Selenium

Berzelius found selenium in 1817 in the sludge from a sulfuric acid plant and named it after Selene, the moon, as a companion to tellurium's earth. Its electrical resistance falls sharply in light, and that property is the whole basis of xerography: Chester Carlson made the first electrophotographic copy in 1938, and the selenium-coated drum in early Xerox machines held an electrostatic charge in the dark and released it wherever light struck. In glassmaking a trace of selenium cancels the green cast that iron impurities give, and more of it makes ruby-red signal glass. Nutritionally it is essential — glutathione peroxidase and the thyroid deiodinases need it, and deficiency in low-selenium soils causes Keshan disease — but the margin is narrow: the recommended intake is around 55 µg a day and toxicity begins around 400 µg.