Although curium follows americium in the periodic system, it was actually the third transuranium element to be discovered. It was identified by Seaborg, James, and Ghiorso in 1944 at the wartime metallurgical laboratory at the University of Chicago as a result of helium-ion bombardment of 239Pu in the Berkeley, California, 60-inch cyclotron. Visible amounts (30 µg) of 242Cm, in the form of the hydroxide, were first isolated by Werner and Perlman of the University of California in 1947. In 1950, Crane, Wallmann, and Cunningham found that the magnetic susceptibility of microgram samples of CmF3 was of the same magnitude as that of GdF3. This provided direct experimental evidence for assigning an electronic configuration to Cm+3. In 1951, the same workers prepared curium in its elemental form for the first time. Fourteen isotopes of curium are now known ranging in mass from 237 to 251. The most stable, 247Cm, with a half-life of 16 million years, is so short compared to the earth's age that any primordial curium must have disappeared long ago from the natural scene.