296 TRANSACTIONS OF SECTION A. 
atomic structure will be acceptable unless it can account for variations such as 
these. 
Besides considering variations in atomic properties such as have been 
referred to, it will be necessary also, in devising a theory of atomic structure, 
to take into account the fact that valency is a ‘ directed function.’ The 
tetrahedron apparently is a complete embodiment of the properties of the 
carbon atom, in so far as these are due to directed forces, if the affinities of the 
atom be thought of as proceeding from the centre of mass to the four apices. 
Or if, instead of representing carbon by a sphere four times the volume of the 
unit sphere representing the hydrogen atom, four unit spheres be piled in 
tetrahedral form, the four hollows into which other similar spheres will fit 
are in positions representing the directions in which affinity acts. The great 
body of facts arrived at by studying optically active ‘asymmetric’ carbon 
compounds are all compatible with such modes of representing carbon; more- 
over, the hypothesis is the only one devised that sets the necessary limit to the 
number of isomerides possible. What is true of carbon is true apparently of 
other elements. But it is very noteworthy that the affinity of carbon atoms 
for carbon atoms, as well as for those of many other elements, is extraordinarily 
strong in comparison with that of other elements for each other; carbon has 
properties which are altogether peculiar. 
Fresh significance has been given to the problems of valency of late years 
owing to the introduction, by Barlow and Pope, of the conception that it is to 
be regarded as a function of the volume occupied by the atom. Assuming 
that the atoms are closely packed, they have succeeded to an extent which is 
altogether remarkable, by means of this hypothesis, in correlating crystalline 
form with molecular structure. Regarding the sphere within which the in- 
fluence of the hydrogen atom is exercised as unity, that of the dyad elements 
is twice, that of the triad three times, that of a tetrad element such as 
carbon four times, as great as that of the hydrogen atom. The halogens 
appear to occupy the same relative volume as hydrogen. A large body of evi- 
dence to this effect is to be found in a recent communication to the Royal 
Society (‘ Proc. R. Soc.’ A, vol. 90, p. 111, 1914). Apparently, when an 
element such as an atom of halogen is introduced in place of hydrogen, the 
alteration in volume which attends the change is not simply due to the dis- 
placement effected by the new atom: the alteration in composition involves 
alterations in the spheres of influence of all the atoms in the molecule, so that 
their relative volumes remain the same though their actual volumes may vary. 
It is to be expected that many of the problems of molecular structure which 
in the past could not be considered, especially in the case of inorganic com- 
pounds, will now be amenable to treatment from the crystallographic side. 
The view originally put forward by Lavoisier and elaborated by Berzelius, that 
acids such as sulphuric acid are compounds of an acid oxide with ‘ water,’ may 
be referred to as a case in point (see ‘Proc. R. Soc.’ A, vol. 90, p. 73, 1914). 
In view of the production of helium in so many cases of ‘atomic’ disrup- ~ 
tion, it must not be forgotten that the problems of ‘elementary’ atomic struc- 
ture still require study on the chemical side. It is not to be supposed that they 
are no longer amenable to chemical treatment and that they are ripe for purely 
physical treatment. 
Professor Hicks: Professor Rutherford has approached the question chiefly 
from the side of radioactive phenomena, whilst Professor Armstrong has dealt 
with certain stereographic properties of the molecule which physicists must 
take account of in forming any theory of the structure of the atom itself. I 
propose to draw attention to certain aspects of the problem when approached 
from the spectroscopic side, i.e., from consideration of the atom as a configura- 
tion capable of emitting definite sets of free vibrations. Before doing so, 
however, I should like to offer some criticism with reference to a point raised 
by Professor Rutherford, viz., the actual value of the effective nuclear charge 
in any case. Moseley’s law indicates that they are consecutive multiples for the 
consecutive elements in the periodic table, and they are known if that for one, 
say He, is known. What evidence we have seems to me rather to weigh in 
favour of He having an atomic number 4 in place of 2 which is assumed by 
Rutherford, Bohr, and Moseley himself. That it is at least 2 is clear from 
