Collier's New Encyclopedia (1921)/Selenium

SELENIUM, a non-metallic hexad element occupying an intermediate place between sulphur and tellurium: Symbol Se; at. wt. 79.5. It was discovered by Berzelius in 1817. Though not very abundant in nature, it enters into the composition of many minerals, and has been found in the free state in certain parts of Mexico. It is prepared from cuproplumbic selenide by heating the pulverized ore with hydrochloric acid, igniting the insoluble residue with an equal weight of black flux and dissolving out the selenide of potassium with boiling water. By exposing this solution to the air selenium is deposited as a gray powder. Like sulphur, it occurs in the amorphus and crystalline states. In the former it may be drawn out into ruby-colored threads, and when melted and quickly cooled becomes vitreous with a sp. gr. of 4:3, and nearly insoluble in bisulphide of carbon. In the crystalline condition it forms monoclinic prisms of sp. gr.=4.5-4.7. It boils below a red heat and gives off a deep yellow vapor which condenses in scarlet flowers, and when thoroughly heated burns with a blue flame forming selenious anhydride. It is oxidized and dissolved by nitric acid, yielding selenious acid. It is used in certain electric contrivances on account of the changes its electric resistance undergoes when it is subjected to light.