(from the French feu, fire, akin to the Latin focus, a hearth or fireplace), a word applied to certain substances which are used in the generation of heat, such as wood, peat, coal, &c., and also sometimes applied to the substances employed in generating light, such as oil, spirits of wine, naphtha, &c. In the present article the former application will alone be considered; for information respecting the latter, see Gas-Lighting, Lamp, &c.

The abundance and consequent cheapness of fuel has a great influence on the prosperity, habits, and manners of a nation. Where fuel is scarce, factories languish, and commerce declines. In cold climates scarcity of fuel is individually a great calamity, for it abridges the hours of labour, causing persons to spend those hours in sleep which under other circumstances would have been turned to profitable account; it also causes persons to crowd together for the sake of warmth in a way that is injurious to health and morals. Abundance of fuel, on the contrary, with good roads and a system of inland navigation for its distribution, forms the basis of national prosperity, not only ministering to the useful arts, but enabling the occupier of every house to create an artificial climate suited to his wants and wishes.

The most common and widely distributed description of fuel is wood, a term applied to the trunk, roots, and larger branches of trees. Recently felled wood consists chiefly of woody fibre, sap, and water. The woody fibre is a compound of carbon, hydrogen, and oxygen, and forms the chief bulk of plants; both it and the sap are combustible—that is, are capable at a high temperature of combining rapidly with the oxygen of the atmosphere and forming gaseous compounds. It is in the act of this formation that heat is generated. The sap, which forms only a small proportion of the bulk of wood, varies in different kinds of trees: the sap of the pine tribe contains resin; that of the oak, tannin; that of the beech and birch, extractive. The quantity of water in wood varies greatly with the kind of tree, and with the time of year when it is felled, it being least in winter. As the water is not combustible but must be got rid of at the expense of the heat generated by the parts which are so, it is obviously desirable to store the wood in a dry and airy situation before using it as fuel. By this means 100 lb. weight of wood have been known to lose 20 lb. weight in ten or twelve months. Wood, as commonly used for fuel, contains about one-third of its weight of water. Wood also contains earthy and alkaline salts in the proportion of $\frac{1}{3}$th to $\frac{1}{4}$th, and these remain as an incombusible ash.

Wood is distinguished from all other fuel by the valuable property of reproduction, and also by the fact that it often passes through various stages of beauty and utility to man before it becomes converted into fuel.

The heating power of wood is considerable, in consequence of its excess of hydrogen, which, in burning and forming water, requires for equal weights three times as much oxygen as the carbon does in forming carbolic acid; and it gives out in burning nearly four times more heat than the carbon. The lighter woods contain more hydrogen than the heavier, so that they burn with flame longer than they incandesce as charcoal; they also burn more easily and give out their heat more quickly than the hard woods.

During the combustion of wood its volatile parts undergo some complicated chemical changes. When wood is burnt out of contact with the air, the carbon is preserved in the form of charcoal (see Charcoal), which is a very useful fuel when an incandescent heat free from flame and smoke is required; but when some of the volatile products are to be collected, the wood is placed in iron retorts which are gradually raised to a red heat; the volatile products form carburetted hydrogen, carbolic acid, carbolic oxide, and other gases, and also certain vapours which condense into liquid or solid products; some of the liquids are soluble in water, such as pyroxylic spirit, pyroligneous acid, &c.; the insoluble products form tar and certain oily substances.

In most countries deposits of peat occur of greater or less extent. In Holland, the north of Germany, Ireland, &c., peat deposits are of immense extent. The origin of peat has been accounted for in those districts where clay occurs near the surface by supposing muddy pools to have formed, round the edges of which aquatic plants have taken root and gradually extended themselves into the centre, thus forming a bed where mosses accumulate, and new plants take growth, while the old are decaying and becoming compressed into a solid mass below. This process goes on until the pools are filled up with vegetable matter, and the surplus water is discharged over the neighbouring lands, where the process is repeated until a peat bog is formed. Even in mountain districts, where the soil is impervious, clouds and mists may supply moisture, and a bog be formed by the growth of one generation of vegetable matter on the ruins of its predecessor. As the plants which form the peat are in different stages of decomposition at various depths, the character of the peat varies greatly. Near the surface it is light-coloured, spongy, and the vegetable character but little changed; lower down it is brown and dense; while at the base of some bogs, which may be as much as forty feet in depth, the peat is black, almost as dense as coal, and resembling coal in chemical composition.

