Those substances which receive and retain fire until they are wholly or partially consumed. Dr. Black divided fuel into five classes. The first comprehends the fluid inflammable bodies; the second, peat or turf; the third charcoal of wood; the fourth, pit coal charred; and the fifth, wood or pit coal in a crude state, and capable of yielding a copious and bright flame. The fluid inflammables are considered as distinct from the solid on this account - that they are capable of burning upon a wick, and become in this way the most manageable sources of heat, though, on account of their price, they are never employed for producing it in great quantities, and are only used when a gentle or small degree of heat is sufficient. The species which belong to this class are alcohol and the different oils. The first of these, alcohol, when pure and free of water, is as convenient and manageable a fuel for producing moderate heats as can be desired; its flame is perfectly clean and free from any kind of soot; it can easily be made to burn slower or faster, and to produce le9s or more heat, by changing the size or number of wicks upon which it burns; - for as long as these are fed with spirit in a proper manner, they continue to yield flame of precisely the same strength.

The cotton, or other materials of which the wick is composed, is not scorched or consumed in the least, because the spirit with which it is constantly soaked is incapable of becoming hotter than 174° Fahr.; it is only the vapour which arises from it that is hotter, and this, too, in the parts most remote from the wick, and where only the combustion is going on, in consequence of communication and contact with the air. At the same time, as the alcohol is totally volatile it does not leave any fixed matter, which, by being accumulated on the wick, might render it foul and fill up its pores; the wick, therefore, continues to imbibe the spirit as freely, after some time, as it did at the first. These are the qualities of alcohol as a fuel: but these qualities belong only to a spirit that is very pure. If it be weak, and contain water, the water does not evaporate so fast from the wick as the more spirituous part, and the wick becomes, after some time, so much soaked with water that it does not imbibe the spirit properly: the flame becomes much weaker, or is altogether extinguished. When alcohol is used as a fuel, therefore, it ought to be made as strong or as free from water as possible.

Oils, though capable of burning in a similar manner to alcohol, are not so convenient in many respects; the soot which they emit accumulates at the bottom of the vessel exposed to it, and cheeks the transmission of heat. By employing numerous very small wicks, or the argand burners, we may chiefly prevent the formation and deposit of soot; but the wicks become scorched or charred, and are soon rendered incapable of absorbing the oil so fast as before. Attempts have been made to obviate this difficulty by making wicks of incombustible matter, as asbestos or wire; nevertheless, as the oil does not totally evaporate, a small quantity of gross carbonaceous matter fixes itself to the wicks which, by degrees, absorb less and less of the fluid, until they become quite useless.

The second class of fuel mentioned, peat or turf, is so spongy, that, compared with the more solid fuels, it is unfit to be employed for producing strong heats. It is too bulky for this; we cannot put into a furnace at a time a quantity that corresponds with the quick consumption that must necessarily go on when the heat is violent. There is, no doubt, a great difference in this respect among different kinds of peat, but this is the general character of it; however, when we desire to produce and keep up by means of cheap fuel an extremely mild uniform heat, we can hardly use any thing better than peat; but it is best to have it previously charred or burnt to a black coal. When prepared in this manner it is capable of being made to burn more slowly and gently, or will bear, without being extinguished altogether, a greater diminution of the quantity of air with which it is supplied than any other of the solid fuels. According to Clement and Desormes peat affords only about one-fifth of the heat that is given out by an equal weight of charcoal.

Mr. Tredgold states, that the weight of a cubic foot varies from 44 to 70 pounds, and that the dense varieties afford about 40 per cent, of charcoal; the other varieties nearly in proportion to their density.

The third class mentioned, the charcoal of wood, is capable of affording an intense heat. Mr. Dalton, by heating water, obtained a result equivalent to melting 40 lbs. of ice with 1 lb. of charcoal. Dr. Crawford's experiments give 69 lbs. of ice melted by 1 lb. of charcoal. Lavoisier, Clement, and Desormes, about 95 lbs.; and Hassenfratz, 92 lbs. Mr. Tredgold considers 47 lbs. of ice melted to be the real average effect of 1 lb. of charcoal: a cubic foot weighs about 15 lbs.

The fourth mentioned class of fuel, pit coal charred or coke, possesses similar properties to wood charcoal, although it is a much stronger fuel, - that is, it contains the combustible matter in a more condensed form; it is, therefore, consumed much more slowly, and is better adapted for long-continued intense heats. It has, however, a defect, from which wood charcoal is free; it leaves dense ashes in the grate, which in time collect in such quantity as to obstruct the passage of the air; and when the heat is intense, these ashes vitrify into a tenacious substance, which clogs the furnace. It is preferable to wood coal for melting metals, as affording a greater quantity of heat before it is consumed, and at a less expense.

The fifth class of fuel, according to Dr. Black, is wood and crude coal; these differ from their charcoals in affording copious and bright flames when plenty of air is admitted to them. If but little air be admitted sooty vapours are given out without flame, and with greatly diminished heat. Wood and candle coal do, however, differ from each other so much, as respects their useful properties in manufacturing operations, that we deem it necessary here to drop the generalization of Dr. Black, and consider wood and coal, and. the varieties of each, separately. First, as respects -

Wood: its effect in producing heat depends greatly on its state of dryness. Several experiments made by Count Rumford show the effect of dry wood to be much greater than that of unseasoned; the latter containing about one-third of its weight of water. The kind of wood is also a cause of some difference; lime-tree wood was found, by Count Rumford, to give out most heat in burning. With l lb. of dry pine-wood, the Count caused 20.10 lbs. of ice-cold water to boil. The same weight of dry beech made only 14.33 lbs. of ice-cold water to boil. A cubic foot of dry beech weighs about 49 lbs. By the experiments of Fossombroni, wood was found capable, by its combustion, to evaporate twice its weight of water, or to prepare two-thirds of its weight of salt. Rumford made the effect about one-third more than Fossombroni, owing, possibly, to superior management in the former. By an experiment made at the Opera House, in Paris, 160 lbs. of wood were found to be equal in effect to 58 lbs. of coke.