The experiments that I made on the absorption of uranium and thorium rays showed that those of thorium are more penetrating than those of uranium, and that the rays emitted by the oxide of thorium in a thick layer are more penetrating than those emitted by a thin layer of the same. The following figures (p. 13) give the fraction of the radiation transmitted by a sheet of aluminium 0·01 thick. With the uranium compounds, the absorption is the same whatever be the compound used, which leads to the conclusion that the rays emitted by the different compounds are of the same nature. Radio-active substance. Fraction of radiation transmitted by the sheet. Uranium 0·18 Uranium oxide, U_{2}O_{5} 0·20 Uranate of ammonium 0·20 Phosphate of uranium and copper 0·21 Thorium oxide of thickness 0·25 m.m. 0·38 Thorium oxide of thickness 0·5 m.m. 0·47 Thorium oxide of thickness 3·0 m.m. 0·70 Thorium oxide of thickness 0·60 m.m. 0·70 Thorium sulphate 0·25 m.m. 0·38 The characteristics of the thorium radiation have formed the subject of very complete publications. Mr. Owens has demonstrated that a uniform current is only obtained after some time has elapsed, with an enclosed apparatus, and that the intensity of the current is greatly reduced under the influence of a current of air (which does not occur with the compounds of uranium). Mr. Rutherford has made similar experiments, and has explained them by the proposition that thorium and its compounds produce, besides the Becquerel rays, another _emanation_, composed of extremely minute particles, which remain radio-active for some time after their emission, and are capable of being swept along by a current of air. The characteristics of the thorium radiation, which have reference to the thickness of the layer employed and to the action of air currents, have an intimate connection with the phenomenon of the _radio-activity induced, and of its propagation from place to place_. This phenomenon was observed for the first time with radium, and will be described later.