Pia, east, ng11 alle, and rolled iron are the ordinary states of the metal met with in commerce ; their manufacture and properties have already been described. [IKON MANUFACTIME AND TIIADE In one or more of these conditions iron has been known from the most remote period. Cutting instruments made of this metal are mentioned in the books of Moses. The method of reducing and forging iron is described in Homer and Healed, and Pliny's ‘CCatural History' contains a long article on the subject The alehymists considered it to be a compound of gold and the hypothetical body acrimony, and symbolised it there fore by the sign 8 ; they gave it the name of Mars, probably because weapons made of it were used in the service of the god of war.
Iron has a white or grayish white colour. When tolerably pure, as in bar-iron, it is very difficult of fusion, but at a white heat possesses the valuable property of welding ; that is, two pieces brought into contact at that temperature, and well pressed together by hammering, unite as perfectly as if they hail been joined by melting. The heat of the oxyhydrogen blowpipe or of the voltaic arc, however, is sufficient to even volatilise iron. Iron can be beaten out into tolerably thin sheets, but is not nearly so malleable as gold or silver. It is very ductile, being capable of being driwn into thinner wire than that of any other metal. In tenacity also it is unsurpassed, a wire of one thirty-sixth of an inch in diameter sustaining a weight of sixty pounds before breaking. Iron readily conducts heat and electricity, but is inferior in these respects to some other metals. It is attracted by the magnet, and may itself be rendered permanently magnetic; the latter property is destroyed on heating the metal to redness, but is recovered again on cooling.
Under the influence of air and moisture iron rapidly attracts oxygen, or ru.as, especially after the first spot has been produced. The exact action that goes on is not very well made out ; it appears probable that the peroxide first formed is reduced to the state of protoxide by the particles of the metal with which it is immediately in contact, the nrotoxide so produced absorbing oxygen from the air and being re-con verted into peroxide ; in short, that the peroxide of iron protoxidises the metal and that the air peroxidises the protoxide. The carbonic acid present in the air also plays an important part in the rusting of iron. This liability of iron to rust is perfectly prevented by keeping it dry ; but the object is most commonly effected by constant removal of the surface of oxide by polishing powders, such as emery or brick dust, or by enveloping the metal in a covering of plumbago, a mechanical process known in domestic life as blacklcading.
Iron decomposes water at a red heat, hydrogen being evolved, and a black oxide of iron produced. Diluted sulphuric, hydrochloric, nitric, and other acids dissolve iron, with evolution of hydrogen or, in the case of nitric acid, of binoxide of nitrogen. Strong nitric acid, how ever, does not act upon iron, the metal remaining in what is termed the passirc conddion; a condition that seems to be intimately connected with the electrical relations of this metal.
1Vrought iron, steel, and cast iron differ from each other chemically chiefly in the amount of carbon they contain. Thus wrought iron contains only from about 0.2 to 0.4 per cent.; steel, from to while east-iron sometimes contains as much as 5.0 per cent. of carbon. From recent researches it appears that to the presence of a greater or smaller quantity of titanium is owing the value of iron used for making steel, and that titanium moreover confers other important properties on iron. Iron also sometimes contains silicon, sulphur, phosphorus, arsenic, and traces of other metals, such as aluminium, calcium, potassium, manganese, &c., but these deteriorate its quality.
The combining equivalent of iron is 23, and its symbol (Fe), (from ferruni, Lat.). It unites with other elements and compounds, chiefly in two proportions, forming respectively protosalts and persalts. In the former the constituents are equal in number; in the latter, two equivalents of iron are associated with three of the other body, and for that reason are sometimes called sesguisalts.
Iron and oxygen form three definite compounds :— 1. Protoxide of iron . . . . . Fc0 2. Peroxide of iron , 3. Ferric acid A fourth is also sometimes described, it contains and is termed the black or magnetic oxide, but it does not form salts, and may be viewed as a combination of One equivalent of protoxide with one of peroxide (FeO + It is in fact formed by precipitating a solution of equivalent quantities of a proto and per-salt by an alkali. The natural form of this oxide is the well-known mineral loadstone, and is the valuable ore from which Swedish iron is smelted.