The lawn iron ore, or hydrated oxide, does not occur crystalline, but in botry oidal masses, or in stalactitic lumps ; sometimes in pieces earthy and friable ; its spec. gray. is 8-922. Its composition is—proxide of iron, 84.00 ; water, 11.00; oxide of manganese, 2.00' silica 2'00. Its most remarkable deposit n the United States is at Salisbury, Conn., where it has been wrought for nearly 100 years. • This is the most extensive mine in the country, 8000 tons per annum. Other localities of brown haematite exist in Litchfield, Conn., ae well as in the vicinal county of Duchess, New-York, and Berkshire Mass. The iron which this variety affords is supe rior in malleability to that yielded by the red ore of iron, and is much esteemed also. This ore is abundant in Pa. yield ing from 45 to 55 per cent. of metal. Haematite is abundant in Wisconsin. Iron was first found in this country in Virginia, in 1715. In many parts of Missouri the iron is so pure as not to re quire the preliminary roasting, and the iron mountain of that state has a circuit of two miles, aid au elevation of 350 feet. It consists of specular iron, yield ing 70 per cent. of metal, and contains only a few crystals of felspar.
The general principles which regulate the treatment of ores, will be found given under the article, Metallurgy. Some general notions of the particular treat ment of iron ores are given here. After raising the ore it has to be picked, to separate valuable from worthless ore, or mere stone. They are next roasted in large heaps in the open air to drive off the sulphur and arsenic which they usually contain, and also render them more fri able and easier to be powdered. In Eng land the roasting is conducted with bitu minous coal, but in this country alto gether with charcoal. Trunks of trees and brushwood are laid down and over laid with charcoal, and ignited. Upon the top of this the ore is heaped several feet high. After being roasted the ore is transferred to the crushing mill, where it undergoes another powdering, when it is transferred to the Smelting furnace to be converted into iron. Ilere it passes through two distinct operations : 1. The reduction of the oxide to the state of pure metal ; 2. The separation of the earthy matters as These processes consist in exposing the ore, generally mixed with fluxes, to the action of carbon at a high temperature in furnaceS, urged by bellows, hence called blast furnaces, or sometimes 'high fur naces.
The height of the blast furnace is very variable ; some being only 36 feet high including the chimney, while others have an elevation of 60 feet. These extreme
limits are very rare : so that the greater part of the furnaces are from 45 to 50 feet high. They are all terminated, by a cyl indrical chimney of from 8 to 12 feet long; being about one fifth of the total height of the furnace. The inside diame ter of this chimney is the same as that of the throat or mouth ; and varies from 4 to 6 feet. The chimney is frequently formed of a single eourse of bricks, and acquires solidity from its hoops of iron, so thickly placed that one half of the sur face is often covered with them. At its lower end, the mouth presents one or two rectangular openings, through which the charge is given. It is built on a base ment circle of cast-iron, which forms the circumference of the throat ; and a slop ing plate of cast-iron is so placed as to make the materials slide over into the furnace, as shown in the figure.
The inside of the blast furnaces of Staf fordshire is most frequently of a circular form, except the hearth and working area. The inner space is divided into four portions, different in their forms, and the functions which they fulfil in the smelting of the ore.
The undermost, called the hearth, or crucible, in which the cast-iron collects, is a right rectangular prism, elongated in a line perpendicular to the axes of the tuyeres. The sides of the hearth consist in general of refractory sandstone (fire stone), obtained mostly from the bed of the coal basin, called millstone grit; and the bottom of the hearth is formed of a large block of the same nature, laid on a cast-iron plate. In this country it is chiefly a mica slate, or gneiss rock, con taining a large mixture of quartz.
The second portion is also made of the same refractory grit stone. It has the form of a quadrangular pyramidal, ap proaching considerably to a prism, from the smallness of the angle included be tween the sides and the axis.
The third portion, or lower body of the furnace, is conical, but here the interior space suddenly expands; the slope out ward.% at this part seems to have a great influence on the quality of the cast-iron obtained from the furnace. When No. 2 of the blackest kind is wanted for cast ings, the inclination of this cavity of the furnace is in general less considerable than when No. 2 east-iron for conversion into bar-iron is required. The inclina tion of this conical chamber, called the boshes, varies from 55 to 60 degrees with the horizon. The diameter of this part is equal to that of the belly, and is from 11 to 13 feet. The boshes arc built of masonry, as shown in the following fig ure.