CHAPTER XXI.

IRON, MANGANESE, NICKEL, AND COBALT.

§ 1. Iron and its Compounds.

IRON.-Symbol, Fe. (Ferrum). Atomic Weight, 56; Quantivalence, II., IV., and VI.; Specific Gravity, 7.8.

450. History and Occurrence.-Were we to seek for that circumstance which might best illustrate the peculiarities of ancient and modern civilization, we should perhaps find it in the history of this metal. The ancients, imbued with a martial spirit and passion for conquest, made iron the symbol of war, and gave it the emblem of Mars. And if it were required also to symbolize the pacific tendencies of modern society, its triumphs of industry and victories of mind over matter, its artistic achievements and scientific discoveries, we should naturally employ the same metal, iron. As gold and jewels have long been the type of barbaric and empty pomp, so iron may now be well regarded as the emblem of beneficent and intelligent industry.

Native iron of meteoric origin has frequently been found, and instances of its occurrence on the earth have been reported, but usually in these cases the iron is combined with nickel. We are, however, acquainted with numerous ores of iron, among which are magnetite, red hematite, and specular iron, brown iron-stones, spathic iron, and clay iron-stone.

451. Preparation.-Metallic iron or wrought-iron has been obtained from iron-ores, and to some extent this is still its source, but by far the largest portion brought into the market is derived from the decomposition of cast-iron, which is essentially a ferric carbide, but also contains varying quantities of other substances. The operation is usually conducted in reverberatory furnaces. In this process,

the cast-iron is melted on a flat hearth by causing the flame to impinge upon it from above on its way through the furnace, as shown in Fig. 171. A workman, with a long, oar-shaped implement of iron, stirs (puddles) the melted mass until the carbon and other impurities of a like

FIG. 171.

nature are burned away or con

verted into a slag, and the metal becomes thick and pasty. This is called puddling. The puddler then rolls up from the mass a ball of about 75 lbs. weight, which he transfers to the tilting or trip hammer, where it is beaten by heavy blows into a crude bar. By this operation the liquid slag, consisting chiefly of ferrous silicate, is squeezed out, as water is expelled from a compressed sponge. The metal, still hot, is then passed between grooved cylinders, where it is rolled out into bar-iron. The quality of metal is greatly improved when these bars are broken up, bound together, reheated to the welding-point, and again passed through the rolling-mill. This latter operation is often repeated several times, and is known as piling or fagoting.

Puddling-Furnace.

452. Properties.-Pure iron is of a silver-white color, while ordinary wrought-iron is grayish-white, and when polished has a perfect lustre. In the absence of impurities, iron is so malleable that books have been made of it with leaves as thin as paper, and so ductile that it may be drawn out into wires as thin as a hair. Its most useful quality, however, is its superior tenacity, or power of resisting strain; no other metal being equal to it in this respect. Hence the value of iron in the manufacture of cannons and mortars, where the immense expansive force of gunpowder is to be resisted, and in the making of wire cables for suspension bridges. So great is

its tenacity that an iron wire 0.075 of an inch in diameter is capable of supporting a weight of 449 pounds.

Wrought-iron has a fibrous text

FIG. 172.

ure, and rough, hackly fracture, Fig. 172. It is said that the effect of constant jarring is to cause it to lose this tough, fibrous character, and to become crystalline. It usually contains a small quantity of carbon, which hardens the iron without affecting its other properties to any great extent; but if the amount exceeds per cent., it renders the iron cold-short, that is, brittle and liable to snap asunder when cold. The presence of sulphur, even in so small a proportion as Tobo, unfits the iron for being worked at a red heat, as it is liable to split when hammered; it is then said to be hot-short.

453. When wrought-iron is heated to whiteness, it becomes soft, pasty, and adhesive, and two pieces in this condition may be incorporated, or hammered into one. This is called welding. During the heating a film of triferric tetroxide is formed upon the surface of the metal, which would obstruct the ready cohesion of the separate masses. To prevent this, the smith sprinkles a little sand upon the hot iron, which gives rise to the formation of a fusible silicate, easily forced out by pressure, leaving clean surfaces that unite without difficulty. This important quality is possessed only by iron, platinum, and sodium. All the other metals pass suddenly from the solid to the liquid state, at their respective melting-points. In its ordinary condition iron oxidizes rapidly in the air, and dissolves in nitric acid. But under several circumstances it assumes different, and peculiar chemical relations. If momentarily immersed in a strong mixture of nitric and sulphuric acids it retains its metallic lustre, but has lost the power of either being oxidized in the air or of dissolving

in nitric acid; it has become passive, or assumed an allotropic form.

454. Uses.-Iron in some of its innumerable forms ministers to the benefit of all. The implements of the miner, the farmer, the carpenter; the mason, the smith, the shipwright, are made of iron and with iron. Roads of iron, traveled by iron steeds, which drag whole townships after them and outstrip the birds, have become our commonest highways. Ponderous iron ships are afloat upon the ocean, with massive iron engines to propel them; iron anchors to stay them in storms; iron needles to guide them, and springs of iron in chronometers by which they measure the time. Ink, pens, and printing-presses, by which knowledge is scattered over the world, are alike made from iron.

455. Ferric Carbides (Cast-Iron).- As already stated, most of the wrought iron of commerce is obtained from the ore indirectly, the latter being first decomposed in such a manner as to yield certain ferric carbides, etc., known as cast or pig-iron.

The operation is conducted in tall chimney-like struct

FIG. 173.

Smelting-Furnace.

ures, termed blast - furnaces. They are constructed of stone, and lined with the most refractory fire-brick, having the form seen in Fig. 173. The top

or mouth of the furnace serves for charging it, and for the escape of smoke; it is both door and chimney. The tubes or tuyere pipes at the bottom serve to supply the air, which is forced in by means of immense blowing cylinders driven by water or steam power. The amount of air thus forced through some large furnaces exceeds 12,000

cubic feet per minute. Formerly the air was used at the ordinary temperature (cold blast), but within a few decades an immense improvement has been effected by heating the air before it enters the furnace (hot blast).

The melted

FIG. 174.

456. In some cases the materials are drawn up an inclined plane to the mouth of the shaft by the same engine that impels the blast mechanism. The furnace is supplied with ore, coal, and limestone, broken into small fragments. When the heat is sufficiently intense the carbon of the fuel deoxidizes the iron, and the limestone being decomposed into carbonic dioxide gas, which escapes, and "burnt lime," which in its turn acts upon the ore, unites with the sand, clay, silica, and other impurities, to form a slag or scoria, a crude semi-vitreous, easily-fusible product. cast-iron, falling to the bottom of the furnace, accumulates and is drawn off by taking out a tap or plug. It is allowed to run into a bed of sand, containing straight channels and furrows running at right angles. The former are called by the workmen the sow, and the latter the pigs; hence the term pig-iron. As the contents of the furnace are removed from below, crude-ore, limestone, and fuel are constantly supplied from above, and the operation goes on day and night uninterruptedly for a course of years, or until the fabric demands repair.

457. Cast-iron has a granular texture (Fig. 174), and is so brittle that it cannot be forged, but may be remelted and cast into moulds. It expands when first poured into the mould, so as to copy it perfectly, but subsequently contracts. The expansion is caused by the particles assuming a crystalline arrangement while consolidating; the contraction by the cooling of the metallic mass when solidified. There are several varieties of cast-iron. The so-called "Spiegeleisen" (mirror-iron) of Germany is a nearly pure