Chemistry. — ''The boiling point. line of the system: hexane-nitro- 

 hencene." By Dr. E. H. Büchner. (Communicated bj Prof. 

 Hou-kman). 



(Communicated in the meeting of June 30, 1917). 



On studjing the experimental data available, it occurred to me 

 that one may easily predict, whether a system of two partially 

 miscible liquids will show a maximum in the vapour pressure curve 

 or not. The following rule, indeed, may be enunciated : when 

 the difference of the boiling points of the two substances is less 

 than 100°, a maximum pressure is found ; when the boiling points 

 differ more than 100°, no maximum occurs in the ;;, .I'-curve. Of 

 course, this limit is not perfectly sharp ; yet, the deviations are 

 remarkably few. 1 have been able to find only three systems ^), which 

 may really' be considered exceptions to the rule, as one observes 

 a maximum, although the boiling points differ from 110° to 120°. 



This made me sufticiently trust in the rule to predict with its 

 help the behaviour of systems not yet investigated, and to expect, 

 for instance, in the system n-hexane-nitrobenzene (boiling points resp. 

 69^ and 210°) a p, .«-curve, continually descending from the hexane 

 side. This is particularly interesting, because a research by Kohnstamm 

 and Timmermans"), in connexion with a rule given by van der Waals, 

 would lead to the conclusion that the p, .f-curve must have a 

 maximum. 



These authors, indeed, reckon the systems, which consist of nitro- 

 benzene and a hydrocarbon, among the group : "splitting up of the 

 plait", whereas van der, Waals has shown that splitting up of a 

 plait is only possible, if a minimum critical temperature occurs in 

 the system (which is equivalent to a maximum in the vapour 

 pressure curve). Kohnstamm and Timmermans, it is true, expressed 

 themselves with some i-eserve. While they were able to observe 

 experimentally the minimum temperature in the plaitpoint curve of 

 the system decane-nitrobenzene, they were prevented in doing the 

 same for hexaue-nitrobenzene by the appearance of solid nitrobenzene. 



1) Cf. Bakhuis Roozeboom, Heterogene Gleichgewichte, zweites Heft, II (in the 

 press). 



2) These Proceedings, 15, November 1912. 



