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.
CHAPTER II
Consideration of Cell Types and Their Characteristics
A selenium cell consists essentially of two electrodes of brass or copper bridged by a thin layer of metallic selenium. When connected into a circuit with batteries and other apparatus the current flows from one electrode to the other thru this selenium bridge. Since the resistance of the selenium to an electric current depends upon the amount of light falling upon it the flow of current thru the cell will be controlled by the brilliancy of the illumination.
Metallic selenium being opaque, the light penetrating but ¹/₅₀,₀₀₀th of an inch as calculated by Marx, it is necessary that the selenium layer be extremely thin in order that the light may affect an appreciable proportion of the total conducting area. This condition is never reached when the electrodes lie parallel to each other with the selenium between them. However by arranging the electrodes so that the current flows at right angles to the plane of the selenium surface we can cause all the current to flow thru the light affected area. This can only be accomplished by making use of a transparent conductor for one electrode.