NITRIDE AND SULPHIDES OF IRON.

607

Wanklyn and Carius have described a hydride of iron procured by the action of iodide of iron on zinc ethyl. It has not been analysed, but is described as a black powder which evolves pure hydrogen when gently heated: hydrochloric acid dissolves it with evolution of hydrogen.

(760) Nitride of Iron.-When iron wire is heated in a current of dry ammoniacal gas for some hours it becomes brittle, but does not usually gain in weight more than about o'2 per cent. Despretz, however, obtained a compound of iron with nitrogen in which 100 parts of iron increased to 1115, becoming less dense, brittle, whitish, and less oxidizable than pure iron. Sulphuric acid dissolves this nitride easily with evolution both of hydrogen and nitrogen, whilst sulphate of ammonium is retained in solution. If heated to redness in a current of dry hydrogen it becomes reduced to metallic iron, whilst ammonia is generated. Fremy found that it becomes easily and permanently magnetized. He found the best mode of preparing this nitride, to which he attributes a composition expressed by the formula (Fe.N2), to consist in transmitting a current of dry ammonia over dried ferrous chloride heated to redness in a porcelain tube. If pure ferric oxide, as obtained from the oxalate by ignition, be heated in a current of ammonia, a brittle nitride is formed, consisting of Fe,N (Rogostadius), which is with much probability regarded by the last-named chemist as the only definite nitride of the metal.

2)

(761) SULPHIDES OF IRON.-Sulphur combines with iron in several proportions: the protosulphide, FeS, and the bisulphide, FeS are the most important; but besides these a disulphide, Fe,S, has been obtained by heating sulphate of iron to redness in a current of hydrogen gas: a sesquisulphide, Fe,S,, may also be formed as a hydrate by precipitating the persalts of iron by the protosulphides of the alkaline metals; and two magnetic sulphides of iron, Fe,S., and Fe,S,, are found native.

Protosulphide, or Sulphuret of Iron (Fe=88, or FeS=44), may be prepared by heating a bar of iron to whiteness and bringing it into contact with a roll of sulphur: immediate union takes place, and the resulting sulphide melts and runs down in drops of a reddish-brown colour: when formed in this manner it usually contains an excess of sulphur. It may also be prepared by projecting in successive portions, into a red-hot earthen crucible, a mixture of 7 parts of iron filings with 4 parts of sulphur in fine powder; vivid deflagration occurs at the moment of combination. Protosulphide of iron dissolves both iron and sulphur if either be

608

PROTOSULPHIDE AND BISULPHIDE OF IRON.

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present in excess; its composition, therefore, is variable. Like carbon, the presence even of a minute portion of sulphur alters the quality of wrought iron, which if it contains even T, of sulphur is rendered red short.' Anhydrous protosulphide of iron is dissolved by diluted sulphuric or hydrochloric acid with evolution of sulphuretted hydrogen: this decomposition is frequently employed in the laboratory as a convenient source of sulphuretted hydrogen. Nitric acid and aqua regia decompose it and form ferric nitrate or ferric chloride, setting part of the sulphur free, and converting the residue into sulphuric acid. When heated in the open air, this sulphide absorbs oxygen and becomes converted into sulphate of iron; at a still higher temperature it is decomposed, sulphurous and sulphuric anhydrides escape, and sesquioxide of iron remains.

Protosulphide of iron may be obtained as a black hydrate by precipitating a solution of a ferrous salt with a solution of a protosulphide of one of the alkaline metals; thus 2 KHS + FeSO 4 =FeS+H2S+K2SO4; in this condition it rapidly attracts oxygen from the air, and assumes a brownish-red colour, sesquioxide of iron being formed and sulphur liberated. When iron is present in very minute quantities in a solution, and is precipitated by a solution of sulphide of ammonium, the very finely divided particles of sulphide of iron are apt to pass through the filter; the liquid then has a peculiar green tint.

If iron filings be mixed with two-thirds of their weight of sulphur in powder, and moistened, the mixture becomes hot when exposed to the air, and absorbs oxygen with sufficient rapidity to cause it in many cases to inflame; sulphide of iron is at first formed, and this quickly becomes converted into sulphate. A valuable lute for the joints of iron vessels is composed of a mixture of 60 parts of iron filings sifted fine, and 2 of sal ammoniac in fine powder intimately blended with I part of flowers of sulphur. This powder is made into a paste with water, and applied immediately; in a few minutes it becomes hot, swells, disengages ammonia and sulphuretted, hydrogen, and soon sets as hard as iron itself.

