10 
too points to increasing association in the vapour of Hg, to Hg,, 
when the temperature gets higher. 
In agreement with the abnormally high value of 7): 7, viz. 2,0 
instead of 1,6, the value of Js in VAN DER Waars’ vapour pressure 
formula 
will be comparatively low. For with p;=t (atm), Pr: 7,=¢ 
we have: 
which (as log pj has little influence as a rule) leads to a too low 
value of f, When g is too large. We really find for mereury with 
p=d for jf, the value /, = log” p. = log” 192 — 2,28, the mean 
value for many normal substances being — 2,90(= 6,7 with Neprr’s log.). 
7). The course of the vapour-tension factor f for mercury. 
Though for some associated substances, as acetic acid, methyl 
alcohol, ete. the value of 7; (hence also that of fj) is found greater 
than for normal substances (mean 3,35 or with Nep. log. 7,7), the 
opposite behaviour can also be expected. For as the vapour-pressure 
curve for substances, which are almost entirely bi-molecular at the 
critical temperature, but gradually become mono-molecular on decrease 
of temperature (as is the case with mercury vapour), will evidently 
lie above the norma! value for Hg, (the critical pressures of Hg, and 
Hg, are namely equal, whereas 7; for Hg, will be double 7% for 
Hg,), the vapour pressure will be comparatively large, hence f too 
small at a temperature (referring to 7). of He.) below (7), For 
associating substances as acetic acid, however, where the association 
decreases at higher temperature, the vapour-pressure curve on 
the other hand — if namely at 7) the association has disappeared 
for the greater part — will lie below that for (Ac),. In consequence 
at a certain value of 7’:(7%), below (7%), the vapour pressure becomes 
relatively too small, hence f too large. 
All this refers to the said extreme cases. In intermediate cases 
— when the substance at 7 is only partly associated, so that 7’ 
must be referred to a temperature between Tr, and FT, — the value 
of f, in reference to the normal value at that reduced temperature 
m, will entirely depend on the value of m, and it is impossible to 
say beforehand whether in a definite case the value of f will be 
smaller or greater than the normal value of f. 