On the banks of the Rhine peat is cut by means of a spade into blocks, and exposed to the air to dry, the upper layer being first separated from the lower and denser portion. In Holland the peat is scooped out by means of spades; or if a considerable quantity of water be present, an instrument is used consisting of a sharp iron ring attached to a handle, a net or cloth being fastened to the ring for draining off the water. The muddy peat thus collected is trodden out by the feet of men, raked, and the stones picked out; it is then thrown into shallow wooden boxes, strewed with hay to prevent the peat from adhering, and the remaining water is allowed to drain off. In the course of a few days when the mass has attained a certain consistence, women with flat boards strapped to their feet stamp down the peat until it has attained such a consistency as not to take an impression from a common tread. It is next stamped with beaters; and the cake, which is eight or nine inches thick, is divided by means of long laths into squares of about four inches, which are removed a few at a time from each box. The cakes are then dried by placing the first taken out transversely on the second, the third upon the fourth, reversing the order when the pieces are piled up in store.

The value of peat depends greatly upon its dryness, density, and firmness; if porous and brittle it crumbles during carriage or after it is stacked and thus becomes nearly worthless. In many cases the value of peat depends on its capability of being alternated with the substances to be heated. Porous and almost valueless peat has been rendered valuable by being passed through a press, in which case a lump of peat may lose as much as one-fourth of its weight of water. Peat may be nearly valueless as a fuel from the quantity of ash which it affords, consisting of vegetable salts and the earthy matter of peat, and amounting in some cases to one-third of the weight of the peat. When this large quantity of ash occurs in peat it renders the fuel very dusty, and in smelting processes it is objectionable on account of its chemical action. It is remarkable that the carbonates of the alkalies are not found in this ash, but phosphates, sulphates, &c.

In some large towns, peat, or turf as it is also called, is imitated by employing the refuse bark of the tan-yard, which is made into flat cakes, and chiefly used as fuel by the poor.

In the sandy plains of the East, camels' dung is dried and used as fuel; it was from the use of such fuel in Egypt that sal ammoniac originated, the salt subliming during combustion. Other descriptions of excrement are also used as fuel. The Chinese have long been accustomed to mix cow dung and other refuse vegetable matter with soft clay and the dust of coal to form balls which when dried in the sun become a cheap and useful fuel, burning with very little smoke. These balls are largely manufactured in the coal districts of China, and are distributed over the empire by means of the canals. It is a curious fact that Sir Hugh Platt, in 1594, indicated a method of making coal balls with loam, and that Ray, in 1663, observed this kind of fuel at Liège (Journey through the Low Countries, &c., 1673, page 58), where they were called hot shots, serving to slake the heat of a fire and keep the coals from burning out too fast. We are informed that in some parts of Wales stone-coal culm is made into balls with clay, and is a common form of fuel in Pembrokeshire. The combustion is slow, and a long steady heat is kept up well adapted for lime burning. In 1853 a patent was taken out in England by M. Ducayla

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1 Hakluyt, in his Voyages, vol. i., p. 348, says, "we were forced to use for fowell the dung of horses and camels, which we bought dears of the pasturing people." A substance in the form of long sticks, said to be made of camel's dung, is sometimes imported from the east under the name of Chine, and is occasionally used instead of the ordinary match for lighting pipes and cigars. It burns slowly without flame, and gives out an odour not unlike that of the burning cuttings of trees and shrubs.

2 Sir Hugh gave an engraving representing a fire of fire-balls, without publishing a description of the method of making them; and he seems to have thought this so valuable that he offered to disclose it for a pecuniary consideration. His book is a curious exponent of the science of his day. It is entitled The Jewell House of Arte and Nature, and was published in 1594. The engraving referred to is contained in an appendix to the work, entitled Divers Chymical Conclusions, p. 69. of Bordeaux for the manufacture of fireballs of such materials as cinders or ashes, wood or lignite, anthracite coal, pit coal, animal black, calcareous earth or clay, mould, &c.

