FUEL. 



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 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 othervapours whileburning, 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 

 on 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 

 substance, which clogs the grate, the 

 sides of the furnace, and the vessels. This 

 last inconvenience is only troublesome, 



C.- 



however, when the heat required is very 

 intense. In ordinary heat 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 tog-ether. 



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 former strength. 



But these great and sudden variations 

 of the heat of a furnace are quite incon- 

 venient in most chemical processes. In 

 the greater number of chemical opera- 

 tions, therefore, it is much more con- 

 venient to use charred fuel, than the same 

 fuel in its natural state. 



There are, at the same time, some 

 kinds of fossil coal, which are exceptions 

 to what has now been delivered in gene- 

 ral. We meet with some of them that 

 leave a smaller proportion of ashes than 

 others, and the ashes of some are not so 

 liable to melt in violent heats. There is 

 one species too, such as the Kilkenny 

 coal of Ireland, and which occurs like- 

 wise in some parts of this country, that 

 does not contain any sensible quantity of 

 water, or other such volatile principles. 

 But this may be called a sort of native 

 charcoal. It has the appearance of or- 

 dinary coal, but, when thrown into the 

 fire, does not emit smoke or soot. It 

 merely becomes red, gives a subtile blue 

 flame, and consumes like charcoal ; only 

 it lasts surprisingly long, or continues to 

 gives heat for a very long time before it 



