Since the uranium is thus continuously radiating energy from itself, without any known source of excitation, the question arises whether any known agent is able to affect the rate of its emission. No alteration was observed when the body was exposed to ultra-violet light or to ultra-red light or to X rays. Becquerel states that the double sulphate of uranium and potassium showed a slight increase of action when exposed to the arc light and to sparks, but he considers that the feeble effect observed was another action superimposed on the constant radiation from uranium. The intensity of the uranium radiation is not affected by a variation of temperature between 200° C. and the temperature of liquid air. This question is discussed in more detail later.
5. In addition to these actions on a photographic plate, Becquerel showed that uranium rays, like Röntgen rays, possess the important property of discharging both positively and negatively electrified bodies. These results were confirmed and extended by Lord Kelvin, Smolan and Beattie8. The writer made a detailed comparison9 of the nature of the discharge produced by uranium with that produced by Röntgen rays, and showed that the discharging property of uranium is due to the production of charged ions by the radiation throughout the volume of the gas. The property has been made the basis of a qualitative and quantitative examination of the radiations from all radio-active bodies, and is discussed in detail in chapter II.
The radiations from uranium are thus analogous, as regards their photographic and electrical actions, to Röntgen rays, but, compared with the rays from an ordinary X ray tube, these actions are extremely feeble. While with Röntgen rays a strong impression is produced on a photographic plate in a few minutes or even seconds, several days’ exposure to the uranium rays is required to produce a well-marked action, even though the uranium compound, enveloped in black paper, is placed close to the plate. The discharging action, while very easily measurable by suitable methods, is also small compared with that produced by X rays from an ordinary tube.
6. The rays from uranium show no evidence of direct reflection, refraction, or polarization10. While there is no direct reflection of the rays, there is apparently a diffuse reflection produced where the rays strike a solid obstacle. This is due in reality to a secondary radiation set up when the primary rays impinge upon matter. The presence of this secondary radiation at first gave rise to the erroneous view that the rays could be reflected and refracted like ordinary light. The absence of reflection, refraction, or polarization in the penetrating rays from uranium necessarily follows in the light of our present knowledge of the rays. It is now known that the uranium rays, mainly responsible for the photographic action, are deviable by a magnetic field, and are similar in all respects to cathode rays, i.e. the rays are composed of small particles projected at great velocities. The absence of the ordinary properties of transverse light waves is thus to be expected.
7. The rays from uranium are complex in character, and, in addition to the penetrating deviable rays, there is also given off a radiation very readily absorbed by passing through thin layers of metal foil, or by traversing a few centimetres of air. The photographic action due to these rays is very feeble in comparison with that of the penetrating rays, although the discharge of electrified bodies is mainly caused by them. Besides these two types of rays, some rays are emitted which are of an extremely penetrating character and are non-deviable by a magnetic field. These rays are difficult to detect photographically, but can readily be examined by the electric method.
8. The question naturally arose whether the property of spontaneously giving out penetrating radiations was confined to uranium and its compounds, or whether it was exhibited to any appreciable extent by other substances.