1 2·91 × 10⁵ 3 2·99 „ 5 3·06 „ 7 3·15 „ 8 3·27 „ 9 3·41 „ The writer (_loc. cit._) showed that the _maximum_ value of _H_ρ for complete deviation of the α rays was 390,000. The results are thus in good agreement. Since _m_ _H_ρ = ----- _V_ _e_ these results show that the values either of _V_ or of _e_/_m_ for the projected particles vary at different distances from the source. Becquerel considered that the rays were homogeneous, and, in order to explain the results, has suggested that the charge on the projected particles may gradually decrease with the distance traversed, so that the radius of curvature of the path steadily increases with the distance from the source. It, however, seems more probable that the rays consist of particles projected with different velocities, and that the slower particles are more quickly absorbed in the gas. In consequence of this, only the swifter particles are present some distance from the source. This conclusion is borne out by some recent experiments of Bragg and Kleeman[146] on the nature of the absorption of α particles by matter, which are discussed in more detail in sections 103 and 104. They found that the α particles from a thick layer of radium are complex, and have a wide range of penetrating power and presumably of velocity. This is due to the fact that the α particles emitted from the radium come from different depths. Since their velocity is reduced in their transit through matter, a pencil of α rays will consist of particles which differ considerably in speed. Those which are just able to emerge from the radium will be absorbed in a very short depth of air, while those that come from the surface will be able to pass through several centimetres of air before they lose their power of ionizing the gas. Since the α particles have different velocities, they will be unequally deflected by the magnetic field, the slower moving particles describing a more curved path than the swifter ones. Consequently, the outer edge of the trace of the pencil of rays on the photographic plate, as obtained by Becquerel, will be the locus of the points where the photographic action of the α particles end. It was found that the α particles are most efficient as ionizers of the gas just before their power of ionizing ends. The loss of ionizing power of the α particles seems to be fairly abrupt, and, for particles of the same velocity, to occur always after traversing a definite distance in air. On the assumption that the photographic as well as the ionizing action is most intense just before the particles are stopped, and ceases fairly abruptly, Bragg has been able to account numerically for the