A metal discovered by Klaproth in 1789, in the mineral called pech blende. In this, it is in the state of sulphuret. But it likewise occurs as an oxide in the green mica, or uranglimmer, and in the uranochre. In obtaining it from pech blende, the mineral is reduced to a fine powder, and digested in a nitric acid till every thing soluble is taken up. The solution is then rendered as neutral as possible by evaporation, and a current of sulphuretted hydrogen gas passed through it as long as any precipitate continues to fall. The liquid is filtered and heated, to drive off all traces of sulphuretted hydrogen. It is now precipitated by caustic ammonia; and the precipitate, after being well washed, is digested, while still moist, in a rather strong solution of carbonate of ammonia. A fine lemon-coloured liquid is obtained, which being set aside for a few days, deposits an abundance of fine yellow crystals, in rectangular prisms. These crystals being exposed to a red heat, give out water, carbonate of ammonia, and oxygen gas, and leave a black oxide of uranium, which is easily reduced to the metallic state, by passing a current of hydrogen gas over it, placed in a glass tube, and heated by a spirit-lamp. The metal presents a liver-brown colour, and remains in the state of powder, being incapable, according to some authors, of reduction by any heat that can be applied to it.
Dr. Ure, however, informs us that 50 grains, after being ignited, were formed into a ball with wax, and exposed in a well closed charcoal crucible to the most vehement heat of a porcelain furnace, the intensity of which gave 170° on Wedgewood's pyrometer. Thus a metallic button was obtained, weighing 28 grains, of a dark grey colour, hard, firmly cohering, finely grained, of very minute pores, and externally glittering: specific gravity, 8.1. A sulphuret of uranium has been formed, which has a black colour, and, when rubbed, a metallic lustre. Its capacity for forming alloys with other metals remains uninvestigated, in consequence of the scarcity of the metal. The oxides of uranium are used in painting upon porcelain yielding a fine orange colour in the enamelling fire, and a black one in that in which the porcelain itself is baked.