323 



They only ascertained that tlie value of -y- is negative, continuallv 



dp 



decreasing, however, in absolute amount: it fails namely from — 0°,0164 

 to — 0°,0031. In analogy with decane-nitrobenzene and petroleum- 

 nitrobenzene ^) they concluded the system hexane-nitrobenzene also 

 to belong to the type : splitting up of the plait. Since, however, 

 according to van der Waals, this goes necessarily together with the 

 existence of a maximum pressure, we get obviously in defiance of 

 the rule enunciated above, which requires no maximum. One of the 

 three statements, therefore, must be wrong ; I thought it, accordingly, 

 of some interest to investigate experimentally, if a maximum exists 

 or not. 



Instead of the vapour pressure curve one may as well determine 

 the boiling point line, which is more convenient. The question then 

 becomes whether the boiling line exhibits a minimum or not. 



The determinations were carried out in Professor Smits' apparatus, 

 formerly described *) ; the substances used were carefully dried and 

 fractionated. The results of the measurements are joined in the table 

 below, and represented moreover in the figure. 



As appears most clearly from the figure, there is no question of a 

 minimum ; the curve shows, on the contrary, the shape, often found in 

 systems with limited miscibility above the critical solution point '). 

 A comparatively quick rise of short duration is followed by a very 

 slow increase of the boiling temperatures over an extensive area *) ; 

 an inflexion point occurs and tinally the curve rises very steeply 

 to the boiling point of pure nitrobenzene. 



This result fully agrees with my expectation ; the question now 

 arises how Kohnstamm and Timmermans' determinations may be 

 made to tally with it. It seems to me most likely that the system 

 belongs to the type "retrogression", as the authors mentioned 

 previously believed themselves also. We have, then, onlj to remark 

 the particularity that the plaitpoint curve does not ascend regularly 

 from the critical endpoint, but rises in the beginning slowly, after- 

 wards more steeply. Thereagainst no objection, indeed, can be 

 raised; although in the single system that is fully investigated 

 (metbylethylcetone-water), a uniform rise is observed, the known 



») For this system too, one gels in conflict with my rule ; but it must not 

 be forgotten, that petroleum itself is already a mixture, and that we have, thus, 

 no binary system. 



2) These Proceedings, April 1917. 



3; In our system 19.2°: Timmermans, Zeitschr. phys. Chem. 58, 186, 1008. 



*) Nearly coinciding with the region of limited miscibility. 



