I made several experiments with radium enclosed in a little glass vessel. The rays emerging from the vessel, after traversing a certain space of air, were received in a condenser, which served to measure their ionising capacity by the usual electrical method. The distance, _d_, from the source to the condenser was varied, and the current of saturation, _i_, obtained in the condenser was measured. The following are the results of one of the series of determinations:— _d_, c.m. _i._ _i_ × _d_^2 × 10^{–3}. 10 127 13 20 38 15 30 17·4 16 40 10·5 17 50 6·9 17 60 4·7 17 70 3·8 19 100 1·65 17 After a certain distance, the intensity of radiation varies inversely as the square of the distance from the condenser. The radiation of polonium is only propagated in air to a distance of a few centimetres (4 to 6 c.m.) from the source of radiation. In the case of the absorption of radiations by solid screens, we find another fundamental difference between radium and polonium. Radium emits rays capable of penetrating great thicknesses of solid matter, _e.g._, several centimetres of lead or of glass. The rays which have passed through a great thickness of a solid body are extremely penetrating, and it is practically impossible to absorb them entirely by any material whatever. But these rays form only a small fraction of the total radiation, the greater part of which is absorbed by a slight thickness of solid matter. Polonium emits rays which are readily absorbed, and which can only pass through extremely thin screens. The following are figures showing the absorption produced by an aluminium lamina of thickness 0·01 m.m. This lamina was placed above and almost in contact with the substance. The direct radiation and that transmitted by the aluminium were measured by the electrical method (apparatus of Fig. 1); the current of saturation was practically obtained in every case. I have represented the activity of the radiating body by _a_, that of uranium being unity.