[Pages from history].

In March 1896, not long after the discovery of X rays by W. Röntgen, H. Becquerel discovers the "uranic rays" emitted by the element uranium. In 1898, M. Curie observes that minerals containing uranium are more radioactive than the uranium they contain; she infers that yet unknown radioactive elements are present in minute quantities in these minerals. A few months later, P. and M. Curie, after an exhausting labour, discover polonium and radium. The "physiological action" of the rays emitted by radium are observed for the first time in 1900-1901. E. Rutherford and F. Soddy show, in Montreal, in 1902, that radioactivity is a spontaneous transmutation of one chemical element in another. The discovery of the atomic nucleus and of nuclear reactions will lead F. and I. Joliot-Curie to discover artificial radioactivity in 1934.