In 1896 Becquerel discovered that certain of the heavier elements emitted penetrating radiation and were unstable; the earlier concept that elements represented the most stable form of matter had therefore to be abandoned. There are three radioactive series known in nature, the parent elements of which are uranium-238, uranium-235 and thorium-232. Each of these decays through a series of daughter elements, which are also radioactive, to a final stable element, which in each series is one of the several isotopes of lead. Radium is one of the daughter elements in the uranium-238 series. The disintegration of the nucleus of a naturallyoccurring radioactive substance is accompanied by the emission of one or more forms of radiation which were named alpha-, beta- and gammarays. Gamma-rays were shown to be penetrating electromagnetic radiation of the same physical nature as X-rays, and it is the radioactive substances emitting gamma-rays which are used in radiography. Alpharays consist of the nuclei of helium and although they may have a considerable kinetic energy they will penetrate only very small thicknesses of material such as, for example, very thin foil. Beta-rays are electrons and also have only a low penetrating power.