FUEL. 
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 
dissipated by it, and nothing now remains 
but the charcoal. The holes being all stop- 
ped 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, char- 
coal 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 in lead 
melted and red hot. 
This is the chief fuel used by the chemists 
abroad, and has many good properties. It 
kindles quickly, emits few watery or other 
vapours while burning, and when consumed 
leaves few ashes, and those very 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 capable 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 kindling 
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 supe- 
rior 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 dis- 
posed 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, 
however, 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 charcoal of wood, 
they seldom choke up the fire considerably, 
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, are nearly in [proportion of 5, 4, and 
3. The reason why both these kinds of 
charcoal are preferred, on most occasions, 
in experimental chemistry, to the crude 
wood, or fossil coal, from which they are 
produced, is, that tire crude fuels are de- 
prived, 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 inconveni- 
ent in most chemical processes. In the 
greater number of chemical operations, 
therefore, it is much more convenient 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 general. 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 likewise 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 ap- 
pearance of ordinary 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 surprizingly long, or continues 
to give heat for a very long time before it 
I 
