At the outset, Roentgen had noticed that although X rays passed through human tissue without causing any immediate sensation, they definitely affected the skin and underlying cells. Soon after exposure, it was evident that X rays could cause redness of the skin, blistering, and even ulceration, either in single doses or in repeated smaller doses. In spite of the hazards[1] involved, early experimenters determined that X rays could destroy cancer tissues more rapidly than they affected healthy organs, so a basis was established quite soon for one of Medicine’s few methods of curing or at least restraining cancer. The work of the Curies in turn stimulated many studies of the effect of radioactivity. It was not long before experimenters learned that naturally radioactive elements—like radium—were also useful in cancer therapy. These elements emitted gamma rays,[2] which are like X rays but usually are even more penetrating, and their application often could be controlled better than X rays. Slowly, over the years, reliable methods were developed for treatment with these radioactive sources, and instruments were designed for measuring the quantity of radiation received by the patient. [Illustration: _Frederic and Irene Joliot-Curie_] The next momentous advance was made by Frederic Joliot, a French chemist who married Irene Curie, daughter of Pierre and Marie Curie. He discovered in 1934 that when aluminum was bombarded with alpha particles[3] from a radioactive source, emission of positrons (positive electrons) was induced. Moreover, the emission continued long after the alpha source was removed. This was the first example of artificially induced radioactivity, and it stimulated a new flood of discoveries. Frederic and Irene Joliot-Curie won the Nobel Prize in chemistry in 1935 for this work. Others who followed this discovery with the development of additional ways to create artificial radioactivity were two Americans, H. Richard Crane and C. C. Lauritsen, the British scientists, John Cockcroft and E. T. S. Walton, and an American, Robert J. Van de Graaff. Ernest O. Lawrence, an American physicist, invented the cyclotron (or “atom smasher”), a powerful source of high-energy particles that induced radioactivity in whatever target materials they impinged upon. Enrico Fermi, an Italian physicist, seized upon the idea of using the newly discovered neutron (an electrically neutral particle) and showed that bombardment with neutrons also could induce radioactivity in a target substance. Cockcroft and Walton, Lawrence, and Fermi all won Nobel Prizes for their work.