dysprosium (dĭsprōˈzēəm) [key] [Gr., = hard to get at], metallic chemical element; symbol Dy; at. no. 66; at. wt. 162.500; m.p. 1,412°C; b.p. 2,562°C; sp. gr. 8.54 at 25°C; valence+3. Dysprosium is a lustrous silvery metal; it is very soft and can be cut with a knife. It is in Group 3 of the periodic table and is a member of the lanthanide series; all members of this series are rare-earth metals and resemble one another in their chemical properties. Dysprosium is stable in air at room temperature. It dissolves in both dilute and concentrated mineral acids; forms a white oxide known as dysprosia; and, with other elements, forms several brightly colored salts. It is commonly found with other rare-earth metals in several minerals, including gadolinite and euxenite. Dysprosium and its compounds are among the most highly susceptible to magnetization of all substances and are used in special magnetic alloys. A cermet of dysprosium oxide and nickel is used in nuclear reactor control rods. Dysprosium is used with argon in mercury-vapor lamps to give a higher light output and balance the color spectrum. Although dysprosium was discovered (but not isolated) in 1886 by P. E. Lecoq de Boisbaudran, a French chemist, it did not become available in relatively pure form until the 1950s.