ii. After the metal in the moulds is cooled, it is to be removed, weighed, and stored; and the sand of the pig bed dug up, wetted, and prepared for another cast. The cinders at small furnaces are easily removed in common carts. At stone coal furnaces, various methods have been devised to remove the large mass of cinder daily produced, of which that at present generally practiced at the anthracite furnace may be considered the best. It is this: Dig two round basins of about five or six feet in diameter, and two feet in depth, at the side of the stack. In the centre of each basin put a piece of pig metal, in an upright position. Around this pig metal, the cinders, which run into the basin, gather. A chain attached to a crane is then fastened to the pig metal, by means of which the cold cinder is placed upon any suitable vehicle, to be carried off.

A whole volume might be written without exhausting what could be said on the management of furnaces, and of blast furnaces in particular. But our space is limited, and we wish to avoid prolixity. Many occasions will arise, in the course of this work, which, we hope, will enable us to supply whatever deficiency our statement may, thus far, have exhibited.

X. Theory of the Blast Furnace.

It would be inconsistent with our object to enter, with scientific minuteness, upon this branch of our investigations. If we shall be able to convey to an intelligent mind a clear and comprehensive view of the operations which take place in the interior of the blast furnace, our design will be accomplished. It is evident our explanations must be somewhat of a speculative nature; but these are illustrated and confirmed by operations performed under the cognizance of our senses. In the previous chapters, we have related and reasoned upon matters which can be tangibly verified; but in the present instance, we are obliged to draw general conclusions from isolated, though well-established facts, by means of pure analogy -an operation frequently and daily needed, and constantly performed by those engaged in the management of blast furnaces.

a. In the second chapter, we have spoken of fuel and its combustion, as well as of the different combinations which oxygen forms with fuel. We are forced to refer to that subject in the present instance, for the process of combustion must be well understood before we can understand the chemical operations which take place in a blast furnace. The fuel used in the blast furnace is composed, to a greater or less degree, of carbon, hydrogen, sulphur, and ashes.

If oxygen or atmospheric air combines with carbon, the result is either carbonic oxide or carbonic acid; at a high temperature, with a sufficient supply of air, always carbonic acid. A suffocated combustion, with an excess of fuel, generally produces carbonic oxide. The result of the combustion of hydrogen and oxygen is always water; that of the combustion of sulphur and oxygen always sulphurous acid.

Fig. 70.

b. Combustion in a blast furnace is, as may well be expected, of a somewhat complicated nature, and requires illustration to be understood. Fig. 70 represents a section of a blast furnace in operation, filled with coal, ore, and fluxes. If we introduce at a, a, the tuyere holes, a current of air or blast, combustion in the lower part will ensue ; and, according to circumstances, the product will be carbonic acid of greater or less durability. But if we have an excess of fuel, and a limited supply of air, the final product of the combustion will be carbonic oxide. The primitive or immediate combination of carbon and oxygen at the tuyere forms carbonic acid; and this carbonic acid, in its progress through the coal, combines with more carbon, and forms carbonic oxide. Carbonic acid can not combine with any more oxygen than it already possesses; but carbonic oxide will combine with as much more as it already contains. Carbonic acid is of no use in reviving iron from the ore, for the ore is a combination of iron and oxygen; and carbonic acid could not abstract any oxygen from the ore. But carbonic oxide will combine with whatever oxygen is present in the interior of the blast furnace.

c. Practical investigation has demonstrated that the more friable and tender the coal is, the more easily oxygen combines with it; and that the more compact it is, that is, the greater its specific gravity, the greater is the difficulty with which it combines with oxygen. Heated air combines more readily with fuel than cold air,

and of course is more inclined to form carbonic oxide. Soft, open fuel and heated air form carbonic oxide, the agent in the reduction of the ore, more readily than hard coal; and we may conclude that charcoal and coke are more useful than anthracite coal in the manufacture of iron. According to this statement, the atmosphere of oxygen and carbonic acid will be a zone of greater or less radius, of which the mouth of the tuyere is the centre, as the circular lines in the engraving indicate. The radius of this zone has been found, by experiments made on furnaces, to vary, according to fuel and blast, from six inches to four or more feet. Applying what we have said to a common furnace, with grate and draft, the column of carbonic acid will be from six inches to four feet in height, if we pass a current of atmospheric air through hot and burning fuel. If the column of fuel is higher than this, the carbonic acid will be gradually converted into carbonic oxide. This process is exactly the same in the blast furnace; the oxygen of the atmosphere is gradually converted into carbonic acid, carbon with much oxygen and then gradually into carbonic oxide, carbon with less oxygen. Where the atmosphere of carbonic acid ceases in the blast furnace, we may conclude that the working of the carbonic oxide upon the ores commences, and that it changes more or less in its course upwards. The ascending current of the gases, in a blast furnace, consists, then, of carbonic oxide, hydrogen, and combinations of hydrogen and carbon. These latter gases are derived directly from the fuel, above the reach of free oxygen, and constitute gaseous combustibles, ready to unite with oxygen. Mixed with the above are steam, carbonic acid, and nitrogen-incombustible gases which have not the least influence upon the ore. The nitrogen is derived from the atmosphere.

