CHROMIUM (Neo-Lat., from Ok. chnima, el dot-). A metallic element discovered by Vauquelin in 1797. It does not occur free, but is found in eombination chiefly with iron as ehromite, which is the principal ore of chro mium, as erocoisite, a lead chromate, and as wolchonskoite. a native chrome ochre. Ch•o mium is also a frequent constituent of meteoric iron, and the green color of emerald. serpentine, peonhaite, and other minerals, is due to this element. Thp metal is readily obtained from chromit e by separating the iron and reducing the remaining chromic oxide with charcoal by heat ing in a lime crucible. It may also be prepared by the electrolysis of a solution of (linimets chloride containing chromic chloride.
Chromium (symbol, Cr, atomic weight, 52.1) is a grayish-white powder consisting of small Instrims, very hard. brittle rhoniliiihedral crys tals whieh have a specific gravity of 5.9 to 6.8, and melt at. a higher temperature than plati num. Owing to its high melting-point, the metal itself has no iniportant uses, although \Olen added in quantities of less than 1 per cent. to steel it yields an alloy called chromium stccl, which has a fine texture, great hardness. tenacity, and elasticity. An alloy of chromium with aIn lb MUM has also been described. With oxygen chromium forms two basic oxides, which yield, respectively, ehromons and chromic compounds, and an acid-forming oxide which yields chro Chromic oxide may be obtained by ignit ing the hydroxide or by heating a mixture of potassium hichromate and sulphur. It is a green pigment of great permanence that is known in commerce as chrome green and ultramarine green. The hydrated oxide, which is made by heating to dull redness 3 parts of boric acid and part of potassium biehromate, is the pigment known as emerald arccn. Guignet's green, Pen
a:W(7-'8 green, and Vcridian. Of similar com position is the green pigment called Arnaudon's green.
Many of the chromate: have considerable com mercial value, especially those of potassium, so dium, and ammonium. Among the biehromates, that of potassium is the most important, as it is used in the preparation of all other chromium salts. Potassium biehromate is prepared from chromite. the ore being, for this purpose, roasted, finely ground. and mixed with half its weight of potassium carbonate and twice its weight of lime; the mixture is heated to bright redness with an oxidizing flame in a reverberatory furnace. and, on cooling. the resulting mass is treated with but water; to the solution thus obtained potas sium sulphate is added for the purpose of precip itating the lime as sulphate. while the potasslam chromate produced remains dissolved; finally, dilute sulphuric acid is added in order to con vert the chromate into biehromate. Potassium hichromate crystallizes in the form of bright-red prisms. It is used for the preparation of chrome pigments for the production of various iotors in calico-printing and dyeing; in the manu facture of safety matches; as a bleaching agent tor tallow, palm oil, etc., with sulphuric acid: for the oxidation of anthracene to alizarin: in tanning leather; and in consequence of its prop erty of rendering gelatin insoluble when mixed with that substance and exposed to light, it finds extensive application in photogelatin processes. Lead chromate is the bright-yellow precipitate obtained when a solution of a lead salt is added to potassium biehromate: it is used as a pigment under the name of chrome yellotc.