FUEL. 
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 consnmed in 
the least, because the spirit with which it is 
constantly soaked is incapable of becoming 
hotter than 174°, Fahrenheit, which is con- 
siderably below the heat of boiling water. 
It is only the vapour that arises from it 
which is hotter, and this too only in its outer 
parts, that are most remote from the wick, 
and where only the combustion is going on, 
in consequence of communication and con- 
tact with the air. At the same time, as the 
alcohol is totally volatile, it does not leave 
any fixed matter which, by being accumu- 
lated on the wick, might render it foul and 
fill up its pores. The wick, therefore, con- 
tinues 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, on the contrary, it be weak, and 
contain water, the water, being less volatile, 
does not evaporate so fast from the wick 
as the more spirituous part ; and the wick 
becomes, after some time, so much soaked 
with watey that it does not imbibe the spirit 
properly. The flame becomes much weaker, 
or is altogether extinguished. When alco- 
hol 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. This was observed by Mus- 
chenbroeck in his experiments upon the 
expansions of metalline rods heated by 
lamps. It is true we can prevent this en- 
tirely, by using very small wicks, and in- 
creasing the number, if necessary, to pro- 
duce the heat required. Or, we may em- 
ploy 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 com- 
plete 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 
<«es not imbibe the ail so fast as before. 
Some have attempted a remedy, by making 
the wick of incombustible materials, as as- 
bestos, or wire; but still, as the oil does 
not totally evaporate, but leaves a small 
quantity of gross fixed carbonaceous mat- 
ter, 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. 
The 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 fur- 
nace, at a time, a quantity that corresponds 
with the quick consumption that must ne- 
cessarily 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. How- 
ever, 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 ex- 
plained. When prepared for use in that 
manner, it is capable of being made to burn 
more slowly and gently, or will bear with- 
out being extinguished altogether, a greater 
diminution of the quantity of air, with 
which it is supplied, than any other of the 
solid fuels. Dr. Boerhaave found it ex- 
tremely convenient and manageable in his 
Furnus Studiosorum. 
The next fuel, in order, is the charcoal of 
wood. This is prepared by piling up billets 
of wood into a pyramidal heap, with seve- 
ral 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 openings at all the spiracles. These, 
are carefully watched ; and, whenever the 
white watery smoke is observed to be suc- 
ceeded by thin blue, and transparent 
smoke, the whole is immediately stopped ; 
