176 Mr Kleeman, The Atomic Constants , 
of substances at the critical point—the most important corre- 
sponding point—and the values of the molecular volumes calculated 
from the expression LECar, where c, denotes the apparent volume 
of an atom relative to that of a hydrogen atom. The values of @ | 
are given at the top of Table I. Those of the H and C were 
obtained from the two substances di-isobutyl and benzene, while” 
the values for the other atoms are the mean values obtained from 
the remaining substances. KK has been put equal to 14:06. A fair 
agreement between calculation and experiment is obtained. 
There is no obvious simple reason why the apparent molecular | 
volume of a molecule should be an additive property of its atoms, 
since an increase in the volume of a substance is not attended by | 
an equal increase in the actual space occupied by the molecules. | 
The molecular volume in the case of a liquid or vapour, it may be | 
pointed out here, is really the outcome of the equilibrium between | 
the intrinsic pressure due to the attraction between the molecules, | 
and the pressure exerted by the molecules due to their motion of | 
translation. We might therefore expect that the constant c, and | 
the atomic attraction constant c, should be connected with one 
another. Thus the writer has shown that the attraction constant 
of an atom is proportional to /m,, where m, denotes its atomic 
weight. Traube has shown that the atomic volume of an atom at 
the absolute zero, which according to the law of corresponding 
states is proportional to that at the critical point, is proportional 
to /m,. But the values of the constants c, and c, (the average 
values determined from the facts) agree on the whole better with 
the facts than the constant ./m,, as might be expected. 
The factors of ./m, that have to be introduced in both cases 
to obtain a better agreement with the facts are of interest. In 
the case of the constant cy, we may accordingly write ¢y=y,/m. 
The values of ry, for a number of atoms corresponding to the values 
of c, deduced from the internal heat of evaporation* are given in 
Table II, the value for the carbon atom being put equal to unity. 
It is very probable that y, depends on the chemical constitution 
of the molecule in which the atom occurs, and should therefore 
strictly not be given the same value for each substance. Probably 
it will be found when more extensive data are available that it has 
the same value for all substances belonging to the same chemical — 
group. Further investigations of the properties of the attraction — 
constants in the law of molecular attraction along these lines will 
probably lead to interesting results. 
In the case of the constant c, we may similarly write 
Cy=Y2Vm,. The values of y, for a number of atoms are given in 
Table II, the value for the carbon atom as before being put equal 
* Phil. Mag., Oct. 1909, p. 507. 
