6 
406 < (156 : 92) = 688° abs. —= 415° C., whereas now the critical 
temperature of ammonia only amounts to 133° C. 
And as last example CH,. Here with the attraction of C Vaz would 
be = 3,1 +4 1,6=—9,5 instead of 6,4, so that 7; would then 
have become = 190,2 Xx (90:41) = 418° abs. = + 145° C., instead 
of — 83° C, as it really is. 
All these substances therefore owe their relative volatility, resp. 
low critical temperature, to the circumstance that the central atoms 
cannot make their attractive action felt towards the outside, in conse- 
quence of the absorbing action of the surrounding atoms. Substances with 
double or multiple bindings (C,H,, C,H,), where the attractive action 
can operate again fully or partially (see I), will, therefore, at once 
be relatively less volatile than those of the category considered just now. 
III. The critical values of mercury. 
«) Calculation of bj, from the densities of the solid halogen compounds. 
According to the property of the straight diameter '/, (d, + d,) = 
= 1 + y(l—m), hence at low temperatures, where the (reduced) 
density of the vapour d, can be neglected: 
a 
1 ee. 2[(1 + y) — ym]. 
ap 
As now rhe by, we have: 
Dy] it > a 4 
b=, X TEE NE |: ae = | ee 
This formula enables us to calculate the value of bj from v, (the 
molecular volume of the liquid), when 7 (coefficient of direction of the 
straigt diameter) and 7 (the reduced temperature) are known. And as at 
low temperatures the volume of the solid phase will not differ much 
from that of the liquid phase, the above formula may be used by 
approximation also for the calculation of 4, from the molecular 
volume of the solid phase. 
Thus for Argon, where y = 0,75, bp =v, X 1,54—'*/.m). For 
84°(abs.), i.e. at the triple point, the density is = 1,413, hence 
y, == 39,88: £443 — 28,9.. The valne of m is: 84°: HAT = 055 
so that 5, == 28 X 1,15 = 32,4; 1e. expressed in the normal volume 
vo, 32,4: 22432 — 144 « 105, whieh is perfectly identical with the 
value of Dj. calculated directly from 7, and p; in Treatise I. 
The factor, therefore, by which v, is to be multiplied to get dz, 
is == 1 5. 