The fuel in every respect the most interesting to the British Islands, and one of the chief sources of their wealth and prosperity, is coal. The very abundance of this article causes it to be used in so lavish and extravagant a manner, that any general attempts to economise it and to fix its value as a fuel scarcely interest the public. There are, however, particular cases in which it is desirable to economise coal as far as possible, as in the case of a steam-ship of a thousand horse power, a single journey of which may require upwards of 2000 tons of coal, or more than 80,000 cubic feet. Hence it is obvious from the details given of the various descriptions of coal in the article Colliery, that some varieties of coal are better fitted for the purposes of steam navigation than others. A few years ago, when the government was establishing a steam navy, Sir H. de la Beche and Dr Lyon Playfair were requested to examine and report on the coal suited to the steam navy. The inquiry was conducted with great ability, and has resulted in two reports published in 1848 and 1849, which the reader interested in the subject will do well to consult. We will however state a few of the chief points elicited by this inquiry.

Bearing in mind the object of the inquiry, the commissioners considered that the chief test of the value of any coal submitted to their examination was its power of converting water into steam, so that if a given weight of coal in a certain time converted a larger proportion of water into steam than the same weight of another coal in the same time, the evaporative power of the one would be greater than that of the other. It was found, however, that the coal best adapted to steam-ships of war should also combine other qualities: for example, the fuel should burn quickly, so that steam may be raised in a short time; it should not be bituminous lest its smoke should betray the position of the ship when it might be desirable to conceal it; it should have such a cohesive power as not to be broken into fragments by the rolling motion of the vessel; it should have such a density and structure as to bear stowing away in a comparatively small space (a condition which in coals of equal evaporative value was found to involve a difference of more than 20 per cent.); lastly, the coal should not contain a large proportion of sulphur, nor be subject to rapid decay, or it might in either case lead to spontaneous combustion. But it was not found possible to unite all these conditions in the same coal. Anthracite, for example, has high evaporative power, but not irritating easily its action is not quick; it is not easily broken by the motion of the ship, but not being a caking coal, it would not cohere in the furnace, and would escape through the grate-bars during the rolling of the ship in a gale; it gives off no smoke, but from the intensity of its combustion it causes the iron of the grate-bars and of the boiler to oxidize rapidly; hence, with many advantages, anthracite has a few defects sufficiently prominent to preclude its use under ordinary circumstances. It was thought that a patent fuel might be formed with some of the anthracites of Wales, which should combine the advantages and elude the defects above referred to; but it was found that the cementing tar of the patent fuel burnt so much more rapidly in the furnace than the anthracite that the latter accumulated on the bars and obstructed the draught, or escaped through the grate unburnt.

In conducting this important inquiry, circulars were sent to the different collieries of Great Britain, explaining the object of the inquiry, and requesting the proprietors to forward two tons of coals for experiments; and in nearly all cases the application was favourably responded to. Each sample of coal was accompanied by a certificate from the owner, and in the report are given various particulars respecting the mine, the geological position of the coal, the method of working it, the nearest port, the extent of the trade in that coal, the physical character of the coal, together with the result of the experiments made upon it in a tabulated form.

One of the first experiments on each coal was to test its cohesive power. This was done by enclosing a portion of coal in a wooden cylinder 3 feet in diameter, and about 4 feet in length, with a bearing or gudgeon at each end; within the cylinder were three shelves each 6 inches wide, tending towards the axis, for the purpose of forming a lodgment for the coals, and carrying them up towards the top of the cylinder during its revolution; thus ensuring a certain amount of fall. The coals to be tested were broken to the size used in the subsequent experiments on their evaporating power, and then thrown on a sieve with one-inch square meshes. 100 lb. of the coal left on the sieve were put into the cylinder which was made to revolve a certain number of times. The coals were afterwards sifted on the same sieve, and the weight remaining gave the percentage of large coals. The mean of two trials with each coal was taken, with fifty revolutions of the cylinder for each.

The coal was next heated under different arrangements of draught so as to ascertain when the gases escaping from the coal were most economically consumed. It was found that experiments made with highly bituminous coal, under different areas for the admission of air, varied much more considerably than the less bituminous coals of South Wales.

