Selenium

Selenium was discovered in 1817 by the Swedish chemist Jons Jacob Berzelius, together with Johan Gottlieb Gahn, as a red deposit in the lead chambers of a sulfuric acid works. It closely resembled the element tellurium, already known and named for the Earth, so Berzelius named the new element after the Moon. Selenium is a metalloid whose electrical conductivity changes with the light falling on it, a property that made it central to early photocells, light meters and xerographic copying. An essential trace nutrient in tiny amounts but toxic in excess, selenium is used in glassmaking to remove unwanted colour or to produce a ruby-red tint, in pigments, and in the light-sensitive drums of photocopiers and laser printers.

Selenium's narrow margin between necessary and poisonous was recognised in agriculture before it was understood in human health: livestock grazing on seleniferous soils of the American West developed a wasting condition called alkali disease or blind staggers, a problem some scholars trace as far back as Marco Polo's thirteenth-century account of horses sickening on certain Chinese grasslands. In humans, selenium deficiency was linked in the mid twentieth century to Keshan disease, a potentially fatal heart condition endemic to selenium-poor regions of China, establishing the element as an essential trace nutrient at almost the same time its toxic threshold was being documented from the opposite direction.

Berzelius drew his name from Selene, the Greek goddess and personification of the Moon, deliberately pairing his new element with tellurium, which had been named from the Latin Tellus, the Earth. Because selenium so resembled tellurium chemically, he reasoned that if one bore the name of the Earth, its companion should bear the name of the Earth's satellite. The choice sets up a small celestial pairing within the periodic table, Moon beside Earth, and it echoes, without deriving from, the ancient association of silver with the Moon; here the connection is purely one of naming by analogy, a nineteenth-century chemist's poetic tidiness rather than any inherited lunar symbolism.

Before silicon diodes took over in the mid twentieth century, selenium rectifiers were a standard component in radios, battery chargers and industrial electronics, exploiting the same light- and current-sensitive behaviour that made selenium useful in photocells. In glassmaking, small amounts of selenium compounds produce a deep ruby red, prized by art glassmakers as selenium ruby, a striking counterpart to selenium's more common role decolorising glass by cancelling the green tint of iron impurities, the same trick manganese has long performed in the same industry.