( 766 ) 
sents the absorption of heat for 7’—O taking place in this transition, 
and in the first member is 8 the degree of dissociation of the double 
molecules. 
Besides various other things I demonstrated that the known discre- 
pancies between the experimental critical data and those derived 
from VAN DER WAALS’ equation of state in its usual form can be 
accounted for by the assumption of an association, which would 
even be still noticeable at the critical point. 
I believe that this opinion was already expressed by van per WAALS 
some years ago (1906) *), and elaborated by van Rw in his Thesis 
for the doctorate (1908); it was, however, demonstrated by me in 
the paper mentioned, that quantitative agreement can be obtained 
only when Ad is not —0. Accordingly this quantity A5 plays an 
important part in my theory, which I drew up as early as 1902 
on the occasion of a paper by the late Prof. Bakuurs RoozrBoom. *) 
But it is not about the above matters, however important, that I 
intend to speak now. I may be allowed to return to what IT said 
on p. 259 of my paper in the Arch. Teyler (p. 25 of the reprint), 
namely that the isotherms — if Ad is not =0 — present very 
“remarkable particularities” in the neighbourhood of v= 6. (See 
fig. 1 of the plate). 
These particularities now consist in this, that the isotherms, 
[for Ab positive only below a certain limiting temperature (critical 
temperature)| after having first risen to comparatively high pressures 
in the neighbourhood of +» = +, bend back again as far as in the region 
of low (even negative) pressures, after whicn they rapidiy ascend 
again for the second time to the highest pressures. We shall 
namely see that if e.g. Ab is positive, the degree of dissociation ~ 
of the double molecules approaches rapidly to 0 in the neighbourhood 
of v—=b=b, + Ab in consequence of the relation (a), which 
causes v— 6, after having first greatly decreased, to increase again. 
In consequence of this the pressure will decrease, and it will only 
increase again when v — 6 begins to diminish again in the neigbourhood 
of the value B=0O. If Ab is negative, 3 will draw near to the 
limiting value 1, and we shall evidently see the same course appear 
in the values of v— 6 and p. 
1) In an address delivered in the Meeting of this Academy of Jan. 1906,- which 
address has unfortunately never been printed, so that only the fact and the subject 
of the address are known to me. (Cf. also the Thesis for the Doctorate of 
Dr. van Ris on “Schijnassociatie ete”, p. 3 (1908). 
2) “Equilibria of phases in the system acetaldehyde + paraldehyde etc.” (These 
Proc. Vol. XI, p. 283). Compare also van peR Waars: “Some observations etc, 
These Proc. Vol. XI, p. 308. 
