HE introduction of the use of iron into the construction of build-T ings has practically revolutionized modern architecture; the introduction of steel promises to make equally great changes. The cost of these materials is comparatively so much greater than the ordinary materials used, such as brick and wood, and, again, the uniformity of their composition and strength is such, that in their use the smallest factors-of-safety are used ; that is, the size of material used is very much more nearly equal to its ultimate strength than is the case when using cheaper or less uniform materials. Where, therefore, we "run so closely to the wind," it is essential that the nature and use of the material be thoroughly understood by the architect. Iron is used in three different kinds in building ; namely, wrought-iron, steel, and east-iron. Each has its uses and merits, and its disadvantages. All are really but iron in different combinations. Their differences depend mainly on the amount of carbon they contain. The more carbon, the more brittle, but harder is the iron. The less carbon, the more flexible and elastic, but softer is the iron.

Wrought or rolled iron is the softest, that is iron in its purest form. As it combines with itself a small amount of carbon, it becomes soft steel. The absorption of more carbon makes harder steel, until finally it becomes cast-iron. Pure or real metallic iron does not occur in nature, in commercial quantities, if at all. It is extracted from the various ores of iron, the chief of which are known as magnetite, red and brown hematite, limanite, siderite, etc., being various combinations of iron, with oxygen, forming oxides; of carbon and oxygen forming carbonates ; and of hydrogen and oxygen forming hydrates. Other minerals, rich in iron ore, are found, but cannot be used in the manufacture of irons, on account of the large percentages of sulphur, copper, phosphorus and other substances they contain, which, if present in the finished product even to the smallest extent, render it unfit for most uses.

Three Kinds of Iron.

Iron Ores.

In the manufacture of pig-iron, the ore - or preferable an intelligent mixture of ores- is smelted in a blast furnace with the addition of coal or coke (or a mixture of both), as fuel, and limestone, or some substitute, as a "flux." The blast furnace itself is roughly of the form of an upright hollow cylinder, sometimes 100 feet high, but usually from 50 to 80 feet high and from 20 feet to 25 feet in diameter. The structure has a strong masonry foundation on which rest about eight cast or wrought iron columns, some 10 to 20 feet in height. These sustain a plate-iron casing enclosing the whole furnace from bottom to top. Inside, the furnace is of the shape of two truncated cones, placed base to base over each other, with a short cylinder at the bottom, being thus somewhat narrower at the top and bottom. The bottom of the furnace is called the "hearth;" about 5 feet to 7 feet above the hearth is the "crucible;" from hearth to crucible the furnace is cylindrical and from 6 feet to 12 feet diameter. From the crucible to the "bosh" which is some 20 feet to 30 feet above the hearth, the furnace enlarges to some 14 feet to 20 feet diameter, sometimes even 25 feet diameter. From here to the "throat" which is the extreme top, the furnace narrows down again, being some 10 feet to 15 feet diameter at the top. The furnace is lined inside with an infusible lining of fire-brick, and the charging of ore, flux and fuel is kept up constantly, and of course the fire and smelting process kept going, without stop, barring accidents, for many months at a time, and until this lining gives out; as a rule, the fire is continuous for from two to four years. The lower end of the furnace is closed save for an orifice at the bottom pierced through the walls about horizontally and known as the " hearth." In this pit the melted iron as it is reduced, being heavier than the flux, impurities or fuel, settles down and collects, until sufficient is obtained to justify the tapping or withdrawal of the plug from the orifice, when, of course, the pressure from above forces out the molten iron, which being thus withdrawn flows off through dikes and furrows in the sand of the casting-house floor. This tapping is done from three to tour times every twenty-four hours. The main or feed channels through which the metal flows off directly from the furnace are known as the "sows"; at right angles to these, at frequent intervals, are the short furrows known as the "pigs." These are of convenient size for handling, and when cooled, are broken from the "sows" and form what is known in commerce as "pig-iron."

Manufacture of Pig-Iron.

Blast Furnace.

Hearth and Crucible.

Just under the crucible, that is, above the level to which the melted iron is allowed to rise in the hearth, or some 3 1/2 feet to 6 feet above the hearth, there are from five to eight radial openings in the walls of the furnace admitting the "tuyeres" which are blast nozzles, cooled by the circulation of water in them, and through which hot or cold air is forced horizontally into the blast furnace. The product is known accordingly as " hot blast" or "cold blast." The pressure under which this annas to be forced in, varies, according to circumstances, between 3 and 13 pounds per square inch.

As already remarked, the walls of the furnace widen out above this forming what is known as the "bosh" or the main body of the furnace. Above this the walls usually narrow down, the narrowing, however, depending upon the ore used or the product desired. This part is called the "stack." In the bosh and in the stack the distinctive phenomena of the blast-furnace mainly take place. The top where the walls always narrow down considerably, forming the "throat" of the furnace, is usually closed nowadays by a cone drawn up against a conical hopper, and only opened to allow of the charging of ore, flux and fuel, which is done in alternate layers, after lowering the cone a little. The cone, of course, being again drawn up tightly into place, after the charging. The furnace is kept constantly full, to the throat, being charged as often as the material settles or is withdrawn. The charge usually consists, first, of from 1 to 3 tons of fuel - (coal, or coke, or both) - and then a mixture of ores in proportion of 1 1/2 ton of ore to each ton of fuel. After this the limestone or flux is put in, being in weight from 40 per cent to 60 per cent of the ore. The materials are hoisted to the top in iron barrows by proper machinery. Where the tops of furnaces are kept closed, the blast - (or heated gases at the top) - is conducted off through flues immediately underneath and around the top. Part of these heated but otherwise waste gases are then passed through iron flues to the brick chambers, called "hot blast stoves," or around iron pipes, and serve to heat the "blast" or feed draught of the furnace, where a "hot blast" is used, thus saving fuel, increasing the output and effecting a considerable change in the nature of the pig-iron.