Selenium, chemical element, symbol Se; for physical constants see Periodic Table. Selenium was discovered by Jöns J. Berzelius in 1817. It occurs in nature in the minerals crooksite and clausthalite. It is obtained commercially from the anode muds left from electrolytic copper refining, from which selenium is prepared by roasting with soda or sulfuric acid. Selenium is a lustrous gray metalloid which exists in several allotropic forms. It can be amorphous or crystalline. The most stable form is the crystalline hexagonal form. Selenium is an essential trace element for humans, however selenides resemble arsenic in their extreme toxicity. Selenium and its compounds are used in photocells, light meters, solar cells, rectifiers, special glasses, and xerography.