bismuth (bĭzˈməth) [key] [Ger. Weisse Masse = white mass], metallic chemical element; symbol Bi; at. no. 83; at. wt. 208.98040; m.p. 271.3°C; b.p. about 1,560°C; sp. gr. 9.75 at 20°C; valence +3 or +5. Bismuth is a silver-white, reddish-tinged, brittle metallic element with a rhombohedral crystalline structure. It exhibits more metallic properties than the other members of Group 15 of the periodic table. It occurs free in nature to a small extent. Bismuth does not tarnish in air, but when heated it burns to form yellow fumes of the trioxide. It reacts with the halogens and with sulfur and is dissolved in nitric acid and hot sulfuric acid. Its soluble compounds are poisonous, but some of its insoluble compounds are used in medicine to treat certain gastric disorders and skin injuries. Bismuth is the poorest heat conductor of all the metals except mercury; it is the most diamagnetic of all metals. The major ores of bismuth, bismuthinite (the sulfide), also called bismuth glance, and bismite (the oxide), are found extensively in South America but are rare in the United States, where bismuth is obtained as a byproduct of lead and copper refining. Bismuth expands upon solidification; this unusual property makes it useful in type-metal alloys and for castings. The most important use of bismuth is in the manufacture of low-melting alloys, such as Wood's metal, used in electrical fuses and in automatic fire alarm and sprinkler systems. Bismuth was recognized as a metal by early observers, including Georg Agricola, in the 16th cent., but was believed to be a kind of lead or tin until Claud J. Geoffroy established it as a separate element in 1753.