Uranium, a metal, the protoxide of which, supposed to be the metal itself, was discovered in 1789 by Klaproth in the mineral pitchblende, and was named by him after the planet Uranus, then lately discovered. The metal itself was not really separated until M. Péligot obtained it about 1840 by decomposing its chloride by means of potassium or sodium. Thus produced, it is partly in the form of a black powder, and in part composed of silvery lamina which can be filed and are somewhat ductile. The metal dissolves in dilute acids, setting free hydrogen gas. Its specific gravity is 18-4. In the air it undergoes no change at common temperatures; but when the metal in the form of a powder is moderately heated, it takes fire and burns with a remarkably white and shining light. The product of this combustion is a deep green oxide. Uranium is represented by the symbol U, and its chemical equivalent is 240 (formerly 60, then 120). It forms two classes of compounds: the uranous, in which it is quadrivalent, as uranous oxide, UO2 (oxygen being bivalent), and uranous chloride, UC14; and the uranic, in which it is sexvalent, as uranic oxide, U03, and uranic oxychloride, UO2C12. Prof. Roscoe in 1875 described a pentachloride, UOl5. There is also a uranoso-uranic oxide, U02, 2UO3, or U308. Uranic oxide unites with basic metallic oxides, forming uranates; and there are salts of the metal uranium with iodine, bromine, and fluorine.
The compounds of uranium are employed chiefly in giving yellowish hues to glass and porcelain. The peculiar yellow tint with greenish or opaline reflections seen in Bohemian glass is derived from them. Uranium glass is remarkable for exhibiting with great distinctness the phenomenon of fluorescence, and this property, together with the absorption spectra afforded by uranium salts, has been recently thoroughly investigated by President Morton of the Stevens institute, Hoboken, and Dr. H. Oarrington Bolton of Columbia college, New York. (See." The American Chemist," 1873.) Uranium compounds are of great value in porcelain painting, mineral pitchblende being used to a considerable extent at Joachimsthal in Bohemia, where it is converted into uranate of soda for this purpose. It produces an orange color in the enamelling fire, and a fine black in the furnace in which the porcelain is baked. The uranate of soda is of fine orange color, and has been proposed as a paint. Uranium is found accompanying various ores of silver and lead in several of the mining districts of Bohemia, Hungary, and Saxony. It also occurs as a sulphate and carbonate.
Torbernite, occurring in beautiful green crystals, is a phosphate of uranium and copper; autunite, found in yellow scales, contains phosphate of uranium and lime. Uranic arsenates of similar composition have been recently discovered. Fine specimens of uranium minerals are found near Redruth in Cornwall, and in various localities in Bohemia and Saxony. - See "Index to the Literature of Uranium," by H. Carrington Bolton, in the "Annals of the Lyceum of Natural History," New York, 1870.