(762) Bisulphide of Iron (FeS,=120, or FeS2=60); Sp. Gr. 498: Composition in 100 parts, Fe, 46·67; §, 53'33.—This compound is found abundantly in the native state, constituting the iron pyrites of mineralogists. It occurs in the strata of every period when found in the older formations it is crystallized in cubes, and sometimes in dodecahedra, of a brassy lustre, and is hard enough to strike fire with steel; but in the tertiary strata it more frequently occurs in fibrous radiated nodules. The forma

BISULPHIDE OF IRON.

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tion of iron pyrites may occasionally be traced to the slow deoxidation of sulphates by organic matter in waters containing carbonate or other salts of iron in solution; it is then frequently deposited in cubes or octohedra. This appears to be the usual mode of its formation in alluvial soils. Some varieties of iron pyrites, especially those found in the tertiary strata, are speedily decomposed by exposure to air; oxygen is absorbed, and ferrous sulphate formed. This decomposition occurs with greater facility if the bisulphide be mixed with other substances, as is the case in the aluminous schists; in which, by the further action of air, a basic ferric sulphate is formed, whilst the liberated sulphuric acid reacts upon the alumina, magnesia, or lime of the soil, and forms sulphates; those of aluminum and magnesium may be extracted by lixiviation. The ordinary crystallized pyrites from the older strata is not thus decomposed, but a variety of a whiter colour is rapidly disintegrated by exposure to the weather; this form of pyrites is known as white iron pyrites; it crystallizes in right rhomboidal prisms, but it possesses the same chemical composition as the yellow cubes.

Iron pyrites is not acted upon by cold sulphuric or by hydrochloric acid, but is rapidly oxidized and dissolved by nitric acid and by aqua regia: boiling oil of vitriol dissolves it gradually with evolution of sulphurous anhydride. When heated in closed vessels it fuses, and sulphur is expelled. If heated in the air it burns with flame, peroxide of iron is formed, whilst sulphurous anhydride escapes in large quantity. This circumstance has been turned to account in the manufacture of oil of vitriol, for which purpose enormous quantities of mundic, as the bisulphide is termed by the workmen, are annually consumed. The acid obtained from that source is usually contaminated with arsenic, which in small quantity is a common impurity in pyrites. Iron pyrites may be prepared artificially by exposing a mixture of powdered protosulphide of iron with half its weight of flowers of sulphur, in a covered crucible to a heat just below redness, as long as sulphurous fumes escape.

(763) Magnetic Sulphide of Iron (Fe,S,, or 6 FeS,FeS1); Sp. Gr. 465.-This compound exhibits a brassy lustre, but is distinguished from ordinary pyrites by its solubility in hydrochloric acid. It is often formed when sulphur and iron are heated together in preparing the protosulphide for use in the laboratory. Another variety of magnetic pyrites consists of Fe,S

Mispickel, or arsenical pyrites, is an arsenio-sulphide of iron (FeSAs, or FeS,FeAs; Sp. Gr. 6·13) which, amongst other

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SULPHIDES AND CHLORIDES OF IRON.

localities, occurs abundantly in the Hartz, in Saxony, and in some of the Cornish mines; it crystallizes in right rhombic prisms, of a steel-grey colour and metallic lustre. When heated in closed vessels it is partially decomposed, and sulphide of arsenic sublimes. If exposed to a high temperature in the open air, it produces sesquioxide of iron, whilst arsenious and sulphurous anhydrides escape. Analogous compounds of cobalt and nickel occur

amongst the ores of these metals.

A remarkable class of compounds, termed by Roussin, who discovered them, nitrosulphides of iron, may be obtained by the reaction of nitrite of potassium and sulphide of ammonium upon the salts of iron. The reaction is complicated, and their properties will be better examined along with those of the nitroprussides.

Subphosphide of iron (Fe,P) may be obtained by reducing the phosphate of the metal with charcoal: it fuses at a red heat, and forms an extremely hard, brittle mass, which unites with both phosphorus and iron in all proportions.

