=107. Nature of the rays.= In addition to their great penetrating power, the γ rays differ from the α and β rays in not being deflected to an appreciable degree by a magnetic or electric field. In a strong magnetic field, it can be shown, using the photographic method, that there is an abrupt discontinuity between the β and γ rays, for the former are bent completely away from the latter. This indicates that, as regards the action of a magnetic field, there is no gradual transition of magnetic properties between the β and γ rays. Paschen[171] has examined the γ rays in a very intense magnetic field, and, from the absence of deflection of these rays, has calculated that, if they consist of electrified particles carrying an ionic charge, and projected with a velocity approaching that of light, their apparent mass must be at least 45 times greater than that of the hydrogen atom. It now remains for us to consider whether the γ rays are corpuscular in character, or whether they are a type of electromagnetic pulse in the ether similar to Röntgen rays. They resemble Röntgen rays in their great penetrating power and in their absence of deflection in a magnetic field. Earlier experiments seemed to indicate an important difference between the action of γ and X rays. It is well known that ordinary X rays produce much greater ionization in gases such as sulphuretted hydrogen and hydrochloric acid gas, than in air, although the differences in density are not large. For example, exposed to X rays, sulphuretted hydrogen has six times the conductivity of air, while with γ rays the conductivity only slightly exceeds that of air. The results obtained by Strutt, in this connection, have already been given in section 45. It is there shown that the relative conductivity of gases exposed to γ rays (and also to α and β rays) is, in most cases, nearly proportional to their relative densities; but, under X rays, the relative conductivity for some gases and vapours is very much greater than for the γ rays. It must be remembered, however, that the results obtained by Strutt were for “soft X rays,” whose penetrating power was very much less than that of the γ rays. In order to see if the relative conductivity of gases produced by X rays depended upon their penetrating power, A. S. Eve[172] made some experiments with a very “hard” X ray bulb, which gave an unusually penetrating type of rays. The results of the measurements are shown in the table below, where the conductivity for each type of rays is expressed relative to air as unity. The results obtained for “soft” X rays by Strutt and by Eve for γ rays are added for comparison.