proper manner, an accurate knowledge may be formed, not only of the probable quantity of iron, but even of its tendency to become carbonated in smelting. The expulsion of the water and acid leaves the combined earths more exposed to determination. The small specula of silex are distinctly discerned; adhesion to the tongue developes the presence of clay; and lime is indicated by its assuming a whitish color, either striated or disseminated, approaching towards the surface of the stone. The last method mentioned, namely, that of depriving a given portion of ore of its iron, is the most consonant to truth and to the ideas of the manufacturer. Acids may be used as a check upon the assay by fusion; but this intricate, or rather this slow process, is chiefly resorted to by chemists, and seldom goes beyond the bounds of the laboratory. In assaying by fusion, not only the quantity of iron may with precision be ascertained, but also the quality of crude iron likely to be produced from the ore, with the local proportions of fuel in the large way. The earths formerly united with the iron now become fused with those added for solvents. These float upon the surface of the extracted metal, and, when cold, may afford information, from their color and transparency, concerning the regulation of future proportions upon a more extended scale. The art of roasting iron-stone (technically called its calcination) has by some been consi 400 grains of iron-stone, 75 grains of finely-pulverised coke, The last reduction may be considered as nearly a perfect assay of the ore, which in many cases, where the earthy matters are nearly in equal proportions, may be accurately performed without the addition of any flux. Some masses of the same substance not previously roasted, weighing 6024 grains, were put into a crucible along with coke-dust, and a pyrometer roll filled at 1°. After an exposure of ten hours to a bright heat, the pyrometer was found to indicate 28°, and the iron-stone had lost in weight forty-two per cent., or ten per cent, more than when roasted in the common way. This may be attributed to the loss of oxygen, in consequence of the metallic particles being exposed in contact with carbonaceous The iron-stone, when washed, and perfectly freed from this substance, was of a grayishblue color, adhesive to the tongue, possessed of a metallic taste, and, when pulverised, deflagrated in flame. matter. 400 grains of the common roasted iron-stone, for the sake of comparison, were fused alone, and afforded a dense, shining, opaque glass, without any metallic separation. 400 grains of the cemented deoxidated ore were reduced to the same size, and fused under the same circumstances, from which resulted a prismatic colored button of iron, weighing 120 grains, or thirty per cent. dered as an operation in which, for the first time, the iron-stone unites with a portion of oxygen, and in so far as this requires to be again removed in the smelting furnace, previous to separation, it has been deprecated or thought unnecessary. Practice, however, has more clearly in this, than in any other metallurgical operation, determined the utility and necessity of roasting the ores, previous to their being smelted. Iron-stones in general, in roasting, lose from thirty to thirty-five per cent. of their weight. Torrefaction commences with a change of color upon the surface, which keeps penetrating as the process advances. When the iron-stone is of one uniform color throughout, the operation may then be considered as perfect; if a mass of the iron-stone be broken in the interim, the unroasted part will be found occupying the centre, and of a blackish color. Some hours are necessary to roast thoroughly, even a small piece of ironstone; the same effects however may be produced in two minutes, by exposing iron-stone, finely pounded, upon a red-hot plate. The change of color in this case is rapid; it almost instantly becomes brown, then black, though in cooling it usually returns to a brownish-red, or dirty-purple color. An iron-stone, that had lost thirty-two per cent. in roasting, was pounded very small, fused, and yielded as follows: 139 grains of iron = 334 per cent. 123 grains of iron 41 per cent 132 grains of iron = 44 per cent. In another experiment 4281 grains of the same iron-stone, in pieces, were exposed for twenty-four hours, in contact with coke-dust, to a heat that, by the pyrometer, indicated 69° of Wedgewood; loss in weight equal to forty-three per cent., so that fourteen hours of longer exposure, and double the temperature, had only produced a further deoxidation of one per cent. beyond that obtained in the first cementation. The pieces of iron-stone were now completely metallic, compact, and brightened under the file. 300 grains of this deoxidated iron-stone yielded by fusion, per se, a mass of soft malleable iron, weighing 113 grains, or thirty-seven and two-thirds per cent. 300 grains, to which were added fifteen of coke-dust, yielded by fusion 163 of iron, or fifty-four and one-third per cent.; five grains of coke were found in the crucible unacted upon, so that ten grains of coke were employed in reviving the additional fifty grains of iron. 200 grains of roasted iron stone were fused, per se, in a black-lead crucible: forty-nine grains of iron, or twenty-four and a half per cent., was the result. 