"These radiations from uranium are persistent, and as far as observations have yet gone, are unchanged either in intensity or character, with lapse of time. Observations to test the constancy of the radiations for long periods of time have been made by Becquerel. ...No perceptible weakening of the radiation has been observed over a period of four years. Mme Curie has made determinations of the activity of uranium over a space of five years... but found no appreciable variation during that period."