The secondary radiation set up by Röntgen rays, like that due to the β and γ rays, consists in part of electrons projected with considerable velocity. These three types of rays seem about equally efficient in causing the expulsion of electrons from the substance through which they pass. We have seen that the X and γ rays are, in all probability, electromagnetic pulses set up by the sudden starting or stopping of electrons, and, since these rays in turn cause the removal of electrons, the process appears to be reversible. Since the β rays pass through some thickness of matter before their energy of motion is arrested, theory would lead us to expect that a type of soft X rays should be generated in the absorbing matter. PART VI. =113. Comparison of the ionization produced by the α and β rays=. With unscreened active material the ionization produced between two parallel plates, placed as in Fig. 17, is mainly due to the α rays. On account of the slight penetrating power of the α rays, the current due to them practically reaches a maximum with a small thickness of radio-active material. The following saturation currents were observed[181] for different thicknesses of uranium oxide between parallel plates sufficiently far apart for all the α rays to be absorbed in the gas between them. _Surface of uranium oxide 38 sq. cms._ Weight of uranium Saturation current oxide in grammes per in amperes per sq. sq. cm. of surface cm. of surface ·0036 1·7 × 10⁻¹³ ·0096 3·2 × 10⁻¹³ ·0189 4·0 × 10⁻¹³ ·0350 4·4 × 10⁻¹³ ·0955 4·7 × 10⁻¹³ The current reached about half its maximum value for a weight of oxide ·0055 gr. per sq. cm. If the α rays are cut off by a metallic screen, the ionization is then mainly due to the β rays, since the ionization produced by the γ rays is small in comparison. For the β rays from uranium oxide it has been shown (section 86) that the current reaches half its maximum value for a thickness of 0·11 gr. per sq. cm. Meyer and Schweidler[182] have found that the radiation from a water solution of uranium nitrate is very nearly proportional to the amount of uranium present in the solution.