200 grains of iron-stone, deoxidated at 69°, were similarly exposed and fused, from which resulted ninety-two grains of iron, or forty-six per cent. The iron-stone subjected to the foregoing experiments, when roasted, yielded forty-four per cent. of iron, and lost in roasting thirty-two per cent.; so that the ore in its native state contained thirty-three per cent. of iron. After the rate of forty per cent. this would furnish us with a quantity of oxygen equal to 13.2 per cent., united originally with the iron. Now, the ponderable matter got rid of by the cementation, in the experiments at 28° of Wedgewood, was the difference between thirty-two and forty-two, or ten, and in that of 69° the difference was equal to 11°; a coincidence sufficiently near to warrant the conclusion, that the iron of our iron-stone is not only in the state of an oxide, but that the dose amounts to forty per cent. at least; for it cannot for a moment be supposed, that any process of cementation, confined to an inferior range of temperature, could separate from the ore the last portions of oxygen, which it is even probable resist the higher temperature, and more perfect operation of the smelting furnace. That a greater portion is removed in a smelting temperature is proved by the following experiments: 934 grains of raw iron-stone, in pieces, were placed in contact with pounded charcoal of wood, and exposed for four hours to a cementing heat of 40°, which was then gradually increased to 120°, at which it remained for nearly eight hours. The result was a complete separation of the iron and earthy matters from each other, in the shape of globules; the former in the state of gray crude iron, the latter in that of glass possessed of some transparency. They weighed collectively 484 grains; the loss, 450 grains, being equal to 48.19 per cent. If thirty-two is taken from this, being the loss sustained in the operation of roasting, the quantity of matter removed by the cementation will be 16-19; so that, in these three experiments by deoxidation, the loss in weight has been analogous to the temperature, viz. În 28° of Wedgewood, loss 42 per cent. do. 43 48.19 In 120° And the quantities of oxygen passed off in the state of carbonic acid or carbonous oxide, will be represented by the numbers ten, eleven, sixteen, nineteen. Some pieces of iron-stone, that had lost in roasting twenty-nine per cent., were exposed for fourteen hours to a heat increasing to 120° of Wedgewood, at which it was stationary for several hours. This experiment was repeated, subject to longer but inferior temperature. The quantity of volatile matter lost was 20-6 per cent. A few small globules of cast-iron had exuded, and appeared upon the surface of the masses; but no separation had taken place between the iron and the earthy matter. In both these experiments forty-one per cent. of oxygen seems to have been removed from the ore. Lancashire ore, when reduced to a small size, and heated on an iron-plate till it becomes of a black color, loses only from four to five per cent. of its weight; iron-stone, in the same time, would lose from twenty-five to thirty-five per cent.; as the ore cools the black passes off, and the native red color returns. In this state it has. acquired a slight, though perceptible, obedience to the magnet. If the ore is heated for two hours, it loses in all from six to seven per cent., and is then strongly magnetic, rendering it probable that the magnetic force in this case is more the result of a new arrangement of the metallic particles, than of the expulsion of so small a quantity of water. If the exposure of this ore be still further continued, and particularly with an increase of temperature, it will regain its lost weight, assume a vitreous sort of fracture, and lose its obedience to the magnet; such increase of weight being undoubtedly owing to a farther dose of oxygen uniting with the iron, which is never found to exceed that of the water expelled in the more moderate temperature; and it may sometimes happen, in experiments of this nature, that the ore, after being roasted for eight or ten hours, will weigh as much as when first introduced into the furnace; though, had it retired at an earlier stage of the operation, the deficiency of weight would have been as before stated. In general, when Lancashire ores (according to the sorts) are cemented in contact with carbonaceous matter, a loss of weight is sustained of from twenty-five to thirty-five per cent.; from which is to be deducted that weight, consisting of water, which the ore would have lost by being exposed to a low red heat, leaving the difference to express the quantity of oxygen removed from the ore during the operation. The following may be considered as a near approach to the constituent parts of the richest and densest species of the Lancashire hematites : The proportion of oxygen, united in this analysis, is under forty per cent. When the ore has been roasted it is taken to the smelting or blast furnace, the lower part of which is filled with either charcoal or coke; the coke is always a fixed quantity, and the proportion of limestone added to the ore is according to the quantity of heterogeneous matter with which the metal is combined. A section of the blast furnace is represented in fig. 1. plate IRON MANUFACTURE. A, at the top of the furnace, is an opening for the introduction of the materials; B the body of the furnace; C the place where the blast is introduced; and D a cavity to receive the metal when released from the earthy matter. The materials in the furnace are, previously to the introduction of the blast, heated simply by the draught of the atmosphere; the coke and limestone to a bright red or white heat, and the iron ore to a melting heat. When the blast is introduced, the metal immediately above it is brought into a state of fusion, and penetrates through the fuel into the cavity D. The ore and fuel that were above it sink down to fill up the space left by the ore melted and the fuel consumed. This next comes under the