The discovery of nuclear energy involved a succession of steps from various people. It started with German physician Wilhelm Roentgen's discovery of the X-ray in 1895. The following year, French physicist Antoine Henri Becquerel observed spontaneous energy emitting from uranium salts. Marie Curie and her husband Pierre studied his work, and they defined radioactivity in 1898. In 1920, Englishman Ernest Rutherford — regarded as the father of nuclear physics — wrote a theory about neutrons, albeit without identifying them. British physicist James Chadwick then identified neutrons in 1932. Otto Hahn and Fritz Strassmann successfully split a uranium atom in 1938, the birth of nuclear fission. Leo Szilard was a Hungarian-American physicist who had theorized about chain reactions. In 1939, Albert Einstein, Szilard, Edward Teller, and Eugene Wigner wrote to U.S. President Franklin Roosevelt, warning him of the dangers posed by nuclear energy if they were to be used in bombs. By 1942, Italian-born Enrico Fermi performed the world's first chain reaction at the University of Chicago.