In their photographic and electric effects, the Becquerel rays approximate to the Röntgen rays. They also, like the latter, possess the faculty of penetrating all matter. But their capacity for penetration is very different; the rays of uranium and of thorium are arrested by some millimetres of solid matter, and cannot traverse in air a distance greater than a few centimetres; this at least is the case for the greater part of the radiation.
The researches of different physicists, and primarily of Mr. Rutherford, have shown that the Becquerel rays undergo neither regular reflection, nor refraction, nor polarisation.
The feeble penetrating power of uranium and thorium rays would point to their similarity to the secondary rays produced by the Röntgen rays, and which have been investigated by M. Sagnac, rather than to the Röntgen rays themselves.
For the rest, the Becquerel rays might be classified as cathode rays propagated in the air. It is now known that these different analogies are all legitimate.
Measurement of the Intensity of Radiation.
The method employed consists in measuring the conductivity acquired by air under the action of radio-active bodies; this method possesses the advantage of being rapid
Fig. 1.
and of furnishing figures which are comparable. The apparatus employed by me for the purpose consists essentially of a plate condenser, a b (Fig. 1). The active body, finely powered, is spread over the plate b, making the air between the plates a conductor. In order to measure the conductivity, the plate b is raised to a high potential by connecting it with one pole of a battery of small accumulators, p, of which the other pole is connected to earth. The plate a being maintained at the potential of the earth by the connection c d, an electric current is set up between the two plates. The potential of plate a is
