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_ρ with distance can be satisfactorily explained as a consequence of the complex character of the pencil of rays[147]. Becquerel states that the amount of deviation, in a given magnetic field, was the same for the α rays of polonium and of radium. This shows that the value of _m_ ---- _V_ _e_ is the same for the α rays from the two substances. Since the α rays from polonium are far more readily absorbed than the α rays from radium, this result would indicate that the value of _m_/_e_ is greater for the α particles of polonium than of radium. Further experimental evidence is required on this important point. =93. Charge carried by the α rays=. We have seen that the negative charge carried by the β particles has been readily measured. Since there is reason to believe (section 229) that four α particles are expelled from radium for each β particle, it is to be expected that the positive charge carried by the α particles should be determined still more readily. All the initial experiments, however, made to detect this charge, gave negative results; and, before successful results were obtained, it was found necessary to eliminate some secondary actions, which at first completely masked the effects to be looked for. In consequence of the importance of this question, a brief account will be given of the methods of measurement adopted and the special experimental difficulties which have arisen. In the first place, it must be remembered that only a small fraction of the α rays, emitted from a layer of powdered radium bromide, escape into the surrounding gas. On account of the ease with which the α rays are stopped in their passage through matter, only those escape which are expelled from a superficial layer, and the rest are absorbed by the radium itself. On the other hand, a much larger proportion of the β rays escape, on account of their greater power of penetration. In the second place, the α particle is a far more efficient ionizer of the gas than the β particle, and, in consequence, if the charge carried by the α rays is to be determined by methods similar to those employed for the β rays (see section 80), the pressure of the gas surrounding the conductor to be charged must be very small in order to eliminate, as far as possible, the loss of charge resulting from the ionization of the residual gas by the α rays[148]. The experimental arrangement used by the writer is shown in Fig. 33.