Ionisation is the process by which ions - particles charged with electricity - are produced in a solid, liquid or gas. This address is confined to the consideration of ions in gases and deals with two questions - (1) the nature of the ions, (2) the process by which they are produced. The evidence as to the nature of ions is derived from experiments on their mobility which have shown: - α. The mobility of a positive ion depends only on the gas through which the ions are moving and not on the nature of the gas out of which the ions are formed.β. The mobility of the ions in a gas at constant density is independent of the temperature.γ. There are a considerable number of gases in which the mobility is very approximately inversely proportional to the square root of the density of the gas.δ. In the case of negative ions there is an abnormal increase of mobility when the pressure is reduced below a certain value, and there is some evidence that this is also the case for positive ions at very low pressures.Two theories of the mobilities of ions were considered, one founded on the view that the action between ions and molecules is analogous to impacts between hard elastic spheres, the other on Maxwell's theory of forces between ions and molecules varying inversely as the fifth power of the distance between them.It is shown that α follows from the second theory provided the ion is a cluster whose mass is considerably greater than that of a molecule of the gas through which it is moving. On this supposition it follows also from the first theory if we suppose in addition that all ions in a given gas are of the same size.(β) follows at once from the second theory; to explain it on the first theory we must suppose that the size of the ion varies with the temperature in a definite way. The necessary relation can be deduced from thermodynamical principles if we suppose that the force between an ion and a molecule is analogous to that between a charged point and a sphere which is either a conductor of electricity or has a high specific inductive capacity.(γ) requires on theory one that the ions in these gases should be of the same size, on theory two that the molecules of these gases should exert the same force on a charged point at a given distance.(δ) follows on either theory if the ion dissociates at low pressures so that free corpuscles are present in the gas.With regard to the process of ionisation, the first stage of this in the vast majority of cases consists in the detachment of an electron or corpuscle. Evidence as to the method by which this takes place is afforded by determinations of the velocity with which the electrons are ejected from the body.In the case of ionisation by light or Röntgen rays, this velocity depends primarily on the wave-length of the radiation, not upon its intensity, nor, at any rate to any great extent, on the nature of the molecule from which the corpucle is ejected. This velocity is far greater, even when every allowance is made for resonance, than can be accounted for if we suppose that the energy of the light is uniformly distributed, and that the corpuscle acquires its velocity by the action on it of the electric force in the wave. Another explanation not open to these objections was put forward.When ionisation is due to the action of moving electrified particles, whether positive or negative, the results are quite different. The velocity of the ejected particles (δ-rays, as they are sometimes called) does not seem to vary much, if at all, with the velocity of the particles which eject them, and is of the same order whether these particles are positive rays or the much swifter cathode rays. The method of ejection by the impact of such particles was considered, and suggestions as to a possible theory and some of its consequences thrown out.Dr. R. T. Glazebrook, in moving a vote of thanks to the President, said that one expected, when listening to Sir J. J. Thomson, to be carried to the bounds of knowledge by one who had taken no small part in the extension of these bounds, and he was confident that none of those present had been disappointed in that expectation. He asked the President, on behalf of the Society, to allow his address to be published in full in the "Proceedings."Dr. C. Chree seconded the vote of thanks, which was carried with enthusiasm.