Selenium is named from the Greek Selene, moon and was first extracted in 1817. Selenium is found in a few rare minerals, such as crooksite and clausthalite. The anode muds from electrolytic copper refineries provide the source of most of the world's selenium. The element is a member of the sulfur family and resembles sulfur both in its form and in its compounds. Selenium exhibits both photovoltaic action, where light is converted directly to electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells. Selenium is also used in the Xerography process as a photographic toner for reproducing and copying documents, and as an additive to stainless steel.