Selenium, an elementary substance discovered by Berzelius in 1817 in the refuse of a sulphuric acid manufactory near Fahlun. It resembles sulphur in many of its physical, and tellurium in many of its chemical characteristics. It is always found in combination, its compounds with the metals being called sele-nides, and they are very rare, the least rare being the selenides of iron, copper, and silver. The symbol of selenium is Se; its atomic weight, 79.5; its specific gravity when crystallized, 4.788; its observed vapor specific gravity at 2,588° F., 5.68. When heated as an amorphous powder to 212° it softens, and when raised a few degrees higher it melts, and on cooling forms a dark brown vitreous solid with a metallic lustre and a specific gravity varying from 4.3 to 4.8. Selenium has neither taste nor smell, and is a bad conductor of electricity. It combines with hydrogen to form seleniuretted hydrogen, which has an offensive odor. The gas is obtained by the action of sulphuric acid on selenide of potassium or iron.

Selenium forms with oxygen and water sele-nious acid, H2SeO3, and selenic acid, H2SeO4, which correspond in composition to sulphurous and sulphuric acids (see Sulphur), and form salts called respectively selenites and seleniates.