in smelting iron. In such cases, however, a certain additional amount of fuel is required, in order to provide the necessary heat for expelling the tar and hydro-carbons. On the other hand there are other varieties of coal, often less rich in volatile matter than those referred to, which are totally useless in the blast furnace. These are known as coking coals, and, when exposed to heat, enter into a state of semi-fusion, which is regarded as interfering with the entry and upward passage of the blast, as well as with the uninterrupted descent of the contents of the furnace. Such coal is usually coked, before being delivered to the iron smelter -an operation which, as it is usually conducted in this country, is attended with a considerable waste of the fixed carbon, and necessarily also with considerable expense for labour, and for the necessary buildings and plant. These two sources of cost are saved when the coal is used raw; but it must be borne in mind, that the value of the fuel in the blast furnace is represented solely by the quantity of coke it is capable of affording. This is the only portion which affords any useful heat in the operation of iron smelting: for the inflammable gas demands, as has been already stated, the combustion of a certain quantity of the fixed carbon to secure its expulsion. For the present we will confine our attention exclusively to the process of coking, without reference to the subsequent use of the coke itself; and here it may be remarked that this country has little reason to be satisfied with the rude and unscientific manner in which this operation in most cases is still conducted. In fact, until very recently, no kind of progress had been made in the process for the last 50 years. In proof of this I propose to describe the practice of the County of Durham, in which something like 6 millions of tons of coal are annually converted into coke, and where, if in any locality, some advance towards perfection might be expected. Before proceeding with this demonstration, it may be well briefly to give the figures upon which the calculations as to heat, frequently hereafter to be referred to, are based. These figures are all made dependent upon the number of thermometric degrees by which a given quantity of the combustible can raise the temperature of a certain weight of water; or, which amounts to the same thing, on the quantity of water which can be raised one degree in temperature by a given amount of the combustible. The use of the decimal system is so much more convenient than any other, that scientific writers in this country now generally adopt the centigrade thermometer, and the decimal weights used in France. On this system of notation a kilogramme of water raised one centigrade degree is considered as equivalent to one calorie or heat unit; the kilogramme of matter burnt being also regarded as a unit of fuel. The heating power of different substances varies greatly; thus a kilogramme of sulphur gives out, during combustion in atmospheric air, only 2,320 calories or units, while the same weight of hydrogen gas affords 34,000 units. Again, the combustion of the same body is accompanied by the evolution of very different quantities of heat, according to the amount of oxygen with which it may be made to combine in different cases. As an example, one unit of carbon can take up either one or two equivalents of oxygen; but it gives out 2,400 heat units when burnt with one equivalent, forming carbonic oxide, and as much as 8,000 units when burnt with two equivalents, forming carbonic acid. It is also essential, for the proper understanding of such calculations as those in question, to bear in mind that when a compound body is split up into its component parts, exactly the same quantity of heat is absorbed by the act of decomposition as was evolved by that of combination. Thus, if a unit of hydrogen in the form of water has to be separated from its combined oxygen, the dissociation is accompanied by an absorption of the 34,000 units mentioned above; or again, if carbonic acid containing a unit of carbon is reduced to the condition of carbonic oxide, the absorption of heat, or cooling effect, is equal to (8,000 2,400) or 5,600 calories. Occasions will arise during the course of these investigations, which will require us to understand the quantity of heat, i.e. the number of heat units, in a given quantity of heated matter. As the amount of heat required to raise the temperature of equal weights of different substances is by no means the same, it becomes necessary to make an allowance for this variation in what may be called their capacity for heat. This is done by taking water as a standard, and assuming as the unit of measurement what is designated the specific heat of water, in other words the quantity of heat required to heat a unit of water by 1 deg. C. say from 0 deg. C. If then W represents weight, t temperature (centigrade), and SH specific heat, taking that of water as unity, we have the equation WxtxSH = x, to give the number of calories or heat units contained in a given weight of any known body at a known temperature. Let us apply this formula first to 10 kilogrammes of water, and then to the same weight of a substance having one-fourth the specific heat of water, each body heated to 100 deg. C. It is almost superfluous to add that the quantity of heat evolved by combustion has no necessary relation to its intensity-the latter being dependent on the rapidity of the combination, and upon other circumstances on which it would be foreign to the objects of this work to dwell. A wide range of information, of the character just