Chemically, iron is a dyad. It has the sym bol Fe (from "ferrum," the Latin name for the metal), and an atomic weight of 56 if 0=16. or 55.6 if H=1. It forms two basic oxides.
(1) ferrous oxide FeO, which gives rise to a series of salts known as "ferrous" salts, and (2) ferric oxide (or ferric sesquioxide), Fe,03, which gives rise to a corresponding series of "ferric" salts. A third oxide, having the form ula Fe,O., is also known, which is magnetic, and occurs in nature, either amorphous or crystal lized in octahedra, as the mineral "lodestone." This oxide is black in color, and is known as the black oxide, magnetic oxide, or ferroso ferric oxide. It may he prepared, artificially. by oxidizing iron at a high temperature, either in air or in steam, or by heating carbonate of iron to 650° F., in a current of carbon dioxide. It is not readily attacked by acids or other chemical agents, and for this reason a coating of it is often formed on articles of iron to pro tect them from further oxidation. The Russia iron that is used for stove-pipes is coated in this way, by a secret process. When in mass, iron does not readily decompose water at ordi nary temperatures, though it does so at high temperatures. Finely divided iron decomposes water at 212° F., and at lower temperatures ac cording to some authorities. When in a suffi ciently fine state of subdivision, iron will burn in the air or in oxygen, with the formation of mixture of and Fe204. Iron forms alloys with many metals, and combines directly with chlorine, bromine, iodine, fluorine, sulphur; carbon, boron, silicon, phosphorus and arsenic. Melted iron dissolves carbon to some extent, and when the molten mass is cooled the carbon is largely deposited in the graphitic form, al though a part of it remains in the iron, com bined with it in the form of a carbide; and it is believed that the presence of varying quanti ties of such carbides has much to do with the physical qualities of iron and steel. When melted iron that is saturated with carbon is allowed to cool under great pressure, the car bon is partly deposited in the form of minute crystals of diamond (q.v.); but the manufac ture of the diamond by this method has not yet been made commercially practicable.
Fm rows Ferrous oxide. FeO, has not yet been prepared in a state of absolute purity, but it may be obtained approxi mately pure by reducing ferric oxide, Fe202, by heating it to 600° F. in a stream of pure hydro gen. It is black in color, and absorbs oxygen with great readiness, passing into the higher oxides. Ferrous sulphate, otherwise known as protosulphate of iron, green vitriol, or copperas, is prepared by dissolving iron wire in dilute sul phuric acid, and crystallizing by evaporation.
It has the formula + 71420, and is green ish in color. It is soluble in water, but the solution oxidizes readily, the salt becoming con verted into ferric sulphate, Fe2(So2)2. Ferrous sulphate is largely used in the manufacture of certain black dyes, in the preparation of writ ing ink, Prussian blue and other pigments. Ferrous chloride, FeCl2, may be prepared by heating excess of iron wire or iron filings in chlorine, or by passing dry hydrochloric acid gas over hot metallic iron. It crystallizes in white, lustrous, six-sided scales, is deliquescent, and is volatile at a yellow heat. In air it oxi dizes readily to a mixture of ferric oxide and ferric chloride. Ferrous carbonate, FeCo., is an insoluble compound, occurring in nature as iron ore," and constituting a valuable source of iron. Ferrous sulphide, FeS, is a lustrous black or grayish-black body, which may be prepared by melting sulphur and iron together in the proportion of 56 parts (by weight) of iron to 32 of sulphur, and stirring with a white-hot rod of soft iron. It is insolu ble in water, but dissolves readily in dilute acids, with copious liberation of sulphureted hydrogen gas. Ferrous sulphite has been found in some meteorites, as the mineral See CHEMICAL ANALYSIS.
Ferric
Ferric oxid, Fe202, occurs native as hematite (q.v.), and it may be prepared artificially by heating ferrous sulphate to redness. Ferric hydrate, Fe (OH), is pre cipitated as a brownish-red powder when aml amnia or caustic potash is added to the solution of a ferric salt. Ferric sulphate and ferric chloride are prepared by dissolving this hydrate in sulphuric and hydrochloric acids, respectively. The chloride is used as a disinfectant for sew age, and the sulphate in dyeing cottons. Ferric nitrate used h' mordant in pi-Meting bUffi and blacks in dyeing. In general, a ferrous salt, when in solution, is converted into the corresponding ferric salt by the action of oxi dizing agents; and the ferric salts, conversely, are reduced to ferrous salts by the action of certain reducing agents. Ferrous salts give a white precipitate with caustic alkalies, and, with potassium ferrocyanide, a light-blue precipitate which quickly turns black. Ferric salts give a reddi‘h-blown precipitate v.ith
a ka lies, and a deep blue precipitate v. ith potassium ferrocyanide. See CAST IRON ; S EEL. Consult Howe, H. M.,
Metallograohy of Steel and Cam. iron' (New York 1916); Hudson, 0. F., and Bengough, G. D.,