( 235 ) 
saturation .curves, i. e. curves which would remain entirely inside the 
metastable region, so at pressures lower than the vapour pressure 
of the mixture (negative pressures); finally to the opinion of many 
other authors, according to whom the existence of a lower critical 
point would often, v if not always, be the proof of a chemical action 
between the components (hydration), or of the presence of a third 
component*). 
Kuenen (loc. cit.) has carried this investigation an important step 
forward by drawing attention to the fact that for the consideration 
of the different types of saturation curves, the vapour phase must 
necessarily be taken into account, as well as the two liquid phases. 
This enabled Buchner (loc. cit.) to give a very interesting classifi¬ 
cation of the different types, which, however, is still incomplete. 
Starting from van der Waals’s theory, van Laar*) has finally 
tried to give an analytical solution of the problem in a long series 
of papers. His results are of importance particularly because they 
showed that under certain circumstances non-miscibility can occur 
for perfectly normal substances, a fact which was generally doubted 
at the time 3 ). On the other hand a serious objection to these in¬ 
vestigations is that starting from a special case, they represent it as 
the general one, and therefore must pronounce as abnormal a type 
occurring for normal substances, while they overlook a great many 
others, and further that they, very onesidedly, lay the stress on the 
existence of open plaits (not closed on the side v = b), a circum¬ 
stance, which by no means can be considered as a result, as it 
immediately follows from the arbitrary, if not erroneous suppo¬ 
sition of the linear dependence of b and x, as van der Waals had 
already shown in the Theorie Moleculaire. For the case at last that 
one of the components is abnormal, van Laar arrives at saturation 
lines of very complicated form, which the experiment has never 
found as yet. We hope to return to this subject later on in connection 
with further experiments 4 ). 
3. Investigation by van der Waals. 1 ) 
At last van der Waals resumed the investigation of the 
v ) Wl. Dolgolenko, Zeitschrift fur physikalische Chemie. 62, 498, 1908. 
2 ) J. j. van! Laar, These Proe. 1905 and 1906. 
s ) R. A. Lehfeldt, Philosophical Magazine, 47, 284, 1899. 
Van der Waals, Continuitat II § 12. 
*1 In this survey we pass Keesom and Kamerungh Onnes’ investigations by, as 
these refer to a range of temperature altogether different from that of our 
experiments. 
_ “) J. D. van der Waals, Archives neerlandaises, (2), 13, 77 and 799, 1908 and 
These Proe. Vol. X and XI (1907—1909; passim. 
