Chromioi (Gr.Chromioi 0400266 color), a metal so named from its tendency to impart beautiful colors to its compounds. About the year 1760 S. Lehmann, a Saxon mineralogist, was puzzled by a red mineral that was thought to contain gypsum, iron, and lead, and which was called red-lead spar, He wrote about it to Button in Latin, De nova Mineralis Plumoi Specie Crystalline: rubra; but no one could tell him what it was. Each successive chemist who examined it detected something new, and the ore was in a fair way of contributing a complete mineralogical cabinet in itself, until Klaproth announced that it contained a new metal, and Vauquelin discovered in it a new acid. As soon as these two discoveries were announced, many of the chemists of the day attacked the mineral; and it was not long before they found that it was composed of chromate of lead. Here the history of this element would have come to an end, if some years later an iron ore had not been discovered in France which also contained it, and thus afforded it in sufficient quantity for the various uses to which it was destined to be applied.

Chromium ore has since been found in many localities, among others in the United States near Baltimore, where it is a source of profit to the owners of the mines. Deville obtained chromium in the fused state by igniting a mixture of chromic oxide and sugar charcoal in a lime crucible, intensely heated in a portable blast furnace. The metal thus prepared is shining, has a steel-gray color, a specific gravity of 6, and is hard enough to scratch glass. J. Loughlin prepares metallic chromium by exposing a mixture of chromic oxide and potassium cyanide to a white heat in a covered porcelain crucible. A solution of chromic chloride or other chromic salt, treated with sodium amalgam, yields an amalgam of chromium, which, when heated in a stream of hydrogen, leaves metallic chromium in the form of a pulverulent sponge. - Chromium forms four well marked oxides: a protoxide, CrO, and a sesquioxide, Cr2O3, both capable of forming salts with acids; an intermediate oxide, CrO, Cr2O3, corresponding to the magnetic oxide of iron; and a stable anhydride, CrO3, which by its action on bases forms salts corresponding to the manganates and ferrates.

It also appears probable that a perchromic acid, HCrO4, exists; at least a blue liquid, which is soluble in ether, is obtained on pouring peroxide of hydrogen into a solution of chromic acid; but none of its compounds are known. The protoxide is known only in the hydrated condition, when it has a dark brown color. It forms a double sulphate with sulphate of potassium, CrSO4, K2SO4 + 6H2O, corresponding to the double sulphate of iron and potassium both in form and composition. The crystals are of a fine blue color. Sesquioxide of chromium is obtained as a grayish green hydrate, by boiling with alcohol a solution of bichromate of potassium, acidulated with sulphuric acid. On the addition of ammonia, a bulky, gelatinous, bluish green precipitate of the hydrated oxide is produced. In this form it is freely soluble in acids, and forms salts, the solutions of which are green, but they do not crystallize. Sesquioxide of chromium gives rise to another set of soluble salts, which are of a violet color, and crystallize readily. - Besides the soluble variety of the salts of chromium, an anhydrous, insoluble series is known, corresponding, it would seem, to the dense and comparatively inert modification of the metal itself.

Anhydrous green oxide of chromium is not decomposed by heat, and hence is used as a green color in enamel painting. For this purpose it is usually prepared by decomposing basic chromate of mercury at a red heat. Sesquioxide of chromium is the coloring ingredient in greenstone, in the emerald, in pyrope, and in several other minerals. The pink color used on earthenware is prepared by heating to redness a mixture of 30 parts of peroxide of tin, 10 of chalk, and 1 of chromate of potassium; the product is then powdered and washed with weak hydrochloric acid. A beautiful green pigment, known as vert de Guignet, is made by calcining bichromate of potassium with three times its weight of crystallized bo-racic acid. - Chrome ironstone, FeO, Cr2O3, is the principal ore of chromium. It corresponds in composition to the brown oxide of chromium, and to the magnetic oxide of iron; part of the iron is, however, generally displaced by the isomorphous metal magnesium, and part of the chromium by aluminum. Of the pure ore, 100 parts contain 48-27 per cent. of chromium, and yield 39.6 of chromic acid. - Chromic anhydride, or chromic acid, CrO3, is obtained by mixing four measures of a cold saturated solution of bichromate of potassium with five of oil of vitriol; as the liquid cools the chromic anhydride separates in beautiful crimson needles.

