selenium

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A nonmetallic element, red in powder form, black in vitreous form, and metallic gray in crystalline form, resembling sulfur and obtained primarily as a byproduct of electrolytic copper refining. It is widely used in rectifiers, as a semiconductor, and in xerography. Its photovoltaic and photoconductive actions make it useful in photocells, photographic exposure meters, and solar cells. Atomic number 34; atomic weight 78.96; melting point (of gray selenium) 217°C; boiling point (gray) 684.9°C; specific gravity (gray) 4.79; (vitreous) 4.28; valence 2, 4, or 6. See Table at element.

Century Dictionary and Cyclopedia

1. n. Chemical symbol, Se; atomic weight, 79.5. A non-metallic element extracted from the pyrite of Fahlun in Sweden, and discovered in 1818 by Berzelius. In its general chemical analogies it is related to sulphur and tellurium. It is found in combination with native tellurium, as in selen-tellurium, with sulphur in selen-sulphur; also in very small quantity in some of the varieties of iron pyrites, and in several rare selenides, as clausthalite, or lead selenide, etc. When precipitated it appears as a red powder, which melts when heated, and on cooling forms a brittle mass, nearly black, but transmitting red light when in thin plates. When heated in the air it takes fire, burns with a blue flame, and produces a gaseous compound, oxid of selenium, which has a most penetrating and characteristic odor of putrid horse-radish. Selenium undergoes a remarkable change in electrical resistance under the action of light: hence the use of selenium-cells. See resistance, 3, and photophone.

Wiktionary

1. n. A nonmetallic chemical element (symbol Se) with an atomic number of 34.

GNU Webster's 1913

1. n. (Chem.) A nonmetallic element of the sulphur group of atomic number 34, analogous to sulphur in its compounds. It is found in small quantities with sulphur and some sulphur ores, and obtained in the free state as a dark reddish powder or crystalline mass, or as a dark metallic-looking substance. It exhibits under the action of light a remarkable variation in electric conductivity, and is used in certain electric apparatus. Symbol Se. Atomic weight 78.96.

WordNet 3.0

1. n. a toxic nonmetallic element related to sulfur and tellurium; occurs in several allotropic forms; a stable grey metallike allotrope conducts electricity better in the light than in the dark and is used in photocells; occurs in sulfide ores (as pyrite)