tetraferric carbide, Fe,C. Other varieties appear to be mixtures of this compound with artificial plumbago (a variety of carbon), or with true metallic iron.

458. Steel. This is a compound of the metal with about one and a half per cent. of carbon. It is produced in different ways. One variety is made by imbedding bars of the best wrought-iron in powdered charcoal, in boxes or sand-furnaces, which exclude the air, and heating it intensely for a week or ten days. The chemical changes are obscure; probably carbonic oxide penetrates the heated metal, is decomposed, surrenders part of its carbon and escapes as carbonic dioxide. The steel when withdrawn has a peculiar rough, blistered appearance, and is therefore known as blistered steel. This method of making steel is called the process of cementation. When this quality of steel is melted and cast into ingots, it constitutes cast-steel.

A great improvement in the manufacture of steel has been introduced, called from its inventor the Bessemer process. By means of this, steel is produced directly from the cast-iron, without previous casting into pigs. The melted cast-iron is run from the shaft-furnace into eggshaped vessels made of boiler-plate, lined with fire-clay, which are termed "converters." An intense blast of compressed air is forced into the molten mass, and ten to twenty-five minutes of this operation suffice to decarbonize the cast-iron so as to convert it into steel, or wrought-iron, as may be desired.

459. In its properties steel combines the fusibility of cast-iron with the malleability of bar-iron. Its value for cutting instruments, springs, etc., depends upon its quality of being tempered. When heated to redness and suddenly quenched in cold water, it becomes so hard as to scratch glass. If again heated and cooled slowly, it becomes ast soft as ordinary iron, and between these two conditions any required degree of hardness can be obtained. As the metal declines in temperature, the thin film of oxide upon

its surface constantly changes its color. The workmen are guided by these tints. Thus a straw-color indicates the degree of hardness for razors; a deep blue for sword-blades, saws, and watch-springs. Steel receives a higher polish than iron, and has less tendency to rust. Nitric acid placed upon steel corrodes it, and leaves the carbon as a dark-gray stain; hence it is often used for writing and ornamental shading upon this metal.

II

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460. Ferrous Oxide, FeO.-This compound is not found native, but is obtained when ferrous oxalate is heated in a close vessel, as a black powder, which in the air ignites spontaneously, burning to ferric oxide, Fe,O, (diferric trioxide), which is of very wide distribution as the minerals red hematite and specular iron, from which a large proportion of the iron of trade is derived. Red hematite is massive, earthy, or fibrous, and brick-red in color. Specular iron is extensively employed under the names of colcothar and jewelers' rouge, as a pigment, and for polishing jewelry, glass, and metallic objects.

461. Triferric Tetroxide, Fe,O,, Magnetic Oxide.This substance occurs native as the mineral magnetite, the most valuable of the ores from which iron is produced. This appears as a black, crystalline powder, in Nature it forms large masses, and is frequently found in distinct octahedral crystals of considerable size. The triferric tetroxide is strongly magnetic, and the black oxide which forms on iron when heated in aqueous vapor consists of this compound, which is also produced by the combustion of iron in oxygen gas.

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462. Ferric Disulphide, FeS,.-This compound occurs native in two isomeric modifications, one, the mineral marcasite, the other, iron pyrites. Both forms are widely distributed. Iron pyrites crystallizes in cubes or other forms of the monometric system, of a golden-yellow color and strong metallic lustre. Heated in the air, iron pyrites burns with evolution of sulphurous oxide, and it is much

used in the manufacture of sulphuric acid. Sulphur and copperas are also obtained from it, but it is never worked for iron. Marcasite is a mineral possessed of a white color and metallic lustre, which in moist air decomposes rapidly, with formation of ferrous sulphate (FeSo, +7H,O) and evolution of heat. It occurs in coal-beds, and sometimes causes their spontaneous ignition.

463. Ferrous Sulphate, FeSo, +7H,O, Green Vitriol, Copperas.-This salt is largely manufactured from iron pyrites. It is used in dyeing, for making ink and Prussian blue, and in medicine. It often exists in soils to a pernicious extent, but is decomposed by lime, gypsum being formed. Ferrous Carbonate, FeCO,. This is a very abundant mineral known as spathic iron. It is grayish-white, opaque, and crystallizes in rhombohedrons. When found in large masses it constitutes one of the most valuable iron-ores. Steel has been made directly from it, hence it is known as steel ore.

