The increasing recognition of the importance of the α-rays in radioactive phenomena led to attempts to determine the nature of this easily absorbed type of radiation. Strutt (Lord Rayleigh) in 1901 and Sir William Crookes in 1902 suggested that they might possibly prove to be projected particles carrying a positive charge. I independently arrived at the same conclusion from consideration of a variety of evidence. If this were the case, the α-rays should be deflected by a magnetic field. Preliminary work showed that the deflection was very slight if it occurred at all. Experiments were continued at intervals over a period of two years and it was not until 1902, when a preparation of radium of activity 19,000 was available, that I was able to show conclusively that the particles were deflected by a magnetic field, though in a very minute degree compared with the β-rays. This showed that the α-rays consisted of projected charged particles while the direction of deflection indicated that each particle carried a positive charge. The α-particles were shown to be deflected also by an electric field and from the magnitude of the deflection, it was deduced that the velocity of the swiftest particles was about 2.5 x 10^9 cm per second, or one-twelfth the velocity of light, while the value of _e/m_--the ratio of the charge carried by the particle to its mass--was found to be 5,000 electromagnetic units. Now it is known from the data of the electrolysis of water that the value of _e/m_ for the hydrogen atom is 9,650. If the α-particle carried the same positive charge as the unit fundamental charge of the hydrogen atom, it was seen that the mass of the α-particle was about twice that of the hydrogen atom. On account of the complexity of the rays it was recognized that the results were only approximate, but the experiments indicated clearly that the α-particle was atomic in mass and might prove ultimately to be either a hydrogen or a helium atom or the atom of some unknown element of light atomic weight. These experiments were repeated by Des Coudres in 1903 with similar results, while Becquerel showed the deflection of the α-rays in a magnetic field by the photographic method.