405 



phase line is so small that it is smaller than the errors of observation. 

 Then we shall be compelled to class such a system with type 2. 

 And that the chance to such a situation may not be so slight as 

 might be supposed at first sight, appears from my investigation of 

 the system ether-water, where two such transitions are found. In 

 this system the distance between the points Q^ and A (see fig. 43 

 of Prof. VAN DER Waals' 14^'> contribution) is so small that I could 

 not possibly decide whether the minimum of the plaitpoint curve 

 occurs in the figure, lies on the axis, or would be found just outside 

 the figure. Besides, it appeared that in the critical endpoint the slopes 

 in the P-T projection of the three-phase line and the plaitpoint curve 

 differed so little that practically we have to do with a transition 

 case here too. 



Of type 1 no example was known as yet. In the 17^'^ contribution 

 Prof. VAN DER Waals has only shortly alluded to this case, and in 

 fig. 51 he has given a P-T projection which corresponds with this 

 case, but omitted a full discussion, particularlj^ also because this 

 case is attended with great complications. I have, however, now 

 succeeded in finding an example of this type in the system hexane- 

 water, the particularities of which I wish to describe in the following 

 pages. 



2. Without further purification the vapour pressure line was 

 determined with Kahlbaum's synthetically prepared hexane, and the 

 three-phase line with a great quantity of water (47 mol. 7^ H^O). 

 For the filling of the test-tube, just as for the following investigation, 

 the apparatus was used which I described in my paper on the 

 system ether-water ^). The following values were found for the hexane 

 tk = 235,3, Pk = 30,1 atm., for the critical endpoint it =: 222,3, 

 Pi^ = 51,95. It appeared from these data that the three-phase pressure 

 lies higher than the vapour tension of the two components (at the 

 same temperature], and that therefore most probably the concentration 

 of the vapour lies between that of the coexisting liquid phases. 

 Whether, however, we had to do here with type 1 or 3, could not 

 be decided from these observations. 



Before proceeding to an accurate investigation I purified the 

 hexane in the following way. A fractionation of the hexane yielded 

 a fairly large middle fraction of the boiling point 68.9° (range 0.1) ; 

 ^^=234.55; Pt = 30.1. Accordingly only the critical temperature 

 had been appreciably changed by this fractionation. 



Then the hexane was heated with reflux bv the side of sodium 



') Those Proc Vol. XV p. 382, 383 and fig. 1. 



