Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/295

IKON 279 melting at near 1500, and being rendered still more fusible by the presence of small quantities of sulpliur and silicon ; whence sulphurized pig irons are often blended with purer varieties in order to produce good casting metal for various purposes. At the ordinary temperature the linear coeffi cient of expansion of wrought iron is near to 00001 25 (values between 0-0000115 and 0-0000144.- having been obtained by Borda, Smeaton, Lavoisier and Laplace, Trough- ton, and Dulong and Petit), so that 1 unit of length at will become on an average 1-00125 units in length at 100. Slightly lower values have been obtained with steel of different qualities by various of these observers, averaging 0000115; whilst cast iron expands less still, averaging O OOOOlll as linear coefficient of expansion; the precise numbers obtainable vary with the conditions, according as the metal has been hammered, rolled, hardened, annealed, &c. At somewhat elevated temperatures the rate of expan sion is higher ; thus Dulong and Petit find that the mean rate of expansion of iron between and 100 is to that between and 300 nearly in the ratio of 4 to 5. The force exerted during expansion is very great, being equal to that requisite to produce an elongation of the bar examined to the extent through which its length increases by heat;, thus, according to Barlow, a weight of 1 ton suspended to an iron bar a square inch in section will extend its length by O OOOl times the original length, so that 1 inch of length will become I OOOl inches; this increase in length would be brought about by a rise in temperature of about 9 C. ; hence for an increase of 3G ; or less than the average differ ence between a cold and warm day in winter and summer respectively, a girder of iron of 20 square inches in section would exert a thrusting strain upon two walls, &c., built firmly up to its ends when coldest, equal to about 20 x s -/ or 80 tons for each inch of its length, were it not that tha pressure is more or less relieved by the giving of the walls long before this strain is reached. In conse quence it is indispensable to allow a space for expansion in all constructions in which iron is employed, e.g., ordinary buildings, railways, furnaces braced together with tie-rods, &c. With large masses of ironwork exposed to the weather, very great strains may be produced through unequal expansion in differently heated parts, e.g., in the portions exposed to sunshine and in the shade respectively; as just indicated, a difference of temperature of 9 between two portions rigidly connected will produce a strain of about 1 ton per square inch. Edwin Clark has calculated that half an hour s sunshine produces more effect in the way of developing strain on the tubes of the Britannia bridge over the Menai Straits than the heaviest rolling loads or the most violent storms. Variations of temperature also exert some effect upon the strength and tenacity of iron ; the numerical values are largely variable with the quality of the metal. At temperatures below a red heat the strength is considerably lessened, and at high temperatures approximating to the welding temperature the tenacity becomes comparatively small (see 43). A peculiar suspension of the chemical activity of iron in refer ence to nitric acid (passive condition) appears to be connected with its electrical relationships ; when placed in nitric acid very slightly diluted (specific gravity about 1 - 4), iron is ordinarily violently attacked ; but if whilst in the acid it be touched with certain sub stances, e.g. , gold, platinum, plumbago, &c. , the action stops (at least under certain conditions, especially when not heated above some particular temperature varying with the strength of the acid Ordway) ; the iron thus rendered passive will induce the same condition in a second piece immersed in the acid by contact ; on exposure to air the passive iron loses its power of remaining un- attacked. Concentrated nitric acid, of specific gravity 1 45, pro duces the passive condition at once, so that a piece of bright metal may be kept for months immersed in the acid without any action being set up ; acid of strength below specific gravity 1 35, on the other hand, is usually incapable of permitting iron to become or remain passive in contact with it. If, whilst passive and immersed in nitric acid, iron be made the positive pole for a voltaic current sent through the acid, oxygen is evolved from its surface without any oxidation being visible ; if on the other hand it be made the negative pole, it immediately loses its passivity, and is attacked by the acid. In consequence of the production of the passive state by contact with concentrated nitric acid, iron is sometimes sub stituted for carbon or for platinum in the forms of voltaic battery