Contemporary Advances in Physics - XII Radioactivity

N the year 1896, which fell near the beginning of the great transformation of modern physics, Henri Becquerel heard that Roentgen had discovered strange rays proceeding from an electric dischargetube while the discharge was passing and the glass walls of the tube were phosphorescing. Suspecting that the new rays were connected with the phosphorescence, Becquerel tested samples of some of the substances which naturally phosphoresce. It happened that one which he tested was a compound of uranium. He wrapped the sample in paper to shut in the light of its phosphorescence, and set it beside a photographic plate; for the rays of Roentgen had disclosed themselves by acting on such plates. Becquerel had made a happy guess; for the compound affected the plate. Yet his original idea was altogether wrong; for the effect had nothing to do with the phosphorescence of the compound, it was due to the uranium itself and faithfully reappeared when other and non-phosphorescent compounds were used instead, and even when a piece of the pure metal was set beside the plate. It was an instance of a fallacious idea having guided a keen observer to a great discovery--not the first in the history of physics, and assuredly not the last. Thereupon Pierre and Marie Curie, having verified that the effect of any quantity of any compound of pure uranium is strictly proportional to the amount of uranium in it, noticed that the effect of certain natural rocks and minerals containing uranium was much greater than that which their content of the metal should produce.