( 600 ) 
d de 
“P —0 has its tangent parallel to the v-axis, and which is therefore 
VU 
larger than 34. So it would seem to follow from this diagram of 
isobars, in connection with the just-mentioned theorem of VAN DER 
Waats that the possibility of a minimum critical temperature is 
excluded on the supposition a,a,=a’,,. However, already in his 
Théorie Moléculaire van per Waars derived the condition for the 
existence of a minimum critical temperature, viz. : 
nes a a Din 
NEER Se 
Oe b, ce b, 
It is clear that it is easy to satisfy this condition also in the case 
of a?,, =a,d,, e.g. — if we assume’) 2b,, = (6, + 6,) — by the values 
bib, anda; = 3a,’ trom-“whieh B, 20,5; dj, = 4, VS; 80 LEEK 
the two conditions (1) become: 
1 
ZVL 
Now it is true that the case will not easily occur that of two 
substances which have the same critical temperature, the one has 
molecules three times as large as those of the other, and a physical 
theory which does not intend to investigate all mathematically possible 
combinations of a’s and #’s, but only those which really occur, need 
perhaps hardly consider this point. It would indeed be very desirable 
for us to bave an insight into the way in which the a’s and 8’s 
of simple substances are connected, and for mixtures into the way in 
which a,, is connected with the a’s of the components, so that the 
theory of the mixtures need only reckon with realisable combinations. 
Now, however, we do not possess this knowledge, and it seems 
hardly possible as yet to indicate in what direction such an insight 
might be gained. Under these circumstances it seems to me most 
advisable to develop as completely as possible the conclusions which 
proceed from the different possible suppositions for the dependence 
of a,, on a, and a,, and to compare these results with the results 
of observation, in order to try and get an indication in this way 
of the last-mentioned dependence. No doubt we shall treat a great 
many suppositions and combinations in this way which will appear 
to be of no physical signification, but it seems to me that under the 
given circumstances this difficulty is unavoidable. In this sense the 
following investigations concerning diagrams of isobars, deviating 
from those examined up to now and cited above, are to be considered. 
1) In fact we must do so, because the theorem of van per Waars mentioned 
only holds for this supposition. 
