_Radio-activity of the Compounds of Uranium and Thorium._ The following are the figures I obtained with different uranium compounds. I have represented the intensity of the current in ampères by the letter _i_:— _i_ × 10^{11}. Metallic uranium (containing a little carbon) 2·3 Black oxide of uranium, U_{2}O_{5} 2·6 Green oxide of uranium, U_{3}O_{4} 1·8 Hydrated uranic acid 0·6 Uranate of sodium 1·2 Uranate of potassium 1·2 Uranate of ammonium 1·3 Uranium sulphate 0·7 Sulphate of uranium and potassium 0·7 Nitrate of uranium 0·7 Phosphate of copper and uranium 0·9 Oxysulphide of uranium 1·2 The thickness of the layer of the uranium compound used has little effect, provided that the layer is uniform. The following illustrate this point:— Thickness of layer. _i_ × 10^{11}. M.m. Uranium oxide 0·5 2·7 Uranium oxide 3·0 3·0 Ammonium uranate 0·5 1·3 Ammonium uranate 3·0 1·4 It may be concluded from this that the absorption of uranium rays by the substance which generates them is very great, since the rays proceeding from deep layers produce no significant effect. The figures I obtained with thorium compounds enable me to state:— Firstly, that the thickness of the layer used has considerable effect, especially in the case of the oxide. Secondly, that the action is only regular if a sufficiently thin layer is used (_e.g._, 0·25 m.m.). On the contrary, when a thick layer of the substance is used (6 m.m.), the figures obtained vary between two extreme limits, especially in the case of the oxide:— Thickness of layer. _i_ × 10^{11}. M.m. Thorium oxide 0·25 2·2 Thorium oxide 0·5 2·5 Thorium oxide 2·5 4·7 Thorium oxide 3·0 5·5 (mean) Thorium oxide 6·0 5·5 (mean) Thorium sulphate 0·25 0·8 (mean) There is here some cause of irregularities which do not exist in the case of the uranium compounds. The figures obtained for a layer of oxide 6 m.m. thick varied between 3·7 and 7·3.