Mr Campbell , The number of Electrons in an Atom. 287 
The number of Electrons in an Atom. By Norman Campbell, 
M.A., Trinity College. 
[Read 6 May, 1907 .] 
1. When the experiments of Kaufmann proved that the 
whole, or the greater part, of the mass of an electron is of electro- 
magnetic origin, it seems to have been generally assumed that 
the mass of an atom is the sum of the masses of the electrons 
contained in it, and consequently that the number of electrons 
in the atom ( N ) is given by the relation 
N= A — , 
p 
where A is the atomic weight, M the mass of a hydrogen atom 
and p the mass of an electron. But at the same time it has been 
recognised that, apart from arguments based on the nature of 
mass, there is no evidence that the number of electrons in the 
atom is nearly so great as would be indicated by that formula. 
Lately, Prof. Thomson* has aroused fresh interest in the 
subject by adducing arguments to prove that the number of 
electrons in an atom is equal to — or at any rate of the same 
order of magnitude as — the atomic weight. His arguments do 
not appear to me entirely conclusive, and it seems desirable that 
any evidence, however slight, that bears on this important 
problem should be brought to light. The arguments that I have 
to offer are extremely speculative, and the only excuse that I 
have for putting them forward is the absence of more definite 
information. 
2. Prof. Thomson’s arguments are three in number. The 
first is based upon the dispersion of light and is similar to that 
advanced by Drudef. The latter author concluded that the 
numbers of electrons in an atom, which influence the dispersion 
of visible light, is of the same order of magnitude as the valency 
of the atom — a result with which Prof. Thomson’s figures are in 
agreement : he also indicated that there must be a larger number 
of electrons, which have free periods so short that they cannot 
influence the dispersion of visible light. 
The second argument is based on the emission of secondary 
Hontgen rays. Since it is well known that the number of atoms 
ionised by the most powerful Rontgen rays is only a very small 
Phil. Mag. 6, xi. p. 769, 1906. 
t An. Phys. 14, p. 677, 1904. 
