1055 
the given method. The material of observation recorded in the tables 
gives rise to the following remarks. 
1. The systems formed by nitrobenzene with a hydroea-bon are 
not simple cases of retreat as we thought at first *); on the con- 
trary they represent cases in which the plait is split up, and belong 
to case 115, in reference to which we had to state in 1909 that we 
had not found an example of it for normal substances, and about 
whose possibility for abnormal substances we then ventured to 
pronounce an opinion only with the greatest reservation *). 
This conclusion appears with perfect certainty for the system 
nitrobenzene + decane; the critical end-point (meeting of three- 
phase line and plaitpoint line) lies here on a branch of the plaitpoint 
dt 
line with negative ; if the plaitpoint line is pursued further, 
dp dp 
becomes zero, and then positive. Accordingly the plaitpoint line 
passes through a minimum, and this minimum is experimentally 
realisable: the branchplait exhibits a point where the closed portion 
gets detached, a homogeneous double plaitpoint, and this lies in the 
absolutely stabie region. The question proposed p. 409 of the Lehr- 
buch der Thermodynamik *) has, therefore, been answered in the 
affirmative by experiment. 
If in this connection the closely related system nitrobenzene-hexane 
is considered, it appears that it is only owing to an accessory cir- 
cumstance that the homogeneous double plaitpoint cannot be realized. 
dt | ‘ 2 5 
For here too a is negative in the critical end-point, but this negative 
value becomes smaller and smaller; but just before it has become 
zero, further investigation is prevented by the appearance of the 
solid phase. So the plaitpoint line becomes inetastable by its meeting 
with the threephase line solid + two liquid phases; we have again 
a eritical end-point, but now the critical point of two “ saturated 
solutions’’. 
The system petroleum + nitrobenzene — if we may compare it 
for a moment in this connection with a binary system — _no 
more belongs to type 115, but to type Ha. In the critical end-point 
dt. ae Eer ete 
— is here positive, the plaitpoint line is intersected by the three-phrase 
ap 
line above the homogeneous double plaitpoint; hence the latter falls 
1) These Proc. XIL p. 243 and table at p. 239. 
2) loc. cit. p. 243. 
8) Van DER WAALS KoHNsTAMM, Vol. Il. Leipzig, Barth. 1912. 
