From this point of view W. K. Bousfield has calculated the sizes of ions on the assumption that Stokes's theory of the motion of a small sphere through a viscous medium might be applied (_Zeits. phys. Chem._, 1905, 53, p. 257; _Phil. Trans._ A, 1906, 206, p. 101). The radius of the potassium or chlorine ion with its envelope of water appears to be about 1.2 × 10^-8 centimetres. For the bibliography of electrolytic conduction see ELECTROLYSIS. The books which deal more especially with the particular subject of the present article are _Das Leitvermögen der Elektrolyte_, by F. Kohlrausch and L. Holborn (Leipzig, 1898), and _The Theory of Solution and Electrolysis_, by W. C. D. Whetham (Cambridge, 1902). (W. C. D. W.) III. ELECTRIC CONDUCTION THROUGH GASES A gas such as air when it is under normal conditions conducts electricity to a small but only to a very small extent, however small the electric force acting on the gas may be. The electrical conductivity of gases not exposed to special conditions is so small that it was only definitely established in the early years of the 20th century, although it had engaged the attention of physicists for more than a hundred years. It had been known for a long time that a body charged with electricity slowly lost its charge even when insulated with the greatest care, and though long ago some physicists believed that part of the leak of electricity took place through the air, the general view seems to have been that it was due to almost unavoidable defects in the insulation or to dust in the air, which after striking the charged body was repelled from it and went off with some of the charge. C. A. Coulomb, who made some very careful experiments which were published in 1785 (_Mém. de l'Acad. des Sciences_, 1785, p. 612), came to the conclusion that after allowing for the leakage along the threads which supported the charged body there was a balance over, which he attributed to leakage through the air. His view was that when the molecules of air come into contact with a charged body some of the electricity goes on to the molecules, which are then repelled from the body carrying their charge with them. We shall see later that this explanation is not tenable. C. Matteucci (_Ann. chim. phys._, 1850, 28, p. 390) in 1850 also came to the conclusion that the electricity from a charged body passes through the air; he was the first to prove that the rate at which electricity escapes is less when the pressure of the gas is low than when it is high. He found that the rate was the same whether the charged body was surrounded by air, carbonic acid or hydrogen. Subsequent investigations have shown that the rate in hydrogen is in general much less than in air. Thus in 1872 E. G. Warburg (_Pogg. Ann._, 1872, 145, p. 578) found that the leak through hydrogen was only about one-half of that through air: he confirmed Matteucci's observations on