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Over a century ago, 1817 to be exact, the Swedish scientist Berzelius discovered a new element in the lead chambers used for the manufacture of sulphuric acid by roasting iron pyrites. Noting its resemblance to Tellurium, the name for which having been derived from the Greek for Earth, Tellus, he named the new element Selenium derived from the Greek for Moon, Selene. The ending ’um being used to indicate a metal according to the practice of naming newly discovered elements. Although believed to be a metal for many years, the chemical reaction of Selenium resembles that of sulphur to such a degree that it is now accepted to be a non-metal in its amorphous and vitreous forms. In its third or crystalline state it has many metallic characteristics and in this form termed metallic selenium. In the Periodic System it occupies the place between Tellurium and Sulphur.

Designated by the symbol Se, selenium has been found in all parts of the globe in small quantities, chiefly in combination with copper, lead and silver forming selenides, in certain pyrites and occasionally in its pure state. It was found in meteoric iron by Warren in 1909.

An idea of its wide distribution may be gained from the following table:

Mineral	Composition	Location
	Sulphur Selenide in natural sulphur	Lispau Islands
Eucarite	Selenide of silver and copper	Chili
Crooksite	Selenide of silver, copper and thallium	Norway and Sweden
Clauthalite	Selenide of lead	Germany
Lehrbachite	Selenide of lead, copper and mercury	Germany
Zorgite	Selenide of lead and copper	Germany

The element is obtained commercially as a by-product from the manufacture of sulphuric acid, various methods of extracting it from the chamber mud being employed. The usual process is to heat the well washed chamber mud with potassium cyanide and nitrate to obtain an alkaline selenate. The element is then precipitated with hydrochloric acid or sulphur dioxide.

Selenium exists in three well defined forms, Amorphous, Vitreous and Metallic.

Amorphous Selenium. This form is obtained as a finely divided brick red precipitate when sulphur dioxide is passed thru selenic acid. It is soluble in sulphuric acid and slightly so in carbon disulphide. It has a Sp. Gr. of 4.26, with no definite melting point, softening gradually and running together between 80° and 100° C. In this state it is an insulator.

Vitreous Selenium. When the amorphous selenium is heated to 217° C and rapidly cooled the vitreous form results. It is now a red vitreous mass, slightly less soluble in carbon bisulphide. When a thin film is held up to the light it shows blood red in color. Sp. Gr. 4.28, Atomic Weight 79.5. This form is practically an insulator having a resistance of 6 × 10⁹ ohms per Cu. Cent. at 75° C or about 3.8 × 10¹⁰ as great as that of copper. It can be electrified by friction. Vitreous selenium has no definite melting point being hard and brittle at 40° C and softening gradually as the temperature rises, becoming fluid at 210° C.

Metallic Selenium. By cooling melted vitreous selenium to 210° C and holding it at that temperature for a short time the metallic form results. The element is now a black glossy opaque mass, a fair conductor of electricity but improving greatly under the influence of light. It melts between 217° and 220° C, Sp. Gr. 4.788, insoluble in carbon disulphide but will dissolve in sulphuric acid to form a green solution. It is the latter form that is used in the construction of selenium cells.

When selenium is vaporized by heat it gives off dark brown fumes having an odor similar to rotting cabbage. These fumes are poisonous and care should be taken that they are not breathed to excess.