Though many attempts have been made to produce a beam of polarised X-radiation and to detect the polarisation by such methods as are applicable to ordinary light, the experiments have proved unsuccessful, and no evidence of polarity has been obtained. An arrangement of molecules such as occurs in crystals does not appear to affect a beam of this radiation transmitted through the crystalline substance. The experiments here described were suggested by the results of an investigation of secondary radiation proceeding from gases and certain solids subject to X-rays, for it was found that the gases experimented upon were the source of a radiation differing little in character from the primary radiation which produced it. In some respects the difference was inappreciable, as, for instance, in the absorbability of the radiations by aluminium. The primary and secondary radiations differed slightly, however, in their ionizing powers in air. The energy of this secondary radiation was found to be proportional to the mass of gas through which the primary beam of definite intensity passed, and to be independent of the nature of the gas.