_Velocity of Ions in an Electric Field._--The velocity of ions in an electric field, which is of fundamental importance in conduction, is very closely related to the coefficient of diffusion. Measurements of this velocity for ions produced by Röntgen rays have been made by Rutherford (_Phil. Mag._ [5], 44, p. 422), Zeleny (_Phil. Mag._ [5], 46, p. 120), Langevin (_Ann. Chim. Phys._, 1903, 28, p. 289), Phillips (_Proc. Roy. Soc._ 78, A, p. 167), and Wellisch (_Phil. Trans._, 1909, 209, p. 249). The ions produced by radioactive substance have been investigated by Rutherford (_Phil. Mag._ [5], 47, p. 109) and by Franck and Pohl (_Verh. deutsch. phys. Gesell._, 1907, 9, p. 69), and the negative ions produced when ultra-violet light falls on a metal plate by Rutherford (_Proc. Camb. Phil. Soc._ 9, p. 401). H. A. Wilson (_Phil. Trans._ 192, p. 4O9), Marx (_Ann. de Phys._ 11, p. 765), Moreau (_Journ. de Phys._ 4, 11, p. 558; _Ann. Chim. Phys._ 7, 30, p. 5) and Gold (_Proc. Roy. Soc._ 79, p. 43) have investigated the velocities of ions produced by putting various salts into flames; McClelland (_Phil. Mag._ 46, p. 29) the velocity of the ions in gases sucked from the neighbourhood of flames and arcs; Townsend (_Proc. Camb. Phil. Soc._ 9, p. 345) and Bloch (_loc. cit._) the velocity of ions produced by chemical reaction; and Chattock (_Phil. Mag._ [5], 48, p. 401) the velocity of the ions produced when electricity escapes from a sharp needle point into a gas. Several methods have been employed to determine these velocities. The one most frequently employed is to find the electromotive intensity required to force an ion against the stream of gas moving with a known velocity parallel to the lines of electric force. Thus, of two perforated plane electrodes vertically over each other, suppose the lower to be positively, the upper negatively electrified, and suppose that the gas is streaming vertically downwards with the velocity V; then unless the upward velocity of the positive ion is greater than V, no positive electricity will reach the upper plate. If we increase the strength of the field between the plates, and hence the upward velocity of the positive ion, until the positive ions just begin to reach the upper plate, we know that with this strength of field the velocity of the positive ion is equal to V. By this method, which has been used by Rutherford, Zeleny and H. A. Wilson, the velocity of ions in fields of various strengths has been determined.