If, for the same ray, the deflection, δ, and the radius of curvature, ρ, be measured in a magnetic field, values could be found from these two experiments for the ratio _e_/_m_ and for the velocity, _v_. The experiments of M. Becquerel threw the first light upon this subject. They gave for the ratio _e_/_m_ a value approximately equal to 10^7 absolute electro-magnetic units, and for v a magnitude of 1·6 × 10^{10}. These values are of the same order of magnitude as those of the cathode rays. Accurate experiments have been made on the same subject by M. Kaufmann. This physicist subjected a narrow beam of radium rays to the simultaneous action of an electric field and a magnetic field, the two fields being uniform and having a similar direction, normal to the original direction of the beam. The impression produced on a plate normal to the primitive beam and placed beyond the limits of the field with reference to the source, has the form of a curve, each point of which corresponds to one of the original beam. The most penetrating and least deflected rays are at the same time those with the greatest velocity. It follows from the experiments of M. Kaufmann, that for the radium rays, of which the velocity is considerably greater than that of the cathode rays, the ratio _e_/_m_ decreases, while the velocity increases. According to the researches of J. J. Thomson and Townsend, we may assume that the moving particle, which constitutes the ray, possesses a charge, _e_, equal to that carried by an atom of hydrogen during electrolysis, this charge being the same for all the rays. We are therefore led to the conclusion that the mass of the particle, _m_, increases with increase of velocity. These theoretical considerations lead to the idea that the inertia of the particle is due to its state of charge during motion, the velocity of an electric charge in motion being incapable of modification without expenditure of energy. To state it otherwise, the inertia of the particle is of electro-magnetic origin, and the mass of the particle is—in part at least—a virtual mass or an electro-magnetic mass. M. Abraham goes further, and assumes that the mass of the particle is entirely an electro-magnetic mass. If, according to this hypothesis, the value of this mass, _m_, be calculated for a known velocity, _v_, we find that _m_ approaches infinity when _v_ approaches the velocity of light, and that _m_ approaches a constant value when the velocity, _v_, is much less than that of light. The experiments of M. Kaufmann are in agreement with the results of this theory, the importance of which is great because it foreshadows the possibility of establishing mechanical bases upon the dynamical of little particles of matter charged in a state of motion. These are the figures obtained by M. Kaufmann for _e_/_m_ and _v_.