\u3ci\u3ePhysics\u3c/i\u3e, Chapter 45: Natural Radioactivity

The discovery of an important phenomenon usually leads to other important discoveries. The discovery of x-rays by Roentgen in 1895 led to the discovery of radioactivity by Becquerel in 1896. In the gas type of x-ray tube used by Roentgen, the glass walls of the tube were observed to fluoresce. Becquerel was interested in determining whether there was any relationship between the fluorescence of the glass of an x-ray tube and the phosphorescence of certain salts which were irradiated by ordinary light. One of the salts used by Becquerel was the double sulphate of uranium and potassium. He wrapped a photographic plate in very thick black paper, placed a crystal of the uranium salt on it, and exposed the whole thing to sunlight. Later, on developing the plate, he found the silhouette of the crystals on the negative; he interpreted this as produced by radiations coming from the crystal. He also performed other experiments in which he placed various absorbing materials between the uranium salt and the photographic plate; in each case, upon developing the plate, he found the shadow of the absorbing material imaged on the plate; he interpreted this as being due to the absorption of the radiation by the substance which had been placed between the uranium salt and the photographic plate. The crowning experiment was the one in which he decided not to irradiate the salt with light from an external source but to determine whether the salt was itself the source of these radiations. For these experiments he built a light tight box that would hold a photographic plate at the bottom. In one experiment some uranium salt crystals were placed on the photographic plate; later, on developing this plate, Becquerel obtained the silhouettes of the individual crystals. In another experiment he put a piece of aluminum between the uranium crystals and the photographic plate, and, on developing the plate, he again found the silhouettes of the crystals, but they were of decreased intensity due to the absorption of some of the radiations by the aluminum. From these experiments Becquerel concluded that the radiations came from the uranium salt, and that the external light had no influence on them