The ascending current of the gases from the tuyeres differs in composition according to height; of course this composition will not be alike at a given height in two furnaces of different construction, and in which different materials are used. Actual experiments on furnaces carried on by hot blast and charcoal have furnished the following results :

Directly above the tuyere. Nitrogen. Carbonic acid. Carbonic oxide. Hydrogen.

We find here, what might have been expected, a gradual inThis is generated by the contact of The relative amount of the different

crease of the carbonic acid. carbonic oxide with the ore.

gases is not equal in different furnaces, for, in another case, the gases were mixed in the following proportions:

The gases of a coke furnace exhibited the following composi

tion:

Directly above the tuyere. Nitrogen. Carbonic acid. Carbonic oxide. Hydrogen.

There are, particularly in coke furnaces, gases of a compound character; but these have little to do with practical results, the aim of our investigations.

From the above, it is apparent that the carbonic acid gas increases as the current of gas ascends; and that, on an average, one-third of the carbonic oxide has been converted into carbonic acid before escaping at the top. If the carbonic oxide is the only reagent in the conversion of ore into iron, we may conclude that one-third of the fuel has been properly applied for the purpose for which it was designed. We here have evidence that all the fuel has not done its duty; otherwise, all the carbonic oxide would have been converted into carbonic acid, and all the hydrogen into water. But such is not the case. If a furnace works well, there will be more carbonic acid at the top of the charges than there will be if a furnace works badly; this circumstance accounts for the different appearance of the trunnel head flame.

d. The theory of the reduction of ore will then be simply this: the gases ascending in the furnace leave a part of their positive elements to combine with the oxygen of the ore, that is, carbonic oxide leaves carbon, and, under peculiar circumstances, hydrogen may be retained. If carbonic oxide absorbs oxygen from the ore, it leaves of course metallic iron or protoxide, and the ore, in descending, will be a mixture of metallic iron and foreign matter. If

that process is not well performed, some oxides of iron will be left in the mixture. If an ore, to this extent prepared, but without any surplus of carbon, descends into the hearth, it cannot produce anything but white iron; for, if the iron is once heated to redness, and melts, it absorbs no more carbon. All the carbon required for making gray iron must be in the ore before it sinks into the hearth. For this and many other reasons, we are forced to assume a surplus of free carbon in the gas mixtures of the blast furnace-carbon, if not chemically, at least mechanically, mixed with the gases, and so finely diffused, that it can penetrate into the pores of the ore. If we adopt this theory, that is, the presence of free carbon, we can account for many apparent irregularities in furnace operations for which we cannot account on the simple assumption that the gases ascend in their constitutional form. By adopting this theory, we account for a circumstance otherwise incomprehensible, that is, the great influence exerted by the pressure of the blast; for if nothing else than carbonic oxide is needed, almost any pressure, even the weakest blast, will accomplish all that is desired. But we know, by experience, that the strongest blast which a given kind of fuel will bear advantageously, is the most profitable. It appears, from this, that the blast works mechanically as well as chemically, in the destruction of coal; and that a certain power will produce particles of coal of a size best calculated to penetrate the pores of the ore. If these particles are too large, they cannot reach the interior of the ore, and the iron will be white. This may be assigned as the reason why a particular pressure of the blast is required to produce gray metal. If the blast is too weak, it produces white iron frem deficiency of carbon in the ore; and if too strong, the consequences are equally injurious. Such an admixture of free carbon will be, of course, uniformly diffused among the gases, and penetrate the porous ores more readily than even the gases themselves, on account of the superior affinity of carbon for oxygen.

A further evidence of the agency of free carbon, in the smelting of gray iron, is in the fact that compact, close ores, of whatever chemical composition, will not produce gray iron. Should an atmosphere of carbonic oxide, or even carbon in any other form, alone be needed to smelt gray metal, there would be no difficulty in manufacturing gray iron from any kind of ore. But this is not the case. From compact specular ore, magnetic ore, the carbonates, and ores too hard burnt, we cannot make gray iron, whatever amount of coal we employ, and whatever kind of blast we use. A certain aggregate