The figures showing the results of the two furnaces in question

The waste of fuel consequent upon irregular passage and distribution of the reducing gases has been already adverted to. If, however, the furnace A were working under such unfavourable conditions as those previously alluded to, there is no reason why carbonic acid once formed should not escape as such. The ratio of carbon as carbonic oxide to carbon as carbonic acid would of course be disturbed, owing to the excessive quantity of coke used in the case of furnace A; because, since the generation of carbonic acid is limited by the oxygen separated from the ore in the process of reduction, all such excess of carbon must find its way out of the furnace as carbonic oxide.

This however constitutes no ground why there should be a greater disappearance of the carbonic acid itself in the one furnace than in the other. The figures representing the difference in this respect may not appear of much importance; but when the results are worked out their significance at once impresses itself upon the mind.

In the two furnaces described as blown with superheated air I ascertained, inclusive of the heat contained in the blast, that each unit of carbon burnt by the air entering the tuyeres, afforded from its superior oxidation nearly 15 per cent. more heat in the case of the furnace B than in that of A.

Not only have we the loss from the difference of combustion above mentioned; but, owing to the larger consumption of coke in the case of the lesser furnace A, the weight of the escaping gases is much

larger than in B, and in consequence the loss from this source is correspondingly greater. This last cause of loss was made so conspicuous upon a former occasion as to require no enlarging upon at present. At page 97, in comparing the duty of an 80 feet with that of a 48 feet furnace, in the case of the former 8,860 calories were carried off in the gases, whereas in the lesser furnace no less than 16,409 were so lost, and this in consequence of the greatly increased volume of the gases, escaping also, it is true, at a somewhat higher temperature.

It is perhaps unnecessary to say that it is not only a waste of heat once generated that we have to deal with in furnace A; but with the cause which has interfered with its generation. In other words, the question is in what manner the formation of carbonic acid in the gases has been impeded, or, if not impeded, what has caused its disappearance.

In comparing the duty of the old furnaces of 48 feet with those of 80 feet, the cause of the disappearance of carbonic acid, or the reduction of the ore by solid carbon forming carbonic oxide-it is immaterial which was considered to arise from reduction being effected in a zone of the furnace where the temperature was too high for carbonic acid to exist in the presence of carbon. The difference between the two furnaces in this respect is shown again in Figs. 1 and 2 Plate IV., in which it will be perceived that the region of lower temperature, coloured dark, is much more extensive in the section of the larger than of the smaller furnace. Let us suppose however that we pour in a vast amount of heat with the blast, a great portion of which is intercepted by the material, and is returned over and over again to the tuyeres in the manner already described; then it is not unreasonable to imagine that the region of less elevated temperature, in an 80 feet furnace, will be so diminished as to approximate in capacity to that of Fig 2, which represents one of 48 feet. In such an event it is clear the same fault which accompanies the reducing action in the lesser furnace, will manifest itself in the larger; viz. that solid carbon will perform this office, or else the carbonic acid generated by the deoxidation of the ore will be resolved into carbonic oxide by dissolving carbon.

There only remains to say a few words on the manner in which the larger furnace B has done its work. The first important point requiring observation is the very small amount of duty performed by

a furnace of such capacity; in point of fact, for like space, it amounts to only 56 per cent. of the work done by its lesser neighbour A, and only to 50 per cent. of that of some furnaces elsewhere. I admit, in the matter of consumption of coke, that this large furnace is producing a ton of iron for something less coke than has been done for months together by furnaces half its size, making twice as much iron per 1,000 cubic feet of space, and blown with air 300° C. (540° F.) lower in temperature.

An instance has recently been brought under my notice where an 80 feet furnace, probably from defects in the stoves or in the condition of the furnace itself, or for both these reasons, was making 350 tons of forge iron per week, with the high rate of consumption of 254 cwts. of coke per ton of iron. Fire-brick stoves now deliver the blast at 1,400° to 1,450° F.; and the fuel has been reduced to 21 cwts., while the make has risen to 500 tons per week. In such a case as this it would of course be unreasonable to give superheated air the credit of saving 3 cwts. of coke; particularly when the reduced consumption-21 cwts.-is in reality by no means an unusually low quantity of fuel for producing forge iron with blast of moderate temperature.

A considerable expenditure of heat, and therefore of fuel, accompanies the use of the unusual quantity of limestone needed in smelting the ironstone of Cleveland; while the slowness with which this mineral is reduced renders the use of large furnaces indispensable for its economical treatment. As a contrast to these results may be adduced the work performed by two furnaces, smelting the easily reduced hematite ore of Bilbao. These examples prove the large amount of work done in a small amount of furnace capacity, and the small amount of heat and therefore of coke required, as compared with Cleveland stone. The figures of the larger furnace also demonstrate the possibility of making iron with almost a theoretical minimum of coke previously insisted on, with blast of moderate temperature. It should be observed that the iron made was for Bessemer steel purposes, and attention is also directed in the case of the loftier furnace to the relation the two oxides of carbon bear to each other.

H