known as Bunsen s and Grove s cells. Passivity may also be brought about in iron by heating the bright metal in the flame of a spirit lamp, &c. , so as to coat it superficially with a iilin of oxide. Preparation of Pure Iron. In order to prepare pure iron, special chemical operations must be gone through, of increasing complexity the greater the purity desired. Berzelius obtained a nearly pure fused substance by mixing filings of the purest soft iron of commerce obtainable with about 20 per cent, of pure ferric oxide and some glass powder (free from lead) as a flux, and exposing for an hour to the highest heat of a smith s forgo in a covered crucible ; in this way the small quantities of carbon and other impurities still retained by the filings are oxidized, and a button of silvery lustre results, of specific gravity 7 844, more tough but softer than ordinary iron. Matthiessen and Szczepanowski found the greatest diffi culty in obtaining iron absolutely free from sulphur by means of the ordinary methods for preparing oxide of iron subsequently reduced by pure hydrogen, but ultimately succeeded iii obtain ing moderately large quantities of metal not containing more than G 00025 to OD007 per cent, of sulphur by the employ ment of a specially prepared ferric oxide made by heating- together pure ferrous sulphate and sodium sulphate (Brit. Assoc. Re.ports, 1868, 1869), and thoroughly washing out the sodium sulphate from the fluxed product. After reduction in platinum vessels by pure hydrogen, and fusion in lime crucibles by the oxyhydrogen flame fed with purified gases, buttons of metal were obtained absolutely free from phosphorus, silicon, and calcium, and practically free from sulphur. By the electrolysis of as nearly as possible neutral solutions of ferrous chloride, or better of double magnesium ferrous sulphate, iron is thrown down in hard brittle films containing a considerable amount of occluded hydrogen (usually about twenty times its volume) ; on annealing, the metal becomes soft, malleable, and silvery white, increasing considerably in density, the specific gravity when first deposited being about 7 67, and rising to 7 81 after annealing ; Lenz finds that the amount of hydrogen occluded is greater the thinner the film of metal, the amount rising in the case of a very thin film to vipwards of 180 volumes ; the metal deprived of the occluded gas by heating in vacuo decomposes water at ordinary temperatures and rusts, par tially reabsorbing hydrogen in so doing (Pcgy. Annalcn, v. 242, 1870) ; whereas before the expulsion of the hydrogen by heating in vacuo the iron is highly brittle and of a fine granular texture, showing no crystalline structure under the microscope (being deposited from solutions containing no free acid), after the expul sion of the hydrogen the metal becomes highly tenacious and capable of resisting repeated bending backwards and forwards without rup ture ; the hardness is lowered from 5 5 to 4 5 on the mineralogical scale, i.e., from something between the hardness of felspar and apatite to something between that of apatite and fluorspar. Under certain conditions iron can be obtained in a crystallized state, the crystalline character being far more readily assumed when small quantities of other substances, notably carbon, are present ; by reducing ferrous chloride by hydrogen at a red heat, Peligot obtained the metal in brilliant crystals belonging to the cubic system ; by reduction with zinc vapour Poumarede transformed ferrous chloride into hollow tetrahedra of specific gravity 7 84. Bessemer iron has been obtained in distinct cubic crystals, whilst Percy has observed solid and skeleton octahedra in cast iron. Malleable iron that has been much rolled and forged during its manufacture exhibits on. etching with acids a fibrous structure ; when pulled asunder by a slowly acting force, this structure is also well seen ; if, however, it be transversely ruptured by a suddenly applied force (e.g. , the impact of a heavy shot on an armour plate), a crystalline fracture usually results. Iron exhibiting fibrous structure on etching is usually considerably more tough and tenacious than that which is crystalline. A change from the former kind of molecular struc ture to the latter, producing comparative brittleness, is believed by many to occur with crank-shafts, axles, &c., exposed to continuous vibration and jolting ; in some cases the acquisition of a high degree of permanent magnetism (e.g., in pump rods) is said to havo been observed as occurring just before rapture of the metal took place. 2. Chemical and Physical Relationships of Iron. I?pn unites with oxygen in several proportions, forming definite oxides, the best marked of which are those indicated by the formulae FeO, Fe ( O 4 , and Fe/) 3 , O standing for^lG parts of oxygen, and Fe for 56 of iron, the value 56 being