The crystals are allowed to dry on a porous tile under a bell glass. It fuses at 400°, and at a higher heat is converted into the sesquioxide. The anhydride is deliquescent, has a sour metallic taste, and possesses considerable oxidizing power. - Chromates. Chromic acid forms three classes of salts: basic, normal, and acid. Bichromate of potash, or anhydro-chromate of potassium, K2Cr2O7 = K2O, 2CrO3, is prepared by heating chrome ironstone to redness and quenching in cold water; it is then reduced to an extremely fine powder, and heated to bright redness in a current of air, in a reverberatory furnace, with a mixture of chalk and carbonate of potassium, the mixture being constantly stirred to hasten the oxidation. When this is complete, the product is digested in water, carbonate of potassium being added if necessary to decompose any chromate of calcium which may have been formed, and the yellow solution is drawn off from the insoluble matter; it is then supersaturated with nitric acid; a portion of silica is thus precipitated, and after this has been separated, the liquid on evaporation yields crystals of the bichromate of potash.

Neutral chromate of potash, K2CrO4, is prepared by adding a solution of carbonate of potassium to the bichromate, until it becomes of a light yellow color. A terchromate of potassium, K2O, 3CrO3, has also been obtained by Mitscherlich. Chromate of lead, PbCrO4, forms a pigment called chrome yellow. It is obtained by precipitating a somewhat dilute solution of acetate of lead by one of chromate or bichromate of potash. It is insoluble in water or acids. At 400° to 500° its color becomes reddish brown; at a higher temperature it fuses. The fused chromate is sometimes used instead of oxide of copper in organic analysis. Dibasic chromate of lead, of a splendid scarlet color, may be obtained by boiling a solution of the yellow chromate of lead with half an equivalent of lime; or by adding to a solution of nitrate of lead a solution of chromate of potassium, with which an additional equivalent of hydrate of potash has been previously mixed. A still more brilliant color may be obtained by fusing 1 part of normal chromate of lead with 5 parts of nitre. This salt is used to impart a permanent orange to calico. - Sulphates of chromium, Cr23SO4. There are three varieties of this salt, the first green, the second violet, and the third red.

The violet modification forms with sulphate of potassium, and with sulphate of ammonium, a beautiful violet double salt (chrome alum), which crystallizes by spontaneous evaporation in bold octahedra, and corresponds in form and composition to ordinary alum. - The following are the most important applications of chromium compounds in the arts. The yellow or neutral chromate of potash is the basis of all other preparations, being made directly from the chrome ironstone. The red or bichromate of potash is obtained from the foregoing salt, and is extensively employed in the arts. In photography it is the basis of most of the printing processes, on account of its property of rendering gelatine' insoluble by exposure to light. In dyeing, it is extensively used as a mordant. It is the material from which the chromic oxide, chromic acid, and the metallic chromates are prepared. Chromic oxide is the most insoluble green pigment known; it is extensively used in printing bank notes and in staining glass and painting porcelain.

Chromic acid is a powerful oxidizing agent, and is used on this account in chemical researches; is found very valuable as an exciting fluid in galvanic batteries; was used for preparing the beautiful mauve red from aniline; is employed in bleaching palm oil, and in destroying the empyreumatic impurities of acetic acid, etc. The chromates of lead, bismuth, baryta, stron-tia, and zinc are employed as pigments, varying in tint from the vermilion red of the basic chromate of lead to the pale straw yellow of the strontia salt. The common chrome green is a mixture of chromate of lead and Prussian blue. The beautiful violet chromic chloride has been proposed as a remedy for cancer. Chromium steel, made by combining about 5 per cent. of chromium with cast iron, possesses most remarkable properties. On account of its excessive hardness, it is the best metal for the construction of safes, while its tensile strength, equal to a strain of 140,000 lbs. to the square inch, especially adapts it to the construction of suspension bridges.