We will now take a glance at the properties of each of the three principal kinds of iron, and the purposes to which it is chiefly applied. Cast-iron, as the crudest. cheapest, and most fusible, is used, for the heavy portions of engineering work, such as bed-plates for machines, cylinders, columns, cisterns, low-pressure boilers, water and gas pipes, Milers, girders, and the like. A large quantity is consumed in the manufacture of "hollow-ware," which includes pots, pans, and other cooking-vessels. , For all kinds of ornamental objects, again, it is almost exclusively used, because here its property of being readily cast into molds gives it a great advantage on the score of cheapness.
Malleable iron differs considerably in its properties from cast-iron. The latter' IS practically incompressible, but it can be comparatively easily torn asunder. Malleable iron, on the contrary, possesses great tenacity; it is, moreover, very malleable and duc tile, especially at a high temperature, so that it can be rolled into sheets as thin as paper, or drawn into the finest wire. Further, it possesses the valuable property of welding- that is, two pieces can be completely united together by hammering at a white heat. Malleable iron is largely employed for the innumerable variety of articles included under the general term " hardware, such as locks, keys, hinges, bolts, nails, screws, wire work, and the so-called tin-plate, which is merely sheet-iron dipped in melted tin. It is the mainstay of the railways and the electric telegraph, and has almost displaced timber as a material for steamships and sailing-vessels. It is also much used for roofs and bridges of large size. Rolled armor-plates for war-ships and fortifications are now made of malleable iron from 5 to 22 in. thick.
Steel possesses several valuable properties which do not belong to either cast or wrought iron. It is harder, denser, and whiter in color. It is also more elastic, takes a higher polish, and rusts less easily. Like malleable iron, it is also weldable. But its most characteristic property consists in its admitting of being tempered at will to any degree of hardness. If, for instance, a piece of steel be heated to redness and plunged into water, it is made hard and brittle; but if it be again heated and slowly cooled, it original softness is restored. By gently reheating the &tee] it will acquire a gradation of tints indicating various degrees of hardness, beginning with pale straw color, and passing successively to full yellow, brown, purple, and finally to blue. The straw color is the result of a temperature of about 440°, and the blue of about 570° F., the former being the hardest and the latter the softest tempering.
The use of steel is no longer confined to such small articles as edge-tools, knives, and other cutlery. By means of improved Machinery and proeesses, steel is at present manufactured on a scale that was little dreamed of thirty years ago, so that such objects as field-guns, heavy shafting, tires, rails, boiler-plates, mind the like are now being made of this material. The superior tensile strength of steel, which is about double that of malleable iron, gives it a great advantage where lightness is required. Large numbers of steamships are now being built of steel.
In 1740 the entire quantity of iron made in Great Britain is believed not to have exceeded 25,000 tons; in 1802 the annual make was estimated at 170,000; in 1828, at 702,581; and in 1839, at 1,512,000 tons.' In 1854, the first year of the carefully collected statistics now published annually by the mining record office, the produce was 3,069,838, and from that time to the present it has gradually risen to nearly 7,000,000 tons. A very large amount of this pig-iron is converted into malleable iron, as there are now upwards of 7,000 puddling furnaces in the country. In the United States about 2,830,000 tons of pig-iron were made in 1872. but the make had fallen in 1876 to 2,093,236 toils. On the continent the iron manufacture is rapidly extending in France, Belgium, Prussia, Aus tria, Sweden, and Russia. It is remarkable that as much 250,000 tons of steel were made in 1875 both in Germany and France by the Bessemer and other processes, a lame quantity being also made in other countries. Notwithstanding the activity of the iron trade abroad, the produce of Great Britain is still about one-half that of all other coun tries put together.
Siernens'8 regenerative gas-furnace is now so much used in the making and melting of steel, as well as for other purposes, that it is desirable to give a short description of it here. No furnace yet designed can be compared with it in respect to economical con sumption of fuel. It consists of two parts: one of these contains the "regenerators," or, as Dr. Percy calls them, the "accumulators;" the other, which may either be quite near or more than 100 ft. apart, contains the "gas-producers" or source of the heat. In the regenerative portion, when the furnace is to be used for the production of iron or steel, there is a inching hearth or bed like that represented at B in fig. 3. Immediately below this hearth there are two pairs of arched chambers filled with fire-bricks placed sufficiently far apart to let air or gases pass freely between them, and at the same time expose a large surface to absorb heat. One pair of these chambers or regenerators communicates by separate flues with one end of the hearth, the other pair with the opposite end of it. Thus we have in duplicate, so to speak, a chamber through which gas and another through which air can be admitted. The furnace being in operation; while the gas and air are being admitted to the hearth through, say, the left pair of these chambers, the highly-heated products of combustion pass through the opep brick-work of the corresponding pair on the right before reaching the chimney. What would pass up the chimney as waste heat in an ordinary furnace is thus absorbed by the bricks of the regenerators. After a given time—nsually frorn 30 to 60 minutes—by means of suit able pipes and valves, the arrangement, or if we may so call it, the current, is reversed. Gas and air are now sent through the freshly-heated pair of regenerators, while the " waste heat" in turn passes into the other pair.- In this way, by reversing the valves at intervals, hot cnrrents of gas and air, in suitable proportions, are always reaching the hearth where combuStion is effected at a temperature.