a blast furnace, with its calcining kilns and other accompaniments, stands on a space not exceeding half an acre of ground. Within this small area will be annually received, and most economically handled in the manner already spoken of, about 120,000 tons of minerals; and from it upwards of 60,000 tons of iron and slag will be conveyed with equal expedition and cheapness. The consequence of such an arrangement is that, in some of our best establishments, for each day's work of the men employed, nearly two tons of pig iron are obtained. It would be difficult, looking at the quantities to be handled, to find an instance where, with so small a number of men, so large an amount of work is performed. We may now proceed to the consideration of those agencies, by the exercise of which the blast furnace performs its duty in the efficient manner claimed for it. In dealing with this question, it is not proposed to enter, in the present Section, into all the details of the chemistry of the smelting process. The leading phenomena only of the theory will now be considered, reserving any further explanation to be discussed under separate Sections. If a piece of carbon, such as charcoal, is heated to redness, and continues to burn, surrounded on all sides by air, it will be so without any visible flame. Each molecule of the combustible is converted at once into carbonic acid, with the evolution of the largest amount of heat its combustion can afford, viz.: about 8,000 centigrade units per unit of carbon. 14,550 B. T.U. If on the other hand several pieces of charcoal are placed in contact with each other, and the air required for their combustion is made to pass upwards through the mass, a blue flame will appear at the top. This is owing to a deficient supply of atmospheric oxygen causing the previous formation of carbonic oxide (CO), which is afterwards burnt to the state of carbonic acid (CO), the moment it meets the external air. This generation of the lower oxide of carbon may take place in two ways. Each equivalent of atmospheric oxygen may meet with one of carbon and unite with it, thus forming carbonic oxide at once; or else the carbon may first be burnt to the condition of carbonic acid, and then this, the highest oxide of carbon, may come in contact with a fresh equivalent of carbon in a highly heated state, and dissolve it, two equivalents of carbonic oxide being the result (CO, x C = 2CO). In a calorific point of view, this action is precisely the same as if the two units of carbon had been burnt directly to the state of carbonic oxide. In all low fires, such as the Catalan hearth and refinery, the real nature of the combustion varies with circumstances, such as the depth of the fire, and the temperature of the fuel, whether it be charcoal or coke. As a rule we may take it that near the tuyeres of these shallow furnaces there is a certain amount of carbonic acid present, only part of which is reduced higher up to the state of carbonic oxide. In this way a portion of the carbon arrives at the surface of the fire in the form of carbonic acid, and there it mingles with that resulting from the combustion of any carbonic oxide which may reach this point as such. It thus happens that in such arrangements as those last mentioned, the whole of the carbon, in the one way or the other, passes up the chimney as carbonic acid, having evolved by its oxidation the greatest amount of heat it is capable of affording. It is clear, however, that when the object of such a fire as the Catalan or refinery is the imparting of heat to some body immersed in the fuel, it cannot be otherwise than an exceedingly wasteful operation. That portion of the carbon which is burnt below the surface of the fuel, passes too rapidly upwards to have time to impart more than a mere fraction of its heat to the matter exposed to its influence. On the other hand, such combustion as takes place on the surface of the mass, is scarcely in contact with the body to be heated, and exercises little or no useful effect. Let us now compare the nature of the combustion as it is effected in the blast furnace, and the application of the resulting heat, with that just described. For this purpose, we will suppose that the fuel is burnt in a furnace having a height of 80 feet. The result of several analyses1 satisfied me that almost all traces of carbonic acid disappear within a foot or two of the level of the tuyeres: we may therefore infer that, in the absence of any subsequent change, the whole of the carbon burnt at the hearth would be given off at the throat as carbonic oxide. Imagine such a furnace filled with coke, along with a neutral substance such as slag, not liable to any chemical change. Fire is communicated below, and the blast applied. Combustion rapidly sets in, and the gases, as they arrive at the top, soon become sensibly 'Chem. Phen. of Iron Smelting, pp. 8 and 9. warmer. Their temperature will continue to rise until at the rate at which the furnace is driven, the refrigerating influence of the cold materials, as they enter, establishes a position of heat equilibrium; and the mean temperature of the gases will then remain stationary. It is easy to note the time when this occurs, and to observe the exact quantity of coke which has been burnt between this epoch and that at which the blast was laid on. The number of heat units evolved by burning this weight of coke is easily computed, and along with it the weight of gases which has been generated by its combustion. The mean temperature of these gases having been noted, we can ascertain with tolerable nicety the quantity of heat they are carrying away with them. The difference between the two sets of figures represents the quantity of heat intercepted by the incoming materials. The amount of