It] IONIZATION THEORY OF GASES 53 
presence of water vapour; for the negative ion undoubtedly has a 
greater mass in moist than in dry gases. At the same time it 1s 
possible that the apparently large size of the ion, as determined 
by diffusion methods, may be in part a result of the charge carried 
by the ion. The presence of a charge on a moving body would 
increase the frequency of collision with the molecules of the gas, 
and consequently diminish the rate of diffusion. The ion on this 
view may not actually be of greater size than the molecule from 
which it is produced. 
The negative and positive ions certainly differ in size, and this 
difference becomes very pronounced for low pressures of the gas. 
At atmospheric pressure, the negative ion, produced by the action 
of ultra-violet light on a negatively charged body, is of the 
same size as the ion produced by X rays, but at low pressures 
J. J. Thomson has shown that it is identical with the corpuscle or 
electron, which has an apparent mass of about 1/1000 of the mass 
of the hydrogen atom. A similar result has been shown by 
Townsend to hold for the negative ion produced by X rays at a 
low pressure. It appears that the negative ion at low pressure 
sheds its attendant cluster. The result of Langevin, that the 
velocity of the negative ion increases more rapidly with the 
diminution of pressure than that of the positive ion, shows that 
this process of removal of the cluster is quite appreciable at a 
pressure of 10 mms. of mercury. 
It must thus be supposed that the process of ionization in 
gases consists in a removal of a negative corpuscle or electron from 
the molecule of the gas. At atmospheric pressure this corpuscle 
immediately becomes the centre of an aggregation of molecules 
which moves with it and 7s the negative ion. After removal of 
the negative ion the molecule retains a positive charge, and probably 
also becomes the centre of a cluster of new molecules. 
The terms electron and ion as used in this work may therefore 
be defined as follows :— 
The electron or corpuscle is the body of smallest mass yet 
known to science. It carries a negative charge of value 3:4 x 10-° 
electrostatic units. Its presence has only been detected when in 
rapid motion, when it has, for speeds up to about 10” cms. a second, 
an apparent mass m given by e/m= 1°86 x 10’ electromagnetic 
