Number 258 (Story #2), February 13, 1996 by Phillip F. Schewe and Ben Stein HENRI BECQUEREL ANNOUNCED THE DISCOVERY OF RADIOACTIVITY at a series of scientific meetings during 1896. Thus the radioactivity centennial comes quickly on the heels of the x-ray centennial. Becquerel's first evidence (presented on February 24) consisted of photographic plates exposed by the rays issuing from a uranium compound. A fuller understanding of radioactivity developed over successive months and years. Only in 1902/03, for instance, did Ernest Rutherford establish that radioactivity was the tangible product of the transmutation of one element into another. Others, such as Marie Curie, added more pieces to the puzzle. Eclipsed for some years by the greater fame of Roentgen's x rays, radioactivity came to have a profound effect on the 20th century through the use of unstable elements in nuclear reactors, nuclear weapons, and in medicine. By the way, the element uranium, discovered in 1789, takes its name from the planet Uranus, which itself had been discovered only a few years earlier in 1781. (Physics Today, February 1996.)