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 10¹⁰ cms. a second, it has an apparent mass _m_ given by _e_/_m_ = 1·86 × 10⁷ electromagnetic units. This apparent mass increases with the speed as the velocity of light is approached (see section 82). The ions which are produced in gases at ordinary pressure have an apparent size, as determined from their rates of diffusion, large compared with the molecule of the gas in which they are produced. The negative ion consists of an electron with a cluster of molecules attached to and moving with it; the positive ion consists of a molecule from which an electron has been expelled, with a cluster of molecules attached. At low pressures under the action of an electric field the electron does not form a cluster. The positive ion is always atomic in size, even at low pressures of the gas. Each of the ions carries a charge of value 3·4 × 10⁻¹⁰ electrostatic units.