839 



alwa.ys at tlie same teiiiperatiires ; it' a slight error should occur in 

 the absolute value of tlie given temperatures, this has no intluence 

 on the pressure differences. The tiiermometers which I used in this 

 investio-ation, have been tested bv a resistance thermometer, tiie 

 resistance of which was determined for boiliiig water, naphthaline, 

 benzophenon, and sulphur. The obtained results are given in table 4, 

 the pressures are gi\en in atmospheres. 



It appears from the last column of table 4 that the difference at 

 150° to 200° is only slightly greater than the errors of observation, 

 that the three-phase tension becomes appreciably greater than the 

 sum of the vapour tensions at 2 HP, and that this difference rapidly 

 increases with ascending temperature. 



4. When we combine the results of the system hexane-water 

 and those of § 2 and 3, it appears that in the three systems the 

 three-phase tension is always greater than the sum of the vapour 

 tensions of the components in the neighbourhood of the critical 

 endpoint. Moreover these three systems present the same shape of 

 the plaitpointline in the 7^-.i-projection ; the ui)per critical endpoint 

 always lies lowei' tha)i the critical points of the two components; 

 the plaitpoint line presents therefore a minimum temperatui-e in the 

 2-.i'-projection (homogeneous double plaitpoint). Though in my opinion 

 it is probable that the systems will behave perfectly analogously, a 

 further in\estigation would have to decide whether for all this 

 homogeneous double plaitpoint lies in the metastable region ; 1 have 

 shown this for the system hexane-water in my cited paper. It is 

 remarkable that in the system ether-water the homogeneous double 

 plaitpoint appears in the immediate neighbourhood of the ether axis 

 or would perhaps lie outside the figure, so that the critical endpoint 

 in contrast with the above discussed systems lies between the critical 

 points of the components. In this system the said peculiarity does 

 not occur. Accordingly I think 1 have to conclude that the systems 

 which present critical endpoints which lie lower than the critical 

 temperatures of the two components possess three-phase pressures 

 which are higher at high temperatures than the sum of the vapour 

 tensions of the pure substances, whereas the opposite is the Case for 

 systems for which the critical endpoint lies between the critical 

 temperatures of the components. Perhaps this conclusion may contribute 

 to account for this remarkable phenomenon. 



^^ ,,,, , Anon/. Cheni. Laboratoru of the 



June 2o, J 914. / ^ a ', 



Universiti/ oj Amsterdam, 



