An element, symbol Se, atomic number 34,
atomic weight 78.96, associated with telurium
and the sulphur family. It occurs as
selenides and in many sulphide ores, and is
used in making red glasses and enamels. As a
semiconductor it is used extensively in
photocells and rectifiers. It exists in
several allotropic forms, the grey being a
conductor of electricity when illuminated and
its conductivity increasing markedly with the
brightness of the incident light. Selenium
was discovered by Berzelius 1817.