Page:Radio-activity.djvu/170

 emitted from the radium come from different depths. Since their velocity is reduced in their transit through matter, a pencil of [Greek: alpha] 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 [Greek: alpha] 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 [Greek: alpha] particles end. It was found that the [Greek: alpha] particles are most efficient as ionizers of the gas just before their power of ionizing ends. The loss of ionizing power of the [Greek: alpha] 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 measurements (see above table) recorded by Becquerel. Quite apart from the special assumptions required for such a quantitative comparison of theory with experiment, there can be little doubt that the increase of value of H[Greek: rho] with distance can be satisfactorily explained as a consequence of the complex character of the pencil of rays.

Becquerel states that the amount of deviation, in a given magnetic field, was the same for the [Greek: alpha] rays of polonium and of radium. This shows that the value of (m/e)V is the same for the [Greek: alpha] rays from the two substances. Since the [Greek: alpha] rays from polonium are far more readily absorbed than the [Greek: alpha] rays from radium, this result would indicate that the value of m/e is greater for the [Greek: alpha] particles of polonium than of radium. Further experimental evidence is required on this important point.