330 



FUEL. 



properly. Tlie flame becomes much weaker, or is 

 ullogeUiei extinguished. When alcohol is used as a 

 fuel, therefore, it ought to be made as strong, or free 

 from water as possible. 



Oil, although fluid like spirit of wine, and capable 

 of burning in a similar manner, is not so convenient 

 in many respects. It is disposed to emit soot ; and 

 this, applying itself to the bottom of the vessel 

 exposed to it, and increasing in thickness, forms, by 

 degrees, a soft and spongy medium, through which 

 heat is not so freely and quickly transmitted. It is 

 true we (an prevent this entirely by using very small 

 wicks, and increasing the number, if necessary, to 

 produce the heat required. Or we may employ one 

 of those lamps, in which a stream of air is allowed to 

 rise through the middle of the flame, or to pass over 

 its surface with such velocity as to produce a more 

 complete inflammation than ordinary. But we shall 

 be as much embarrassed in another way ; for the 

 oils commonly used, being capable of assuming 

 a heat greatly above that of boiling water, scorch 

 and burn the wick, and change its texture, so that it 

 does not imbibe the oil so fast as before. Some have 

 attempted a remedy, by making the wick of incom- 

 bustible materials, as asbestos or wire ; but still, as 

 the oil does not totally evaporate, but leaves a small 

 quantity of gross, fixed, carbonaceous matter, this, 

 constantly accumulating, clogs the wick to such a 

 degree, that the oil cannot ascend, the flames become 

 weaker, and, in some cases, are entirely extinguished. 

 There is, however, a difference among the different 

 oils in this respect, some being more totally volatile 

 than others. But the best are troublesome in this 

 way, and the only remedy is, to change the wicks 

 often, though we can hardly do this and be sure of 

 keeping always an equal flame. 



Tlie second kind of fuel mentioned, peat, is so 

 spongy, that, compared with the more solid fuels, it 

 is unfit to be employed 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 the desire to produce and keep up, 

 oy 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 are considerable. When it is 

 prepared for use in that 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. 



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 

 as to kindle throughout in a very short time. It 

 would burst out into a blaze, and be quickly con- 

 sumed to ashes, were it not covered all over with 

 earth or clay, beaten close, leaving openings at all 

 the spiracles. These are carefully watched; and 

 whenever the white, watery smoke is observed to 

 be succeeded 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 pretty strong red heat raised through the 

 whole mass, and all the volatile matters are dissi- 

 pated by it, and nothing now remains but the char- 

 coaL The holes being 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 vessels. Little pieces may be very finely 

 prepared, at any time, by plunging the wood into 

 lead melted and red hot. This kind of fuel is very 

 much used by chemists, 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 very light. They are, there- 

 fore 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 capable of producing as intense a heat 

 as can be obtained by any ; but in violent heats it is 

 quickly consumed, and needs to be frequently sup- 

 plied. 



Fossil coals charred, called cinders, or coatcs, 

 have, in many respects, the same properties as char- 

 coal of wood ; as kindling more readily in furnaces 

 than when they are not charred, and not emitting 

 watery, or other gross smoke, while they burn. 

 Tliis sort of charcoal is even greatly superior to the 

 other in some properties. It is a much stronger 

 fuel, or contains the combustible matter in greater 

 quantity, or in a more condensed state. It is, 

 therefore, consumed much more slowly on all occa- 

 sions, and particularly when employed for producing 

 intense melting heats. The only inconveniences that 

 attend it are, that, as it consumes, it leaves much 

 more ashes than the other, and these much heavier 

 too, which are, therefore, 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 substance, which clogs the 

 grate, the sides of the furnace, and the vessels. 

 This last inconvenience is only troublesome, how- 

 ever, when the heat required is very intense. In 

 ordinary heat, the ashes do not melt, and though 

 they are more copious and heavy than those of char- 

 coal of wood, they seldom choke up the fire consider- 

 ably, 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 consumed. The heat produced by equal 

 quantities, by weight, of pit-coal, wood-charcoal, 

 and wood itself, is nearly in the proportion of 5, 4, 

 and 3. The reason why both these kinds of char- 

 coal are preferred, on most occasions, in experi- 

 mental 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 former strength. But 

 these great and sudden variations of the heat of a 

 furnace are quite inconvenient in most chemical 

 processes. In the great number of chemical opera- 

 tions, therefore, it is much more convenient to use 

 charred fuel, than the same fuel in its natural state. 



It is proper to be on our guard against the dan- 



