In a recent work, M. Bloch has demonstrated that phosphorus, undergoing oxidation in air, gives rise to slightly motile ions, which make the air conduct, and cause condensation of aqueous vapour. Uranium and thorium are elements which possess the highest atomic weights (240 and 232); they occur frequently in the same minerals. _Radio-active Minerals._ I have examined many minerals in my apparatus; certain of them gave evidence of radio-activity, _e.g._, pitchblende, thorite, orangite, fergusonite, cleveite, chalcolite, autunite, monazite, &c. The following is a table giving in ampères the intensity, _i_, of the current obtained with metallic uranium and with different minerals:— │_i_ × 10^{11}. Uranium │2·3 Pitchblende from Johanngeorgenstadt│8·3 Pitchblende from Joachimsthal │7·0 Pitchblende from Pzibran │6·5 Pitchblende from Cornwallis │1·6 Cleveite │1·4 Chalcolite │5·2 Autunite │2·7 ───────────────────────────────────┼────────────── Various thorites │0·1 〃 │0·3 〃 │0·7 〃 │1·3 〃 │1·4 ───────────────────────────────────┼────────────── Orangite │2·0 Monazite │0·5 Xenotime │0·03 Æschynite │0·7 ───────────────────────────────────┼────────────── Fergusonite (two samples) │0·4 〃 │0·1 ───────────────────────────────────┼────────────── Samarskite │1·1 ───────────────────────────────────┼────────────── Niobite (two samples) │0·1 〃 │0·3 ───────────────────────────────────┼────────────── Tantalite │0·02 Carnotite │6·2 The current obtained with orangite (native oxide of thorium) varied greatly with the thickness of the layer. By increasing this thickness from 0·25 m.m. to 6 m.m. the current increased from 1·8 to 2·3.