. Phenol and 

 water have this property, as shown by their boiling-point, and 

 van der Lee, as well as Lehfeldt J, confirmed this by measure- 

 , ments of the vapour-pressures ; but it was afterwards found 

 by Schreinemakers § that in this case the maximum already 

 exists below the critical point amongst those mixtures which 

 contain more water than those which take part in the three- 

 phase equilibrium. We are not aware of other experimental 

 contributions to the subject having appeared since our former 

 paper. 



a. Etliane and Methyl-alcohol. 



First of all, the iuAestigation of mixtures of ethane and 

 methyl-alcohol had to be completed ; this combination had 

 been found to differ from those with the higher alcohols in 

 that no lower critical point for the liquids exists, and this 

 difference involves the further one, that the character of one 

 branch of the critical curve in the pressure-temperature 

 diagram remained uncertain, and had to be separately investi- 

 gated. Referring again to the mixtures with the higher 



* Van der Lee, Dissertation Amst. 1898 ; Zeitsch. FhysiJc. Chemie, 

 vol. xxxiii. p. 622. 



t Kuenen & liobson, I. c. p. 184. For a full discussion of this im- 

 portant point see van der Lee's paper, I. c, and a paper b}'' the present 

 "writer in the Proceedings of the Kon. Ak. van Wetensch. Amsterdam, 

 October 1903. 



t Lehfeldt, Phil. Mag (5) vol. xlvii. p. 289. 



§ Schreinemakers, Zeitsch. Physih, Chemie, vol. xxxv. p. 461. 



