of heat escaping from the latest constructed blast furnaces is sufficient to expel the water and carbonic acid from the whole of the ironstone used. The intensity of the temperature of the gases is however on the whole much below that required for the operation; and hence the advantage of the change is very questionable. Besides this, there is a further objection, connected with the subsequent use of the escaping gases. By weight the combustible portion of this gaseous fuel only amounts to 15 per cent. of the whole; and any material addition to the uninflammable ingredients might, and we know does, tend seriously to diminish the combustibility of the useful portion of the gases. The actual labour in calcining is too insignificant to demand consideration; and the quantity of coal consumed in the operation is so small, as not to render it advisable to affect in saving it, the burning of the gases which have to serve for driving the machinery, and heating the blast for the furnaces. It may also be added that the calcining of the ironstone, with the excess of free air which passes through the kilns, effects a more perfect separation of the sulphur than can be accomplished in the blast furnace, where at the proper period for expelling this noxious element there is no free oxygen to be found. PRELIMINARY TREATMENT OF LIMESTONE. In the great majority of cases, the purer the iron smelter receives his limestone, the better it suits the purpose he has in view. It generally happens that the silica and alumina of clay ironstones are found in such proportions, that the mere addition of pure lime suffices as a flux for them. The presence therefore of either of these substances in the limestone itself merely adds to the inert matter to be fused. Fortunately for successful work in the furnace fusible slags can be produced, in which the constituents coexist in a vast variety of proportions, the extent of which will be most conveniently explained in a future Section. Many of the great geological deposits of limestone consist of carbonate of lime almost pure; thus chalk, when dried, will contain as much as 99 per cent. of this substance, and the mountain limestone 95 to 97 per cent., while the limestone of the Silurian age is often nearly as pure as chalk. The impurities are a trace of oxide of iron, alumina, and silica; and occasionally magnesia, existing from a mere trace up to 10 or 20 per cent. or more. In the formation known as the magnesian limestone, the last mentioned earth is almost invariably present in greater or less quantity; sometimes nearly half the weight of the stone consisting of carbonate of magnesia. As a mere question of fusibility, the presence of magnesia is the reverse of a disadvantage; because not only do certain mixtures of silica, lime and magnesia, even without any alumina, form compounds which are easily melted, but the presence of a fourth ingredient, or even of more, frequently promotes fusion in the remainder. Generally speaking however, magnesia can be dispensed with; because the earths of our clay ironstones are sufficiently easily melted without its aid, and it is at all times better to use lime, on account of the sulphur which is almost invariably found both with the iron and with the fuel. Lime has at high temperatures a certain affinity for this element, whereas magnesia possesses little or no action on it. To free iron as far as possible from so objectionable an ingredient as sulphur, the lime should be in excess; hence the substitution of a flux inert as regards this metalloid, which is the case with magnesia, is not desirable. At the same time it often happens that economy of cost, generally determined by transport charges, leaves no alternative but to employ limestone containing a considerable percentage of the earth in question. Thus in the United States the lime in the flux is often associated with so much carbonate of magnesia, that in smelting the magnetic ores of New Jersey, yielding 48 per cent. of iron, as much as 25 to 26 cwts. of flux is required per ton of metal produced. The carbon wasted by the reaction of the carbonic acid, entering the furnace as carbonate of magnesia, and the fusion of the needless amount of cinder, will probably require the presence of an addition of 5 to 7 per cent. to the coal, as was the case at the works at which I obtained my information on this head. Among certain iron works, using furnaces of 4,000 to 6,000 cubic feet capacity, it has been the fashion to calcine the limestone before charging it; and inasmuch as the presence of carbonic acid in such cases is, for the reasons assigned, undesirable, such preliminary treatment of the flux may on certain occasions prove useful. So far, however, as it is possible to compare the performances of furnaces, engaged in an operation where there are so many disturbing causes, I have never been able to satisfy myself that the full measure of economy was effected, due to the difference between employing carbonate of lime raw, and in its calcined condition. The objection to the use of raw limestone in the furnace is mainly one involving consumption of fuel, which may be summarised as follows: = ... Calories. 