Uranium (U. 240) is a metal which does not occur to a great extent in the crust of the earth, and of which the chief ore is pitchblende. Its existence in this source was first shown by Klaproth in 1789, but the pure metal was not prepared until over a half a century later. Pitchblende consists chiefly of oxides of uranium, which may be present to the extent of ninety per cent., and is found in many localities, notably Cornwall. Combined with other rarer metals, uranium also occurs in the minerals samarskite and euxenite. As obtained from its salts by reduction with sodium, it is a white metal, malleable and hard, which does not oxidize and tarnish in air, and which possesses the high specific gravity of 18.33. It forms a large number of oxides, of which the trioxide acts as an acid forming salts, the uranates, which somewhat resembled the chromates. It forms also three chlorides by direct union of the elements. Of these pentachloride is interesting as existing in two varieties, both very hygroscopic, and decomposed by water. The metal also forms a number of oxysalts, the uranyl salts, which are usually yellow soluble compounds. The nitrate is also called uranium nitrate, and is used in photography as an intensifier, while printing processes dependent upon the use of the same salt may be employed.