1800 Dalton firmly establishes atomic theory of matter. 1890-1900 Thomson’s experiments with cathode rays prove the existence of electrons. Atoms are found to contain negative electrons and positive electric charge. Becquerel discovers unstable (radioactive) atoms. 1905 Einstein postulates the equivalence of mass and energy. 1911 Rutherford recognizes nucleus. 1919 Rutherford achieves transmutation of one stable chemical element (nitrogen) into another (oxygen). 1920-1925 Improved mass spectrographs show that changes in mass per nuclear particle accompanying transmutation account for energy released by nucleus. 1932 Chadwick identifies neutrons. 1939 Discovery of uranium fission by German scientists. 1940 Discovery of neptunium by Edwin M. McMillan and Philip H. Abelson and of plutonium by Glenn T. Seaborg and associates at the University of California. 1942 Achievement of first self-sustaining nuclear reaction, University of Chicago. 1945 First successful test of an atomic device, near Alamagordo, New Mexico, followed by the dropping of atomic bombs on Hiroshima and Nagasaki, Japan. 1946 U. S. Atomic Energy Commission established by Act of Congress. First shipment of radioisotopes from Oak Ridge goes to hospital in St. Louis, Missouri. 1951 First significant amount of electricity (100 kilowatts) produced from atomic energy at testing station in Idaho. 1952 First detonation of a thermonuclear bomb, Eniwetok Atoll, Pacific Ocean. 1953 President Eisenhower announces U. S. Atoms-for-Peace program and proposes establishment of an international atomic energy agency. 1954 First nuclear-powered submarine, _Nautilus_, commissioned. 1955 First United Nations International Conference on Peaceful Uses of Atomic Energy held in Geneva, Switzerland. 1957 First commercial use of power from a civilian reactor takes place in California. Shippingport Atomic Power Plant in Pennsylvania reaches full power of 60,000 kilowatts. International Atomic Energy Agency formally established. 1959 First nuclear-powered merchant ship, the _Savannah_, launched at Camden, New Jersey. Commissioning of first nuclear-powered Polaris missile-launching submarine _George Washington_. 1961 A radioisotope-powered electric power generator placed in orbit, the first use of nuclear power in space.