alluded to, has been placed at the service of industry by scientific investigation. This of itself leaves the Durham coke manufacturer almost, if not entirely, without excuse for the rude manner in which he has conducted, and in many instances continues to conduct, his business. It has been urged that to obtain a product of the quality sought for by the ironsmelter some sacrifice was unavoidable. The answer to this is that for the last twenty-five years or more the operation has been carried on in Belgium and France, so as to avoid much of the needless waste incurred in the North of England, and without the change, it is alleged, being accompanied by any of those disadvantages, the apprehension of which would seem to have impeded progress in this country. We will proceed in the first place to ascertain the extent and nature of the waste, thus condemned as unnecessary, accompanying the method pursued in this, the chief coking district of Great Britain. The coal of the County of Durham may be regarded as containing 30 per cent. of volatile substances; or, in other words, as being capable of furnishing about 70 per cent. of its weight of coke. As a matter of fact scarcely 60 per cent. is the average yield, implying that oneseventh, or nearly 15 per cent., of the desired product is lost. In addition to this source of waste, imperceptible perhaps to the casual observer, there is another which forces itself upon the attention of D the most indifferent spectator. I allude, of course, to the vast loss of heat which accompanies the immense volumes of smoke and flame which issue from a Durham coke-work, blackening and desolating the country around it. The following figures exhibit an approximate statement of the actual quantity of heat which is evolved, but from which no use is derived, in the coking of 100 kilogrammes of coal in the old so-called bee-hive oven :— ... Heat evolved by the combustion of the 30 kilogr. of hydro- Calories. = 222,000 = ... ... 80,000 302,000 From which has to be deducted the heat required for expelling Giving as net loss 60,000 The heat emitted by the combustion of different specimens of raw coal varies in amount, but for our present purpose we will take it at 8,500 calories per unit, or 850,000 for the 100 kilogr. From this statement we are justified in the assertion that something like 28 per cent. of the entire heating power of the fuel is lost in coking-a loss which, on the six million tons annually coked in the County of Durham, is equivalent to about 12 million tons of coal. It is not to be expected that anything like the whole of this immense loss can be usefully intercepted; for in almost every operation where heat is required great waste is incurred by radiation, by loss at the chimneys, and from analogous causes. The vast area covered by coke ovens, and the extent of their heat-emitting surface, adds greatly to the unavoidable loss which accompanies their use. Greatly as the temperature of the products of combustion from coking must fall, before they reach points where they can be usefully applied, it remains sufficiently intense to burn bricks; this having been effected, at a considerable distance from the ovens, with complete success. The purpose, however, to which this waste heat most readily lends itself is the raising of steam for the engines required at the mine. At one establishment, where nearly the whole output is coked, it is calculated that 8 per cent. thereof would formerly have been burnt to ventilate the mine and draw the coal and water-duties now entirely performed by the ovens themselves. The manner in which the waste gases are applied for obtaining steam is simple. A boiler is placed on a main flue, into which as many ovens as experience has shown to be required for its use, discharge their surplus heat. At the colliery in question twenty-five ovens, having each a diameter of 11 feet and producing 7 tons of coke per week, are attached to a boiler 60 feet long and 5 feet in diameter. The quantity of water evaporated averages about 24 cwts. per hour per boiler, a margin of power being required, owing to the somewhat intermittent nature of the intensity of the heat given off during the distillation of the coal. A collateral advantage attending the system is the entire absence of smoke; but to secure this the flues must have ample capacity. The sulphur thrown into the atmosphere from the coke ovens in the County of Durham, in the form of sulphurous acid, has been estimated at 45,000 tons per annum, which accounts for the immense destruction of vegetation which takes place, when the gas leaves the oven within ten feet of the surface of the land. By the use of chimneys of be, and now often is, so sufficient height, this poisonous gas can diluted before it reaches the earth that its effects, as it is believed, cease to be visible. A few words now as regards the serious loss involved in the destruction of the 10 kilogr. of coke, for each 100 kilogr. of coal employed. It is only proper to observe that the process of coking has been, and is still, somewhat extensively conducted in Durham by means of "flued ovens," in which the burning gases, after they leave the dome, are made to heat the charge of coal externally. This greatly accelerates the process, and by so much shortens the time during which the coke is wasting. But uniform testimony, in which I am disposed to concur, goes to prove that the coke thus obtained is not as good for blast furnace work as that burnt in the ordinary way. It is less dense and less silvery in appearance, and in consequence, for reasons best explained when dealing with the smelting process itself, is |