Recent research on radioactivity

In 1896 Becquerel discovered that uranium and its compounds emit these new rays continuously. Schmidt and Mme. Curie then found almost simultaneously that thorium compounds are also radioactive. The radiations emitted by thorium compounds are comparable in intensity with those from the compounds of uranium. Radioactivity is an atomic property that accompanies the atoms of uranium and thorium wherever they are found; in a compound or a mixture its intensity is proportional to the amount of the metal present...