In testing the evaporative power of the coal a cylindrical boiler was used 12 feet in length and 4 feet in diameter, with flat ends, and an internal flue 2 feet 6 inches in diameter, in one end of which the grate was placed. The flues were on the split, or bridle draught plan, and the column of heated air after leaving the fire was made to pass through the internal flue to the rear of the boiler, where it divided, and returned along the outside of the boiler on both sides to the front; the two branches, each 2 feet 6 inches deep, then turned down at right angles to their former course, and united under the boiler in the bottom flue, traversed its whole length again and entered the base of the chimney, after exposing, during a course of about 36 feet, an area of 197½ square feet of boiler surface to the heating action. The height of the chimney was 35 feet 6 inches, its internal dimensions being 182½ square inches; it was furnished with dampers and with apertures for the purpose of making observations on the temperatures of the currents, and of obtaining samples of the gases for analysis, also for drawing up the soot. The furnace was closed with Sylvester's fire-doors, and a variety of ingenious arrangements were made for estimating temperature, pressure, &c. The method of conducting the experiments was as follows:—Supposing the water in the boiler to be cold, and to stand about one inch below the normal level, the fire was lighted, and the steam got up in the afternoon of the day preceding the commencement of the experiments. The fire was then allowed to burn out, when the fire and ash-pit doors and the damper were all closed. The next morning the first thing done was to open the safety-valve to equalize the external and internal pressures, and then sufficient water was let down from the tanks to raise that in the boiler to the normal level. The depth of the water in the tanks was then gauged, and the first observation of its temperature made. The ashes, cinders, and soot were next cleared out, and after noting the temperature of the water in the boiler, the fire was lighted with a weighed portion of wood, and the exact time was then observed. The coals were then

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1 The experiments were conducted at the college for Civil Engineers at Putney by Professor John Wilson and Mr J. Arthur Phillips, assisted by other gentlemen. gradually added till the fire was of the proper size and form.

The form of fire was slightly varied according to the kind of coal employed, the object being to burn the coal to the best advantage with as little smoke as possible at the chimney. The observations of the temperatures of the two side and escape flues, and of the water in the tanks, then succeeded each other at regular intervals of about one hour each.

When the steam raised the safety-valve the time was observed, and entered under the heading steam up. The damper was adjusted as soon as the fire was sufficiently established, and was not disturbed during the day, except under peculiar circumstances. When by evaporation the water had sunk about one inch below the normal level, the deficiency was supplied from the tanks above; or by the plan afterwards adopted, the water was allowed to flow in continuously, so as to maintain the water in the boiler at a constant level. The fire was supplied with coals, in pieces not exceeding 1 lb. weight each, and not more than one or two shovels full at a time, and were usually spread evenly over the fire; but in the case of anthracite, it was found that the sudden application of heat caused the pieces to split and fall through the bars. They were therefore gradually heated on the dead plate before being put on. With the bituminous coals a preparatory process of partial coking on the dead plate prevented them from coking in the fire (which would have impeded the passage of air through the grate), besides giving better opportunity for burning the smoke and gases by passing them over a large surface of ignited fuel. The duration of the experiment was reckoned from the time the steam was up to about that of the last application of fuel, after which the fire was allowed gradually to burn out, when the damper and furnace and ash-pit doors were closed. During the day ashes were thrown up in small quantities from time to time when the fire was burning clear and well. The weight of coals consumed was then ascertained, by deducting the weight left from the gross weight provided for the day's trial, when the experiment terminated. The next morning when the level of the water in the boiler was adjusted, by turning down a supply from the tanks, their depth was gauged, and the quantity evaporated the previous day was thus ascertained. The coke and cinders were then removed, the clinkers, if any, were separated, and the weight of each was taken. The soot was cleared out at the end of the last day's experiment, and the total weight recorded, which divided by the number of trials, gave the average weight. Samples of the ashes, cinders, and soot, were then put aside in bottles for the purpose of ascertaining the percentage of combustible matter present in the residue. The barometer was observed at about 11 o'clock in the day, being generally about two hours after the steam was up. The quantity of combustible matter in the residue was estimated by heating the powdered substance in a stream of oxygen gas, by which the organic matter was got rid of chiefly in the form of carbonic acid and water, and the loss was estimated as combustible matter.

