Page:Radio-activity.djvu/575

 α particle may be expelled with a velocity less than 1·5 × 10^9 cms. per second and so fail to produce much electrical effect.

These considerations have an important bearing on the question whether matter in general is radio-active. The property of emitting α particles above the critical velocity may well be a property only of a special class of substances, and need not be exhibited by matter in general. At the same time the results suggest that ordinary matter may be undergoing transformation accompanied by the expulsion of α particles at a rate much greater than that shown by uranium, without producing appreciable electrical or photographic action.