=88. The nature of the α rays=. The nature of the α rays was difficult to determine, for a magnetic field sufficient to cause considerable deviation of the β rays produced no appreciable effect on the α rays. It was suggested by several observers that they were, in reality, secondary rays set up by the β or cathode rays in the active matter from which they were produced. Such a view, however, failed to explain the radio-activity of polonium, which gave out α rays only. Later work also showed that the matter, which gave rise to the β rays from uranium, could be chemically separated from the uranium, while the intensity of the α rays was unaffected. These and other results show that the α and β rays are produced quite independently of one another. The view that they are an easily absorbed type of Röntgen rays fails to explain a characteristic property of the α rays, viz. that the absorption of the rays in a given thickness of matter, determined by the electrical method, increases with the thickness of matter previously traversed. It does not seem probable that such an effect could be produced by a radiation like X rays, but the result is to be expected if the rays consist of projected bodies, which fail to ionize the gas when their velocity is reduced below a certain value. From observations of the relative ionization produced in gases by the α and β rays, Strutt[138] suggested in 1901 that the α rays might consist of positively charged bodies projected with great velocity. Sir William Crookes[139], in 1902, advanced the same hypothesis. From a study of the α rays of polonium Mme. Curie[140] in 1900 suggested the probability that these rays consisted of bodies, projected with great velocity, which lost their energy by passing through matter. The writer was led independently to the same view by a mass of indirect evidence which received an explanation only on the hypothesis that the rays consisted of matter projected with great velocity. Preliminary experiments with radium of activity 1000 showed that it was very difficult to determine the magnetic deviation of the α rays. When the rays were passed through slits sufficiently narrow to enable a minute deviation of the rays to be detected, the ionizing effect of the issuing rays was too small to be measured with certainty. It was not until radium of activity 19,000 was obtained that it was possible to detect the deviation of these rays in an intense magnetic field. How small the magnetic deviation is may be judged from the fact that the α rays, projected at right angles to a magnetic field of 10,000 C.G.S. units, describe the arc of a circle of about 39 cms. radius, while under the same conditions the cathode rays produced in a vacuum tube would describe a circle of about ·01 cm. radius. It is therefore not surprising that the α rays were for some time thought to be non-deviable in a magnetic field.