Is Selenium Radio-active?

Abstract

IT occurred to me recently that a possible method of deciding between the two hypotheses which have been brought forward to explain radio-activity, namely, that of atomic degradation (Rutherford and Soddy, Ramsay, &c.) and that of molecular change (Armstrong and Lowry, Proc. Roy. Soc., 1903), lay in attempting to realise radio-activity in the case of an element well known to undergo molecular change readily, but with an atomic weight small enough to exclude the probability of an atomic instability such as is assumed for radium and thorium. Such an element is selenium (at. wt. 79), which suggested itself to me as a suitable material to experiment with because, under the influence of light, it undergoes a remarkable alteration in its electrical resistance and E.M.F. of contact, suggesting an allotropic change of an altogether unusual character. As this change, whatever be its real nature, occurs almost instantaneously (Bellati and Romanese, Atti R. Ist. Veneto, 1881; Maiorana, Atti dei Lincei, 1894 and 1896), it seemed possible that the rapidity of the intermolecular vibration might be sufficient to cause a radiation similar to that of radium and thorium which, by “ionising” the selenium, would render it conducting. In order to ascertain whether such a hypothetical radiation could be detected photographically, I exposed a piece of selenium, placed on a photographic plate, wrapped in three thicknesses of black glazed paper, during thirty-six hours to the bright sunshine of July. On developing the plate a distinct black stain on a background of clear glass indicated the position the selenium had occupied. The first experiment was made with ordinary vitreous selenium, and the stain, although distinct, was not very pronounced. A second experiment with freshly prepared “metallic” selenium, obtained by heating the vitreous variety at 190° for half an hour and then cooling very gradually to the ordinary temperature, gave a much more intense stain on the negative.