The commissioners found that the qualities which distinguish particular kinds of fuel are very varied, so that it is difficult to deduce general results. But the data furnished by their experiments enable us to contrast the actual value of a particular coal with its theoretical value, supposing its combustion to be attended with no loss of heat. The actual duty obtained by 1 lb. of coal from the boiler employed may be expressed by the number of pounds raised to the height of one foot, a result which may be obtained by the formula $W \eta \times 9657 \times 782 = x$, in which $W$ represents water, of which $\eta$ pounds are evaporated by one pound of coal. This formula is deduced from the fact that $\eta$ pounds of coal multiplied by 9657, or the coefficient for the latent heat of steam at 212°, indicates the number of pounds of water which would be raised 1° Fahr.; and the number 782 arises from experiment on the mechanical force denoted by the elevation of 1 lb. of water 1° Fahr.; that force being equal to 782 lb. raised to the height of one foot, according to the experiments of Mr Joule. The best Cornish engines are said to be capable of raising 1,000,000 lb. to the height of one foot for every pound of coal consumed, but this is only about $\frac{1}{3}$th of the actual force generated, and only $\frac{1}{4}$th or $\frac{1}{5}$th of the theoretical force. Experiments on the evaporative power of coal made by different observers give very dissimilar results. Smeaton in 1772 evaporated 788 lb. of water from 212° with 1 lb. of Newcastle coal; Wall in 1788 evaporated 862 lb.; Wickstead in 1840 evaporated 9493 lb. of water from 80° with 1 lb. of Merthyr coal, which is equal to 10,746 lb. from 212°. In some experiments made at the united mines in Cornwall, it was found, after a trial of six months, that every pound of coal evaporated 10,29 lb. of water from 212°; and according to some experiments made in Cornwall, at the request of the commissioners, it was found that 11-42 lb. of water were evaporated by every pound of Welsh coal of similar chemical composition to that of Mynydd Newydd.

At ordinary temperatures coal undergoes a slow combustion under the action of the oxygen of the atmosphere, evolving carbonic acid, nitrogen, and inflammable gases, and in some cases leading to dangerous explosions. This slow combustion is facilitated by the higher temperature of hot climates, and by the presence of moisture. If the coal contain much sulphur or iron pyrites the chemical action may become so intense as to ignite the coals. In stowing coals it is therefore important that they should be as dry as possible, and such a variety should be selected as is least liable to this progressive decomposition. When coal is kept in iron bunkers, and is liable to be wetted with sea water, the iron rapidly corrodes from the carbon or coal forming a voltaic circuit with the iron, and thus promoting oxidation.

In the Great Exhibition of 1851, Messrs Berard and Co. in the French department, No. 51, exhibited "small purified coals, and residue of the same, the produce of a system for purifying coals, patented in France, England, Belgium, and Germany." This plan appears to be well adapted to the purification of sulphurous coal, or coal containing much iron pyrites; also where the coal deposits are in numerous small seams, and cannot be got out without being mixed with slaty and stony matter. The coal used on the Chemin de Fer du Nord was so sulphurous as to injure the locomotives; but by using the purified coal, the evil was for the most part remedied; the quantity of ash was also greatly reduced. The apparatus employed for purifying the coal is similar in principle to the jigging machine used in dressing ores, which, after being stamped in order to separate stony matter, are agitated in water and allowed to rest, when the various portions become arranged in layers, according to their specific gravities. This purified coal yields a very pure coke.

Mr Crace Calvert of Manchester has taken out a patent for purifying coke from sulphur. It consists in mixing the coal before coking with from $\frac{1}{2}$ to $\frac{3}{2}$ per cent. of common salt, the proportion varying with the quantity of sulphur. The coking is then conducted as usual. By this contrivance, coal, which was formerly unserviceable in smelting operations, can now be used with effect.