4,070 The expulsion of carbonic acid from the quantity of flux used Making a total theoretical gain of 8.030 This number divided by 3,000, the number of calories afforded by fuel in the hearth, gives an estimated saving of 2.67 units of fuel on the 20 of iron produced. Of course from this saving, namely 2.67 cwts. of coke per ton of iron, must be deducted the value of a much larger weight of coal, practically used in the kilns for calcining the limestone. Inasmuch however as the refuse coal used for the latter operation costs, in the case alluded to, only one-third of the price of coke, there remained a margin of economy; although in reality the saving was not more than, if indeed it reached, one-half of the estimated weight mentioned above. This difference between the real and calculated saving seems to me to be due partly to the fact that burnt lime, even as it is used by builders, often contains a notable quantity of carbonic acid. In the case of that employed at the blast furnaces, the quantity of carbonic acid unexpelled by the act of calcination must have been considerable, looking at the relative quantity of the two which was used. Thus, when raw limestone was employed, 12.8 cwts. sufficed per ton of iron, equal therefore to 7:17 cwts. of actual lime. Or again, 14 cwts. of limestone sufficed in furnaces only 48 feet high, which would be equivalent to 7.84 cwts. of pure lime. To maintain the cinder however of a similar composition as regards lime, when the calcined lime was used, the weights were 10 and 11 cwts. respectively. It may therefore be in ferred that there was still present (10-7-17) or 2.83 cwts. and (11-7.84) or 3.16 cwts. of carbonic acid respectively. Now as the full equivalent of carbonic acid was only 5.63 and 6.16 cwts., for the two quantities of limestone, it looks as if the actual quantity of this volatile constituent had only been removed by calcination to the extent of about one-half. Another cause of the want of economy of fuel is probably due to the behaviour of carbonic acid as regards lime. Under certain conditions, as is well known, a high temperature dissociates the two substances, while at ordinary temperatures they combine with marked readiness. It has to be observed, however, that a heat up to that of redness greatly adds to the rapidity with which lime absorbs carbonic acid. In such furnaces as those to which the present remarks apply, the calcined lime would commence to absorb carbonic acid immediately after being charged; and it would continue so to act until it reached that zone of the furnace where elevation of temperature and other conditions would put a stop to further absorption. Such carbonic acid, however, as was thus taken up by the lime, would act on the carbon, and give rise to the same expenditure of heat for its subsequent expulsion, as if it had existed naturally in the flux. When the larger furnaces were erected in Cleveland, the same practice as regards the limestone was applied to them; but the results were not such as to justify its continuance, for there was scarcely if any perceptible economy of fuel. The cause of this was probably due to the prolonged period of exposure to the more moderate temperature, extending as this does over a much wider space in a lofty than in a low furnace. This change would be accompanied by a more complete return of the calcined lime to its natural state of carbonate of lime. No doubt the reabsorption of carbonic acid by lime would be accompanied by a rise of temperature; but this probably takes place so near the point where the gases leave the furnace that they have not sufficient opportunity of imparting the heat thus required to the materials filling its upper region. The only case I am acquainted with, when caustic lime is employed in a modern large furnace, is that where a portion of the ore (clay ironstone) is used in a raw state. Such treatment therefore only means expelling carbonic acid from limestone instead of from ironstone. SECTION V. THE BLAST FURNACE. THE difficulty any form of the direct process labours under, in taking the place of the blast furnace, meets us from almost every point of view in which the subject may be regarded. For the founder the blast furnace is indispensable; for so far as our experience goes, it constitutes the only available means by which he can be supplied with cast iron. No doubt, for malleable iron purposes, the blast furnace, as has been admitted, is a circuitous mode of procedure, for in it the metal takes up substances which have to be got rid of by a subsequent process or processes. This disadvantage, as has been already explained, is more than counterbalanced by the very economical application of labour permitted by the character of the operation, and by the much higher duty performed by the fuel, when compared with that obtained elsewhere, whenever a high temperature is required. Along with these two claims for a position of superiority over the direct process, is the practical absence of all waste of iron in the operation of smelting. Doubtless against this last named ground of preference has to be set the loss of metal incurred during the subsequent process of separation from the contracted impurities. After allowing for this, however, a considerable balance remains, even in the item of waste, as against the production of malleable iron direct from the ore, and in favour of the use of pig iron. For a proper appreciation of the nature and efficiency of the process, as performed by the blast furnace, a somewhat careful examination of its work, chemical and otherwise, is indispensable. Before entering into this question, it may be well to refer again to the circumstances which render it easy for the pig iron maker to economise in human labour. One of the most important of these is the compactness of his plant: for, including moderate but sufficient room for stocks of materials, |