§ 2. Manganese, Nickel, and Cobalt.

MANGANESE.-Symbol, Mn. Atomic Weight, 55; Quantivalence, II., IV., and VI.; Specific Gravity, 8.

464.-This metal never occurs in Nature, but can be obtained by making manganic oxide into a paste with oil and lamp-black and heating this mixture to whiteness in a covered crucible. It is a hard, brittle metal of a grayishwhite color, and rapidly oxidizes when exposed to the air. It is best preserved in naphtha. Manganic Dioxide, MnO2, occurs in Nature as the mineral pyrolusite. It is an iron-black or steel-gray, brittle substance, crystallizing in forms of the trimetric system. It is mined extensively, being employed in the manufacture of chlorine and bleaching-powders, as a source of oxygen, and for discharging the brown and green tints of glass.

465. Nickel and Cobalt.-These two metals are closely

related by their properties. Their atoms have identical weights, and their reactions are so similar that there is difficulty in separating one from the other. They occur associated together, and are found alloyed with the iron of meteoric origin.

They may be obtained by the decomposition of ores, chiefly nickelic and cobaltic arsenides, sulphides, and sulph-arsenides. They are both magnetic, and resemble iron in many of their properties. Nickel is a silver-white, ductile and malleable metal, of about 8.4 spec. grav., not much more fusible than iron. It is used principally in the manufacture of german silver, of coinage, and other similar alloys. Cobalt is a reddish or grayish-white metal, of about 8.9 spec. grav.; hard and somewhat malleable at a red heat. It has not been applied to any useful purpose. Cobaltous Chloride, CoCl,+6H,O, may be obtained in ruby-red octahedral crystals from solutions of cobaltous oxide or carbonate, in hydric chloride. The dilute solution of these is used as a sympathetic ink, the characters written with it being so pale as to be invisible till warmed, when they appear blue, owing to the formation of the anhydrous chloride (CoCl,). On cooling, they absorb moisture and again disappear.

CHAPTER XXII.

CHROMIUM, ALUMINIUM, AND THE PLATINUM GROUP.

§ 1. Chromium and its Compounds.

CHROMIUM.-Symbol, Cr. Atomic Weight, 52.5; Quantivalence, II., IV., and VI.; Specific Gravity, 6.8.

466. Occurrence. This metal may be prepared by exposing chromic compounds to intense heat in a current of hydrogen gas or by fusing its oxide with charcoal in a

charcoal-lined crucible. When the oxide is reduced by carbon the metal obtained is of steel-gray color, exceedingly hard, and not easily fused. Many of the compounds have a brilliant color and are used as paints. paints. It gives color to the emerald.

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467. Dichromic Trioxide, Cr,O,, Chromic Oxide.-This cómpound may be obtained, by strongly heating a chromic hydrate, as a bright-green powder, or in the form of greenish-black rhombohedral crystals of metallic lustre, 5.2 spec. grav., and great hardness. It is used in coloring glass and porcelain, and also in ordinary painting under the name of chrome-green. Chromic Trioxide, CrO,, is obtained in splendid crimson needle-shaped crystals often an inch in length, easily soluble in water, melting at 190° C., and decomposing at 250° C. It is a powerful oxidizing agent.

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§ 2. Aluminium and its Compounds.

ALUMINIUM.-Symbol, Al. Atomic Weight, 27.5; Quantivalence (Al。)VI; Specific Gravity, 2.5.

468. History. This important metal was discovered by the German chemist Wöhler in 1827. It is not found native, but may be obtained by decomposing either the chloride or the fluoride with metallic sodium. It is one of the most abundant elements, being the metallic base of alumina, which forms the argillaceous rocks, beds of clay, and a large proportion of granite. It is a shining, white metal, of a shade between silver and platinum, harder than zinc and of remarkable strength and stiffness. It resists, like silver, the oxidizing influence of moist air, melts at a still lower temperature than that metal, and, pound for pound, occupies four times its space. It is the most sonorous of metals, giving forth a clear musical sound when struck. It is malleable and ductile like iron, exceeds it in tenacity, and combines with carbon, forming a cast metal, which is not malleable.