We cannot conclude this article without a brief notice of some of the earlier attempts to ascertain the heating value of fuel. The economy of manufactures, our domestic arrangements, health and comfort, depend so much on the judicious application and regulation of fuel, that any information tending to its better expenditure ought to command

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1 Report, p. 21, 22, slightly abridged. Our expenditure of coal is of so prodigal a nature, and is every day so increasing, that we look forward to the time when the supply may fail. The calculations made many years ago by geologists on the probable duration of our fossil fuel were not only based on incorrect data, but on a comparatively small consumption. The ten-yard coal of Dudley is nearly exhausted, and other districts are becoming more scanty in their supplies, or more difficult to work.

As various kinds of fuel afford different amounts of heat, and as heat cannot be measured or weighed, and its quantity ascertained by direct experiment, the relative values of fuels are ascertained by comparing them with each other under similar circumstances. The heating power of a fuel is the quantity of effect produced by it in a certain time, and this in conjunction with its market price gives its value as a fuel. One fuel may produce a certain greater or less effect than another fuel, and thus its relative superiority or inferiority may be accurately ascertained, although the actual amount of heat furnished by it may be entirely unknown.

Lavoisier and Laplace fixed these values by making the substance under examination act on ice, and the quantity of ice melted gave the value in each case. Count Rumford measured the value of fuel by the increased temperature which it produced in a given quantity of water. Now, as the same quantity of heat which melts one pound of ice at 0° Cent. is sufficient to raise the temperature of as much water 79° Cent., or 0°79 lb. of water 100° Cent., so also an equal weight of aqueous vapour of any given temperature and elasticity is always formed from the same amount of heat, and always contains the same quantity of heat, and the quantity of heat which water at 100° Cent. renders latent in order to become steam is 5½ times sufficient to heat the same weight of water from 0° to 100° Cent., hence the water converted into vapour by the heat required to melt one pound of ice is the 5½th part of the same pound, that is, it can convert into vapour 0°154 lb. of water.

It was found by Despretz and Welter that the quantities of fuel which require equal amounts of oxygen for combustion, give out equal quantities of heat; thus, 1 lb. of oxygen in combining respectively with hydrogen, charcoal, alcohol, &c., raised 29 lb. of water from 32° to 212°. A given weight of the different combustibles has its heating power represented by the number of pounds of water raised in temperature, as in the following table:

| Fuel | lbs. of water | |-----------------------|--------------| | 1 lb. of pure charcoal raised... | 78 from 32° to 212° | | common wood charcoal | 75 | | baked wood | 36 | | wood holding 20 per cent. of water | 27 | | bituminous coal | 60 | | turpentine | 25 to 30 | | alcohol | 68 | | oil, wax | 50 | | ether | 80 | | hydrogen | 236 |

More recent researches have, however, cast considerable doubt upon the law that any given quantity of oxygen evolves the same quantity of heat with whatever combustible body it may combine. From a series of tabulated results given in Gmelin's Hand-Book of Chemistry, vol. i., page 292 (Cavendish Society's translation), it would rather appear that oxygen develops a larger quantity of heat the stronger its affinity for the combustible substance.

Such experiments as the above tend to confirm the modern view of combustion which regards oxygen as a combustible as much as the fuel with which it combines (see Chemistry, vol. v., page 456); so that when oxygen burns by means of any fuel, the heat evolved increases with the quantity of oxygen consumed. It was on this view that Berthier based his process for detecting the quantity of oxygen required for combustion, and the heating power of the combustible in one experiment. His plan is to heat to redness a known quantity of the combustible with a considerable excess of pure litharge until the combustible is entirely consumed by the oxygen of the oxide of lead. On weighing the lead reduced by this process the amount of oxygen consumed is ascertained, and also the heating power of the fuel under examination. In calculations of this kind it will be remembered that 6 parts, or 1 equivalent of carbon, require 16 parts, or 2 equivalents of oxygen, for combustion; that one part of hydrogen requires 8 parts of oxygen; that by subtracting from the hydrogen a quantity corresponding to the oxygen in the coal, the calculation can be made for the carbon only. Now, 1 part of pure carbon requires for combustion 2:666 of oxygen, and is capable, according to Despretz, of heating 78:15 parts of water from freezing to boiling. By multiplying each part of lead obtained by 2:665, the weight of water is obtained which is capable of being heated between these temperatures by a unit of the coal used in reducing the litharge.

