IRON, a common and exceedin& useful metallic element, which has been known and used in the arts for many centuries. Articles made of iron found in the pyramid of Cheops are believed to be 5,000years old. It occurs in nature in the metallic form, both in meteor ites and in certain lavas and volcanic rocks; but the commercial supply is obtained by the reduction of the oxides (or other ores) of the metal, by strongly heating them in a blast fur nace with carbon. Iron is grayish in color, with a marked lustre. If cooled slowly from a molten condition it is grayer; if rapidly, it is whiter. It crystallizes in the isometric system, usually in the form of cubes, sometimes as octahedra. The melting point of iron varies to a considerable extent, according to the impuri ties with which the metal is associated, and also, apparently, according to the physical con dition of the iron itself. Pictet gives it as about 2900° F. for iron that is sensibly pure. The specific gravity of the metal also varies to a considerable extent, the determinations rang ing from 6.95 to 82. The specific gravity of pure iron, at 60° F., may be accepted as 7.85. The specific heat of the metal is about 0.112 at ordinary temperatures, and its coefficient of ex pansion (on the Fahrenheit scale) is about 0.0000068. Taking the electrical conductivity of mercury (at 32° F.) as unity, the conductivity of iron is about 9.68 at 32° F., and 6.19 at 212° F. Chemically pure iron is not found in commerce, the nearest approach to it being electrolytic iron, which is about 99.95 per cent pure. Even the most carefully made iron of the laboratories has some admixture or alloy which cannot be dislodged. The common com mercial forms of iron are wrought iron and cast iron. The former is measurably free of carbon, and is known also as "soft trono and °malleable It contains a small percentage of slag and minute proportions of phosphorus, sulphur, silicon and manganese. It is obtained from cast iron (pig iron) by first melting and boiling the pig metal, then allowing it to cool till in a pasty condition, and then "puddling" it into balls which are hammered to remove the slag content. Cast iron is the product of the blast furnaces in which iron ores are smelted., It is commonly called pig iron. It contains from to 4 per cent of carbon in the form of carbide of iron, or of graphite. Where the
carbon is in the form of carbide, the iron is hard and brittle; when in the form of graphite the iron is soft and tough. Frequently both forms exist in the same sample in varying pro portions. Other common constituents are sil icon, from 0.2 up to 18 per cent; phosphorus from 0.02 up to 3 per cent; a minute percentage of sulphur, and manganese, which may run up to 20 per cent, when the compound is called ferromanganese. On the market pig iron is graded as follows: No. 1, when very gray and with large crystals; No. 2, the same, when with small crystals; No. 3 and No. 4 are closer grained types of No. 1 and No. 2; when of white and gray iron mingled. Nos. 1, 2 and 3 are used in foundry work. No. 4 is used generally for puddling into wrought iron and also for the production of heavy castings and for chilled castings. Mottled iron is added to, the other types to give density and hardness. Repeated meltings of iron increase its hardness. Iron is the most magnetic substance known. Soft iron is capable of being magnetized very highly when surrounded by a solenoid of wire that is conveying an electrical current ; but its magnetization persists only while the electric current is flowing, falling off, upon the cessa tion of the current, to a value that is tically negligeable. It is upon this property of temporary magnetization that the action of the telephone, the telegraph and many other useful electrical inventions depends. (See MAGNET ISM ; and for a full discussion of the phenomena of Magnetization, consult Ewing, 'Magnetic In duction in Iron and Other Metals'). Hardened steel, when magnetized by the action of the electric current (or otherwise), retains a large proportion of its magnetism permanently. Iron becomes nonmagnetic at a red heat, but regains its magnetic possibilities upon cooling again. Wrought iron, when pure, is malleable to a cer tain extent at all temperatures; but it yields to the hammer with special readinesi when heated to whiteness, and it may then be forged and welded without difficulty. The presence of any considerable proportion of sulphur or phos phorus makes the metal "short') or brittle When phosphorus is present in too great a pro portion, the iron is brittle in the cold (that is, 'cold-short") ; while if sulphur is present in excess it is brittle when hot (that is, it is "hot short").