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 presence of water vapour; for the negative ion undoubtedly has a greater mass in moist than in dry gases. At the same time it is 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.

We must suppose 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 is 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 × 10^{-10} electrostatic units. Its presence has only been detected when in rapid motion, when, for speeds up to about 10^{10} cms. a second, it has an apparent mass m given by e/m = 1·86 × 10^7 electromagnetic