The investigation of the α-rays led me to the reflection that these rays behave like projectiles having a certain initial velocity, and which lose their force on encountering obstacles. These rays, moreover, travel by rectilinear propagation, as has been shown by M. Becquerel in the following experiment:—Polonium emitting rays was placed in a very narrow straight cavity hollowed in a sheet of cardboard. Thus a linear source of radiation was produced. A copper wire, 1·5 m.m. in diameter, was placed parallel and opposite to the source at a distance of 4·9 m.m. Beyond was placed a parallel photographic plate at a distance of 8·65 m.m. After an exposure of ten minutes, the geometric shadow of the wire was perfectly reproduced, with a narrow penumbra corresponding to the size of the source. The same experiment succeeded equally well when a double leaf of beaten aluminium was placed against the wire, through which the rays must pass. There are therefore rays capable of giving perfect geometric shadows. The experiment with the aluminium shows that these rays are not scattered in traversing the screen, and that this screen does not give rise to any noticeable extent to secondary rays similar to the secondary rays of the Röntgen rays. The experiments of Mr. Rutherford show that the projectiles which constitute the α-rays are deflected by a magnetic field, as if they were positively charged. The deflection in a magnetic field becomes less as the product _mv_/_e_ becomes greater; _m_ being the mass of the particle, _v_ its velocity, and _e_ its charge. The cathode rays of radium are but slightly deflected, because their velocity is enormous; they are, on the other hand, very penetrating, because each particle has a very small mass together with a great velocity. But particles which, with an equal charge and a less velocity, have a greater mass, would be also only slightly influenced by the action of the field, and would give rise to very absorbable rays. From the results of Mr. Rutherford’s experiments, this seems to take place in the case of the α-rays. The penetrating γ-rays appear to be of quite another nature and similar to Röntgen rays. We have now seen how complex a phenomenon is the radiation of radio-active bodies. The difficulties of investigation are increased by the question as to whether the radiation undergoes a merely selective absorption on the part of the material, or whether a more or less radical transformation.