(764) CHLORIDES OF IRON.-Iron forms with chlorine two compounds-ferrous chloride, FeCl, and ferric chloride, Fe,Cl,which correspond in composition to the two basic oxides of the metal.

Ferrous Chloride, Protochloride of Iron (FeCl, 127, or FeCl =63'5); Sp. Gr. anhydrous, 2.528; cryst. 1°926.—By passing dry hydrochloric acid gas over ignited metallic iron, the acid is decomposed, hydrogen gas escapes, chlorine combines with the iron, and the white anhydrous ferrous chloride sublimes at a temperature at which glass begins to soften. Its solution may be formed by dissolving iron in hydrochloric acid; the hot, saturated liquid deposits the salt, on cooling, in green crystals, which contain FeCl2, 4 H2O. It is very soluble in water, and is taken up in considerable quantity by alcohol. If heated in the open air, chlorine escapes and peroxide of iron remains.

Ferrous chloride unites with chloride of ammonium and forms a double salt, from which the iron may be deposited upon various metallic articles, by boiling them in this solution with scraps of zinc; the zinc displaces the iron, which is deposited in a coherent lamina upon the other metals, in consequence of a voltaic action.

(765) Sesquichloride or Perchloride of Iron (FeCl = 325); Sp. Gr.of Vapour, 11'39; Mol. Vol. (or Fe2Cl=162×5).—This compound sublimes in anhydrous brown scales when dry chlorine gas is transmitted over iron heated to redness. The anhydrous chloride is very deliquescent, and hisses when thrown into water,

CHLORIDE, BROMIDE, AND IODIDE OF IRON.

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forming a red solution. It is also soluble both in alcohol and ether. In a hydrated condition it may be procured by evaporating a solution of ferrous chloride through which chlorine has been passed to saturation, or by dissolving hydrated peroxide of iron in hydrochloric acid: the solution, by concentration, yields large, red, deliquescent crystals (Fe,Cl, 6 H,O), but the salt cannot be rendered anhydrous by evaporation, as it is decomposed into hydrochloric acid and peroxide of iron. It also crystallizes in stellate, orange-coloured groups, with 12 H,, which are less deliquescent than the other hydrate. Perchloride of iron forms a double salt with chloride of ammonium, which crystallizes readily in cubes, and is known in pharmacy as the ammonio-chloride of iron. The composition of this salt varies considerably: it is of a ruby-red colour, and seldom contains more than 2 per cent. of iron. A double salt of perchloride of iron with chloride of sodium and with chloride of potassium may also be formed.

2

A hydrated oxychloride of iron is formed when a solution of ferrous chloride is exposed to the air, or when the perchloride is precipitated by a small quantity of caustic alkali. It is insoluble in water containing salts, but is partially soluble in pure water.

(766) The BROMIDES of iron correspond in composition to the chlorides.

Ferrous Iodide, or Protoiodide of Iron (Fel=310, or Fel=155) is formed by digesting iron wire or filings, in a closed vessel, with four times their weight of iodine suspended in water; direct combination with the elements takes place, the iodide is dissolved and forms a pale-green solution, which, by evaporation in vacuo, yields crystals containing Fel,, 4 H,O. By a continued heat it may be rendered anhydrous, and in that state is fusible. Its solution, if exposed to the air, absorbs oxygen and is decomposed; iodine is set free, and a hydrated oxyiodide of iron falls. This change is retarded by mixing the solution with strong syrup; and as the compound is employed in medicine, this method is frequently adopted to preserve uniformity in its composition. No definite ferric iodide is crystallizable.

(767) FERROUS SULPHATE, Protosulphate of Iron, Copperas, or Green Vitriol, (FeSO4.7 H2O=152+126, or FeO,SO ̧ . 7 HO= 76+63); Sp. Gr. anhydrous, 3'138, cryst. 1857: Comp. cryst. in 100 parts, Fe0, 25'91; SO, 28.77; H2O, 45'32.-This salt is prepared in a state of purity by dissolving I part of pure iron, or 1 of its protosulphide, by the aid of heat, in 1 parts of oil of vitriol diluted with 4 of water. On filtering the solution quickly, it deposits beautiful transparent, bluish-green, rhomboidal crys