probably being between 21 lb. and 3 lb to the square inch."

(6.) The process of reducing the ore to the iron of commerce is as follows:-The ore being prepared and assorted, if of various kinds, and, if necessary, subjected to the preliminary process of roasting or calcining, as described in paragraph (4.) and the blast furnace having been put into operation, and brought up to the proper heat—all of which operations require the exercise of great skill on the part of the ironmaster -the ores are mixed with the "flux," presently to be described, and charged into the blast furnace in a proper quantity, and the fuel, coal, or coke thrown in over them. Another charge of ore is then put in, followed by a second supply of coal or coke, and these operations are repeated at the proper intervals. The intense heat of the furnace being kept up by the action of the blast, the products are-first, the iron, which is a carburet or carbide of iron, composed of pure iron and carbon; and, second, the clay and other impurities, which are more or less separated from it, and which float upon the top, being of lighter specific gravity than the iron, which, of course, descends to the bottom of the furnace, from which it is withdrawn at intervals, and run into rough moulds or channels made

in sand, and to which the name of "pig" is given— hence the name, "pig-iron." The clay or scoriæ in a fluid state are drawn off by a proper aperture previous to withdrawing the melted iron.

(7.) At the end of paragraph (4.) we alluded to the material called "gangue," with which almost all iron ores are in combination, and the presence of which creates a difficulty in the reduction of the ores, arising from its infusibility. To render it fusible, a flux, alluded to in the last paragraph, is mixed with the ore; this is carbonate of lime, or ordinary limestone, which, in its passage through the upper and heated portion of the blast furnace, parts with its carbonic acid, and becomes what is called caustic lime. This enters into combination with the silica and alumina present in the ore, forming a double silicate of lime and alumina, and, being fusible, allows the iron to pass through and descend to the bottom of the furnace. Here, if not for the high temperature, the iron, freed thus from its impurities, to a greater or less degree, would be presented in the form of an oxide, but, in consequence of the high heat, it is completely deoxodised, and the oxygen is replaced by carbon; hence the iron becomes a carbide of iron, which is the condition of what is known as cast-iron. The air is supplied to the blast furnace by means of a

blowing engine, and is either hot or cold according as the system of hot blast or cold blast is used. Previous to the introduction of the hot blast, coke or charcoal were the only fuels used, as by these only could the temperature be obtained necessary to fuse the gangue. By the introduction of the hot-blast—a discovery of James Beaumont Neilson, of Glasgow-much higher temperatures were capable of being obtained with the use of uncoked coal-that is, with coal in its ordinary condition-than with coke or charcoal used in conjunction with the cold blast. In the hot blast, the air in its passage to the blast or smelting furnace is passed through a series of iron pipes, heated to a high temperature in a special furnace, and enters the smelting furnace at a temperature ranging from 500° to 700° Fah. The effect of the hot blast upon the quantity of the iron produced by it has given rise to a discussion which is still maintained, and to which we shall afterwards refer; but whatever be the effect, there can be no doubt that the introduction of the hot-blast system has created quite a new era in the history of the manufacture of iron: it has not only increased the quantity produced, but cheapened it, and enabled some districts, as those of Scotland, where the native fuel formed bad coke unfit for blasting purposes, to com

pete successfully with others where the fuel was of better quality. The arrangements of furnaces or ovens for heating the air for hot-blast furnaces are exceedingly numerous. The reader will find a large amount of information in connection with the subject in a paper read before the Institution of Mechanical Engineers by Mr Henry Marten, of Wolverhampton, under the title of "On the Construction of Hot-Blast Ovens for Iron Furnaces." From this paper, as given in che pages of the Engineer, we take the following igures, illustrative of an apparatus erected at the

orks of the Messrs Lloyds, Fosters, & Co. As will e seen from the illustration fig. 3, the furnace A is overed over, so to speak, with bent pipes, through

26

METALS USED IN CONSTRUCTION.

which the air is forced by the engine, the end connected with iron saddles resting upon iron to admit of the expansion and contraction of th Fig. 2 is the plan; fig. 3 the section.

(8.) From what we have said at the conclu the last paragraph, it will have been perceived quality of the coal exercises an influence of some tance upon the smelting process; of such imp indeed, that, in the view of some authorities, "is even of greater importance than the ore." used in its raw or ordinary condition, or mix coke, its merely mechanical condition was a m much importance; the better able to bear the w the surrounding ores and flux, the better for t poses of the blast. The ash of the coal exerci an influence upon the smelting process, for, b close connection with the carbon of the coal, a jected to a high temperature, its constitue placed in a most favourable condition to be and therefore to be ready to act upon the ore less prejudicially. The following tables, by M give analyses of the blast-furnace coals and ashes: