For various reasons the determination of the velocities in an electric field of the ions produced in gases by the action of Röntgen rays is of fundamental importance in electrical theory. A knowledge of the ionic mobilities the velocities under unit electric intensity) affords information with regard to the structure of the ion; if, in addition, the diffusion coefficients of the ions in various gases are known, the charge associated with the ion can be compared with that carried by the monovalent ion in the electrolysis of solutions. Experimental methods of determining the mobilities of the positive and negative ions were devised not long after the ionising action of the Röntgen rays was known. Rutherford determined the values of the sum of the positive and negative mobilities in a series of gases. Zeleny, by comparing the velocity acquired by the ions in an electric field with that of a gaseous current parallel to the field, succeeded in deducing the values of the difference of the ionic mobilities and also their ratio. In later experiments Zeleny employed a current of gas in a direction perpendicular to the electric field and deduced the absolute values of the mobilities in air, oxygen, carbon dioxide, and hydrogen.