The heating power of a particular fuel is the same, however, that fuel may be burnt. It is true, that the power may be more or less economically applied; the power may be expended with greater or less rapidity, greater, for example, in a furnace than in an open grate, but as the fuel during combustion combines with equivalent portions of oxygen, the same amount of heat is liberated whether the combination be rapid or slow. The rapidity of combustion depends not only upon the mode of arranging the draught or supply of air to the fuel, but also on the state of division of the fuel itself. A given weight of wood in the state of shavings will, from the large extent of surface exposed, burn rapidly, and produce its full heating effect in a few minutes, while the same weight of wood, in the form of a log, may keep up a moderate temperature for some hours. The division of a fuel may, however, be carried so far that the air necessary for its combustion cannot penetrate it. Such is the case with saw-dust, powdered charcoal, or peat, slack coal, &c. If the powdered coal be of caking quality it may be burnt into compact coke, and thus be more useful than a fuel which in its first form is compact but which falls to powder on being heated in the furnace. Small fuel may sometimes be advantageously applied by covering the furnace-bars with pieces of sand-stone or lime-stone for the purpose of preventing the fuel from falling through, and for distributing the supply of air among it. In the roasting of copper ores in South Wales a flaming coal is necessary in the reverberatory furnace where the operation is carried on. But as the flameless anthracite is much more abundant in this district than the bituminous coal, it is turned to account in an ingenious manner. When burnt under ordinary circumstances it crumbles to powder, as already noticed, and either slips through the bars of the grate, or chokes them up. But when anthracite is raised to a very high temperature it forms a vitreous scoria or clinker, which in the ordinary furnace combines with the iron of the bars and chokes up the grate. In the Welsh furnaces the clinkers themselves are ingeniously arranged, so as to perform the office of grate bars, namely, to support the fuel, and to limit the supply of air from below. The clinkers are supported on iron bars placed at a considerable distance apart, and are arranged in a layer twelve or sixteen inches in depth. Above this layer the fuel of the furnace is in full combustion; this fuel consists of anthracite mixed with about one-fourth of its weight of small bituminous coal, and also forms a layer of a depth about equal to that of the clinkers; it is in this the hottest part of the fire that fresh clinkers are being continually formed, and while forming they cake with the numerous fragments of bituminous coal heaped up above them. As fresh portions of the fuel come into operation the clinkers descend towards the bottom of the grate, where meeting with the numerous jets of air which stream up through the bottom the vitrified mass splits and cracks in all directions, forming new channels for the ascent of the draught, but not large enough to allow the small coal to escape. As the calciner-man heaps up fresh fuel above, he hooks out a few clinkers from the bottom to make way for the descent of others. Under this arrangement the oxygen of the air traversing the multitude of channels formed by the cracks in the clinkers, combines with a portion of the fuel and forms carbonic acid, which is uninflammable, but before reaching the vault of the furnace it is deprived of a portion of its oxygen, and becomes converted into carbonic oxide, which is inflammable. But in order that this gas may undergo combustion, air is admitted through apertures in the sides of the furnace just above the ore, and in this way the whole surface of the ore, occupying an area of nearly 23 feet square, is played upon by a thin sheet of flame, produced from fuel which gives scarcely any flame at all.

Common coal gas is sometimes used as fuel, in which case it is calculated that 1 lb. or 24 cubic feet thereof in burning will raise 76 lb. of water from the temperature of freezing to that of boiling. Extending this comparison to the other forms of fuel, it is stated that 1 lb. of dry wood will similarly heat 35 lb. of water, but only 26 lb. if the wood be not dry, or contain moisture to the extent of from 20 to 25 per cent. One lb. of good dry charcoal will similarly heat 73 lb. of water, but if exposed to the air it absorbs at least 10 per cent. of moisture, and in burning gives a flame of carburetted hydrogen (or rather, probably, a mixture of carbonic oxide and hydrogen), arising from the decomposition of the moisture. One lb. of good pit coal is said to raise 60 lb. of water from freezing to boiling, 1 lb. of coke 65 lb., and 1 lb. of turf or peat from 25 to 30 lb.