Charge on the α Particles of Polonium and Radium

WITH reference to the interesting letter on this subject by Prof. Rutherford in last week's NATURE, I should like to point out that in my paper “On the positive electrification of α rays and the emission of slowly moving kathode rays by radio-active substances” (Proc. Camb. Phil. Soc., xiii., p. 49) I have described experiments which demonstrate the communication of a positive charge of electricity to bodies struck by α rays from polonium or radium. I had considerable difficulty in disentangling this positive charge from the copious streams of slowly moving negatively electrified corpuscles which I found were given out by these substances, and the experiments in which I finally succeeded in doing this were not completed until a few days after the reading of the paper on November 14, and are not referred to in the abstract quoted by Prof. Rutherford. A description of them will be found in the paper which has lately been published. I may take this opportunity of saying that I have recently found that uranium also gives out slowly moving corpuscles, so that this effect seems a general property of radio-active substances. The velocity of these corpuscles is very small compared with that of the β rays, and is more nearly of the order of the velocity of the corpuscles emitted by metals when exposed to light.