this difference I ascertained upon two occasions, when blowing in a furnace, and found it to be such that, for every calorie originally evolved in the hearth by the direct combustion of the fuel, 2.33 calories were brought back thither by the materials descending from the upper region of the furnaces. From these figures we have the following statement of the heat development in the hearth, per unit of carbon consumed therein : ... Heat Calories. One unit of carbon burnt at the hearth to carbonic oxide 2,400 5,592 Practically therefore the combustion of a unit of carbon burnt to carbonic oxide in a blast furnace of 80 feet gives nearly as good an effective result, although it evolves only 2,400 calories, as the same quantity of carbon burnt to carbonic acid in a low fire although in the latter case 8,000 calories per unit of carbon are generated. There is however this marked difference between the two examples, that whereas 1 Certain disturbing influences must be taken into the account, but they need not be considered at the present moment. 2 Chem. Phen. of Iron Smelting, p. 293. the 7,992 heat units referred to in the case of the blast furnace are almost all usefully employed, a very large proportion of the 8,000 evolved in the low hearth escapes into the air unutilized. In the low fire, as experience tells us, there is an enormous waste of heat, which is indeed visible in the flame and incandescence at the surface of the fuel. On the other hand, in a blast furnace of 80 feet, the materials are, it is true, red hot, for more than 50 feet above the hearth; but the upper surface of the materials, instead of being red hot, exhibit little or no signs of incandescence, proving a comparative freedom from waste due to this cause. Hitherto, in considering the behaviour of the blast furnace, regard has only been paid to its power of raising the temperature of any substance exposed to its action. But the duty of the blast furnace is of a two-fold kind: it has of course to fuse every non-volatilized substance which enters it, but, before this is done, it has also a chemical function to discharge, viz. the reduction of the ore. This duty is also admirably performed, as will be perceived on a short consideration of its nature, effected as it is by means of carbonic oxide gas. Although an oxide of iron is easily reduced by solid heated carbon, it is greatly to be preferred, for reasons which will be explained in detail, that the deoxidation should be performed by carbonic oxide; and therefore by carbon which in this case, by its combustion at the tuyeres, has already rendered valuable service in melting the slag and iron. In order to enable the reducing gas (CO) to perform its office, an elevation of temperature is necessary. For the removal of the first part at least of the oxygen in the ore, a very moderate heat suffices420° F. (215° C.) being enough for the purpose; but at anything below a low red heat reduction goes on slowly. The effect of the action upon the gas itself, is its conversion from carbonic oxide (CO) into carbonic acid (CO)—a change effected, as will be seen from the symbols, by the absorption by the carbon of an additional equivalent of oxygen, derived of course from the ore, CO + 0 = CO2. It is a fact well known to chemists that reduced iron in its spongy state, when heated in the presence of oxygen, unites readily with this element; indeed so strong is the affinity between the two substances that, under certain conditions, iron in this form is capable of splitting up the very carbonic acid produced from its own previous reduction by E means of carbonic oxide. The conditions for this reversed action are dependent on differences of heat; the power of carbonic acid to oxidize iron increasing, at certain temperatures, more rapidly than does the reducing power of carbonic oxide. Hence, when certain mixtures of the two gases are passed over oxide of iron at certain temperatures, reduction is suspended. As an example it was found impossible to obtain metallic iron when, 1.5 vols. of carb. acid were added to 1 vol. of carb. oxide, the heat being a low red. Further trials were made with the two gases in equal volumes, the mixture being passed over spongy iron, and over peroxide of iron at a white heat. The metal was oxidized to the condition of protoxide (FeO) and the peroxide (Fe,O) was found to have lost exactly as much of its oxygen as brought it down to the same state of combination (FeO) as the other had attained, when it ceased to absorb more of the gas. With a natural law such as that demonstrated by the results of these experiments, it is easy to imagine what must be the fate of ore thrown into a low fire. It speedily reaches a point where the temperature, and the proportion of carbonic acid to carbonic oxide, are such as to render reduction of the whole of the oxide of iron physically impossible. Or, even admitting the gases at the point in question to possess a deoxidizing power, under other conditions, sufficient for complete reduction, that power is so diminished in intensity, and therefore in rapidity of action, by the presence of intensely heated carbonic acid, that fusion of the earths and slagging of the iron oxide become inevitable. By a parity of reasoning, it is difficult to see how the direct process, as carried on in a reverberatory furnace, with an intensely heated atmosphere containing a large quantity of carbonic acid, can be entirely satisfactory, so far as an approach even to complete reduction is concerned. Against the advantage of a saturated oxidation of the fuel, more or less complete in its character, has to be placed the loss of heat which takes place at the chimney of an ordinary furnace, or in the producers employed for generating the gas used in Siemens' furnace, |