operation of the blast, and is similarly reduced. The men who attend the furnace keep adding fuel, ore, and limestone, through the opening A, at the top, and the operation of smelting goes on, until the melted iron, in the cavity D, rises nearly to a level with the tuyere irons, or blast pipes. The melted iron is then tapped, by driving a round-pointed bar into a sort of loam, with which the hole is stopped, and runs into moulds made in sand; in this state it is called pig or cast iron. When the slag, in smelting, has a greenish-gray appearance, it is a certain sign that the furnace is in excellent order. Many methods have been adopted to obtain a regular and uniform blast. The first of these has already been noticed under the article BLOWING MACHINE; but, as this method of regulating the blast has been found to be far from perfect, other means have been resorted to with a view of obtaining the desired end. The one called the water-regulator consists of a large cistern, in which another of less area and capacity is inverted. Through the bottom of the smaller cylinder, which is, from its being inverted, uppermost, a pipe communicates with the blowing cylinder. This inner cistern is filled with water, as is also the space between the inner and the outer cistern to the same level. Now, supposing the air to be forced from the blowing cylinders through the above-mentioned pipe into the inner cistern, the water, being displaced by the air, will descend in the inner cistern, and rise up between the two vessels till the column of water on the outside is equal to the required force of the blast; this column would be about four pounds upon a square inch, and about nine feet. Another pipe proceeds from the same cavity in the inner vessel to the furnace, and communicates nearly a uniform blast, varying only with the outer column of water, which will be less as the outer surface of the water is greater. This contrivance, though for some time considered an important discovery, has, in many instances, been abandoned, owing to its carrying water, both in a state of spray, produced by the agitation, and in a state of vapor, into the furnace, by which both the quantity and the quality of the iron was materially affected. Another mode has been attempted to equalise the blast, called the air vault. The first experiment of this nature was tried at the Clyde ironworks, by excavating a large cavity in a rock; but the trial was unattended by success, partly from the vault not being air-tight, and partly from the VOL, XII. moisture which exuded from the rock mixing with the air. A more successful experiment was made at the Carron works. An air-vault of wrought iron plate has been employed in one of the furnaces at Bradley, in Staffordshire, which appears to answer very well. Its form is a cylinder, about ten or twelve feet diameter, and fifty or sixty feet long. According to an average deduced from a series of experiments made by Mr. Mushett, it appears, that, when the outer air was from 63° to 68°, the air, immediately after its escape from the blowing cylinders into a receiving vessel, was increased from 63° to 90°, and from 68° to 991°. In an average of thirty experiments the air in the act of condensing was raised 30°. This would have the effect of increasing its volume not less than one thirty-eighth of the whole, and the increased pressure of the blast, by this cause alone, would be nearly half a pound upon an inch. Or, in other words, if the air were introduced into the furnace at 60°, the same quantity would be admitted with half a pound less pressure upon an inch than if it were 90°. Hence any means of cooling the air after its condensation, in all seasons of the year, must be attended with beneficial consequences. If the air-vault were made of wrought iron, and its surface constantly kept wet, the evaporation from so great a surface, if freely exposed on all sides to the air, would cool the air very considerably. Indeed, without the aid of the moisture, the effect would be such as to recommend its adoption. It was supposed, that in the summer season there would be some advantage in bringing the air under ground for a considerable distance before it entered the blowing machine; but the resistance arising from the friction on the sides of the channels through which it must pass has been found an insurmountable obstacle. The puddling furnace was originally invented by Mr. Cort, and is a most important appendage to the iron works. In its general form it nearly resembles the ordinary air furnace. It is heated by means of pit-coal, on a grate; and has a chimney of considerable height, in which there is a damper to regulate the degree of heat while puddling. A considerable portion of the space between the grates and the chimney is formed flat, and covered with a peculiar sand, possessing the properties, when heated, of becoming very hard and infusible. On this space is placed three or three and a half hundred weight of finers' metal, and the flame allowed to pass over it with the full force of the fire. In twenty minutes the iron assumes a yellowish-white color, and marks of fusion appear on the angles of the pieces; the puddler then turns up new surfaces to the flame, and keeps breaking those which have reached a softened state. This he continues, at intervals, till the charge has subsided into a thick clotted sort of fusion. The furnace, at this period, is reduced to its lowest temperature; part of the furnace bars and fire are withdrawn, and the damper nearly shut; the puddler keeps stirring and moving the iron backwards and forwards, which now begins to ferment and emit flashes of a bluish colored flame. This operation is conti |