_e_/_m_ _v_c.m./sec. │ Electro-magnetic units. │ 1·865 × 10^7 0·7 × 10^{10}│For cathode rays (Simon). ──────────────────────────────────────┼─────────────────────────── 1·31 × 10^7 2·36 × 10^{10}│For radium rays (Kaufmann). 1·17 × 10^7 2·48 × 10^{10}│ 〃 0·97 × 10^7 2·59 × 10^{10}│ 〃 0·77 × 10^7 2·72 × 10^{10}│ 〃 0·63 × 10^7 2·83 × 10^{10}│ 〃 M. Kaufmann concludes, from comparison of his experiments with the theory, that the limiting value of the ratio _e_/_m_ for radium rays of relatively small velocity would be the same as the value _e_/_m_ for cathode rays. The most complete experiments of M. Kaufmann were made with a minute quantity of pure radium chloride, with which we provided him. According to M. Kaufmann’s experiments, certain β-rays of radium possess a velocity very near to that of light. These rapid rays seem to possess great penetrating capacity towards matter. _Action of the Magnetic Field upon the α-Rays._ In a recent work, Mr. Rutherford announced that, in a powerful electric or magnetic field, the α-rays of radium are slightly deflected, in the manner of particles positively electrified and possessing great velocity. Mr. Rutherford concludes from his experiments that the velocity of the α-rays is of the order of magnitude 2·5 × 10^9c.m./sec. and that the ratio _e_/_m_ for these rays is of the order of magnitude 6 × 10^3, which is 10^4 times as great as for the deflected β-rays. We shall see later that these conclusions of Mr. Rutherford are in agreement with the properties already known of the α-radiation, and that they account, in part at least, for the law of absorption of this radiation. The experiments of Mr. Rutherford have been confirmed by M. Becquerel. M. Becquerel has further demonstrated that polonium rays behave in a magnetic field like the α-rays of radium, and that, for the same field, they seem to have the same curvature as the latter. It also appears from M. Becquerel’s experiments that the α-rays do not form a magnetic spectrum, but act rather like a homogeneous radiation, all the rays being equally deflected. _Action of the Magnetic Field on the Rays of other Radio-active Substances._ We have just seen that radium gives off α-rays comparable to the tube rays, β-rays comparable to cathode rays, and γ-rays which are penetrating and not deflected. Polonium gives off α-rays only. Amongst the other radio-active substances, actinium seems to behave like radium, but the study of its radiation has not yet advanced so far as in the case of radium. As regards the faintly radio-active bodies, we know to-day that uranium and thorium give rise to α-rays as well as β-rays (Becquerel, Rutherford). _Proportion of β-Rays in the Radiation of Radium._