Immediately after Röntgen’s discovery of the production of X rays, several physicists were led to examine if any natural bodies possessed the property of giving out radiations which could penetrate metals and other substances opaque to light. As the production of X rays seemed to be connected in some way with cathode rays, which cause strong fluorescent and phosphorescent effects on various bodies, the substances first examined were those that were phosphorescent when exposed to light. The first observation in this direction was made by Niewenglowski[1], who found that sulphide of calcium exposed to the sun’s rays gave out some rays which were able to pass through black paper. A little later a similar result was recorded by H. Becquerel[2] for a special calcium sulphide preparation, and by Troost[3] for a specimen of hexagonal blend. These results were confirmed and extended in a later paper by Arnold[4]. No satisfactory explanations of these somewhat doubtful results have yet been given, except on the view that the black paper was transparent to some of the light waves. At the same time Le Bon[5] showed that, by the action of sunlight on certain bodies, a radiation was given out, invisible to the eye, but active with regard to a photographic plate. These results have been the subject of much discussion; but there seems to be little doubt that the effects are due to short ultra-violet light waves, capable of passing through certain substances opaque to ordinary light. These effects, while interesting in themselves, are quite distinct in character from those shown by the radio-active bodies which will now be considered. =3. Uranium.= The first important discovery in the subject of radio-activity was made in February, 1896, by M. Henri Becquerel[6], who found that a uranium salt, the double sulphate of uranium and potassium, emitted some rays which gave an impression on a photographic plate enveloped in black paper. These rays were also able to pass through thin plates of metals and other substances opaque to light. The impressions on the plate could not have been due to vapours given off by the substances, since the same effect was produced whether the salt was placed directly on the black paper or on a thin plate of glass lying upon it.