CHROME, a metal discovered by Vau quelin. It exists in the state of an acid, combined with oxide of lead, in a beauti ful mineral named red lead, found in Si beria, and with regard to which very dis cordant analyses had been given by dif ferent chemists. Vauquelin reduced the metallic acid which he discovered in it to the metallic acid, and his researches have been confirmed by those of Klaproth and Gmelin. It derives its name from the splendid and numerous colours which it presents in its saline combinations. It has since been discovered in various mi nerals. The native chromate of lead, or the red lead of Siberia, is generally crys tallized in oblique tetrahedral prisms. Its colour is a fine aurora red ; its lustre: shining, and intermediate between ada mantine and resinous ; the crystals are translucid; the fracture is uneven; the specific gravity 6.0269. It depreciates before the blow-pipe, and melts into a blackish scoria. It colours borax green by fusion. According to Vauquelin, it is composed of 57.10 of lead, 6.86 of oxy gen, and 36.04 of chromic acid. There is found with the chromate of lead, a mi neral of a green colour, in minute crys tals,' which Vauquelin lound to be com posed of the oxides of chrome and lead, and which, as he conjectures, has pfoba bly originated in the decomposition of the perfect chromate, from some process by which part of its oxygen has been ab stracted.
Native chromate of iron has more late ly been found in the department of Var in France, and likewise in Siberia. This mineral is massive, of a blackish brown colour, with no great lustre, and opaque; its fracture is uneven,. and it is hard and difficult to break ; its specific gravity is 4.0. It is scarcely fusible before the blow-pipe, but with borax it melts into a glass of a fine green colour. According to an accurate analysis of it, it consists of 63.6 of chromic acid, or perhaps rather oxide of chrome, and 36 of oxide of iron.
Chrome has been also found in smaller quantities in other minerals, 'particularly in some gems, of which it appears to be the colouring principle. It exists in the emerald, in the state of green oxide, and in the spinal ruby, in the state of acid.
Vauquelin extracted the metal from the red lead ore, by adding to it muriatic acid, which combines with the oxide of lead, and forms a compound that is pre cipitated, the chromic acid remaining in solution. To abstract a little muriatic
acid combined with it, oxide of silver is cautiously added, and the pure chromic acid being decanted from the precipitate of muriate of silver, and evaporated, is exposed to a very strong heat, excited by a forge, in a crucible of charcoal, placed within another of porcelain. It is thus reduced to the metallic state. It is to this chemist that we are indebted, principally, for a knowledge of its pro perties.
Chrome is of a white colour inclining to grey: it is very brittle ; its fracture presents a radiated appearance, needles crossing in different directions, with in terstices between them Its other physi cal qualities have not been determined. This metal is difficult of fusion. Expos ed to the heat of the blow-pipe, it does not melt. When fused, by having been exposed to the intense heat necessary to its reduction, it presents crystalline fila ments, which rise above the metallic mass. Chrome is not easily acted on by the acids. Even when reduced to a fine powder, and treated with concentrated boiling, nitric acid, it is oxydized with much difficulty, and communicates to the acid only a green tinge.
Chrome, in the state of acid, appears to be more susceptible of combination, and this acid being obtained without dif ficulty from its native combinations, its chemical relations have been more ex amined. Chromic acid is very soluble in water ; the taste of the solution is sharp and metallic; it is of an orange-red co lour ; by evaporation, either spontaneous or with a gentle heat, it affords crystals in long slender prisms, of a ruby-red colour. This acid combines with the alkalies, earths, and metallic oxides, forming neutral salts, which are named chromates.
The combinations of this acid with me tallic oxides are in general possessed of very beautiful colours, and are well adapted to form the finest paints. That with oxide of lead has an orange yellow, of various shades; that with mercury, a vermilion red ; with silver, a carmine red; with zinc and bismuth, the colours are yellow ; with copper, cobalt, and an timony, they are dull.