The second kind of fuel mentioned, peat, is so spongy, that, compared with the more solid fuels, it is unfit to be em ployed for producing very 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 this fuel ; 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 gentle heat, we can hardly use any thing better than peat. But it is best to have it previously charred, that is scorched, or burnt to black coal. The advantages gained by charring have been already explained. When prepared for use in that manner, it is capable of be ing made to burn more slowly and gent ly, or will bear, without being extinguish ed altogether, a greater diminution of the quantity of air with which it is sup plied, than any other of the solid fuels. Dr. Boerhaave found it extremely conve nient and manageable in his Furnus Stu diosorum.
The next fuel in order is the charcoal of wood. This is prepared by piling up billets of wood into a pyramidal heap, with several spiracles, or flues, formed through the pile. Chips and brushwood are put into those below, and the whole is so constructed, that, when kindled, it kindles almost over the whole pile in a very short time. It would burst out into a blaze, and be quickly consumed to ashes, were it not covered all over with earth or clay, beaten close, leaving open ings at all the spiracles. These are care fully watched ; and whenever the white watery smoke is observed to be succeed ed by thin blue and transparent smoke, the hole is immediately stopped ; this being the indication of all the watery vapour being gone, and the burning of the true coaly matter commencing. Thus is a p'retty strong red heat raised through the whole mass, and all the volatile mat ters are dissipated by it, and nothing now remains but the charcoal. The holes be ing all stopped in succession, as this change of the smoke is observed, the fire goes out for want of air. The pile is now allowed to cool This requires many days ; for, charcoal being a very bad conductor of heat, the pile long remains red hot in the centre, and, if opened in this state, would instantly burn with fury.
Small quantities may be procured at any time, by burning wood in close ves sels. Little pieces may be very finely prepared, at any time, by plunging the wood in lead melted and red hot.
This is the chief fuel used by the che mists abroad, and has many good proper ties. It kindles quickly, emits few watery or other vapours while burning, and when consumed leaves few ashes, and those ve ry light. They are, therefore, easily
blown away, so that the fire continues open, or pervious to the current of air which must pass through it to keep it burning. This sort of fuel, too, is capa ble of producing as intense a heat as can be obtained by any ; but in those violent heats it is quickly consumed, and needs to be frequently supplied.
Fossil coals charred, called cinders, or coaks, have, in many respects, the same properties as charcoal of wood ; as kind ling more readily in furnaces than when they are not charred, and not emitting watery, or other gross smoke, while they burn. This sort of charcoal is even greatly superior to the other in some pro perties.
It is a much stronger fuel, or contains the combustible matter in greater quan tity, or in a more condensed state. It is, therefore, consumed much more slowly all occasions, and particularly when employed for producing intense melting heats. The only inconveniences that at tend it are, that, as it consumes, it leaves ;much more ashes than the other, and these much heavier too, which are, there fore, liable to collect in such quantity as to obstruct the free passage of air through the fire ; and further, that when the heat is very intense, these ashes are disposed to melt or vitrify into a tenacious drossy i substance, which clogs the grate, the sides of the furnace, and the vessels. This Oast inconvenience is only troublesome, however, when the heat required is very intense. In ordinary beat the ashes do not melt, and though they are more co pious and heavy than those of charcoal of wood, they seldom choke up the fire con siderably, unless the bars of the grate be too close together.
This fuel, therefore, is preferable, in most cases, to the charcoal of wood, on account of its burning much longer, or giving much more heat before it is con sumed. The heat produced by equal quantities, by weight, of pit-coal, wood charcoal, and wood itself, are nearly in proportion of 5, 4, and 3. The reason why both these kinds of charcoal are pre ferred, on most occasions, in experimen tal chemistry, to the crude wood, or fossil coal, from which they are produced, is, that the crude fuels are deprived, by charring, of a considerable quantity of water, and some other volatile principles, which are evaporated during the process of charring, in the form of sooty smoke or flame. These volatile parts, while they remain in the fuel, make it unfit (or less fit) for many purposes in chemistry. For, besides obstructing the vents with sooty matter, they require much heat to evaporate them ; and therefore, the heat of the furnace, in which they are burnt, is much diminished and wasted by every addition of fresh fuel, until the fresh fuel is completely inflamed, and restores the heat to its farmer strength.