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839 
COAL. 
and coarse coal in like manner, only the com- 
ponent particles are smaller, and approach to a 
granular appearance ; canned coal, without visible 
composition, and having a flat, conchoidal frac- 
ture in every direction, with but little lustre, by 
which it is distinguished from pitch coal. All 
these kinds are joined by numerous transitions, 
so that it often becomes doubtful to which of 
them we should ascribe certain specimens, though 
they undoubtedly are members of this species. 
As the preceding varieties of coal consist of 
variable proportions of bitumen and carbon, they, 
of course, must vary in their inflammability. 
Several varieties become soft, and others coke, 
when kindled, or, in other words, allow of the 
separation of the bituminous from the carbona- 
ceous part. We perceive this separation in its 
combustion in a common fire; the coal, when 
kindled, swelling and softening, exhaling a kind 
of bitumen, and burning with smoke and light ; 
while, after a certain period, these appearances 
cease, and it burns only with a red light. The 
separation is effected more completely by the 
application of heat in close vessels: the bitu- 
men is melted out, and there is disengaged 
ammonia, partly in the state of carbonate with 
empyreumatic oil, and the coal gas (a variety 
of carburetted hydrogen), often mixed with car- 
bonic acid and sulphuretted hydrogen, the car- 
bonaceous matter being, in a great measure, 
left, forming coke-—The decomposition of coal 
is carried on, on a large scale, with a view 
to collect the products; the gas being used to 
afford an artificial light, which is clear, steady, 
easily regulated, and economical; the bituminous 
matter, or mineral tar, being applied to the uses 
for which vegetable tar and pitch are employed, 
and the coked coal being used in the smelting of 
metallic ores, and for various other purposes, 
where an elevated and steady temperature is 
needed, 
Coal, excluding anthracite, has been supposed 
to be of vegetable origin. There is a remarkable 
graduation from bituminated wood to perfect 
coal. In some varieties, the structure, and even 
the remains, of plants are apparent, and its che- 
mical composition agrees with that of vegetable 
matter. It is difficult to determine, however, in 
what manner it has been formed, or by what 
operations the vegetable matter, from which it 
has originated, has been so far modified, as to 
have assumed the properties under which it 
exists. And there are many geologists who re- 
gard it, in common with anthracite, as an ori- 
ginal mineral deposit.—The varieties called slate 
coal, foliated coal, coarse coal, cannel coal, and pitch 
coal, occur chiefly in the coal formation; some 
varieties of pitch coal, also the moor cval, bitu- 
minous wood, and common brown coal, are met 
with in the formations above the chalk; the 
earthy coal, and some varieties of bituminous 
wood and common brown coal, are often included 
in diluvial and alluvial detritus. The coal seams 
COAT. 
alternate with beds of slaty clay and common 
clay, sandstone, limestone, sand, &c. They are 
often associated with vegetable organic remains, 
in slaty clay, sometimes, also, with shells, and 
having iron pyrites intermixed with them. Bitu- 
minous coal is so universally distributed, that it 
is unnecessary to attempt the enumeration of its 
localities. 
COAL-ASHES. See Asuuzs. 
COAL-GAS. The combustible aeriform pro- 
duct of the destructive distillation of mineral 
coal. Though often supposed to bea very simple 
and uniform gas, and though now almost univer- | 
sally known in Britain, it is both compound and 
exceedingly variable, and might easily be classi- 
fied into a number of varieties. Its chief consti- 
tuents are olefiant gas and light carburetted hy- 
drogen, in very variable proportions; and its 
most frequent secondary constituents are car- 
bonic acid, carbonic oxide, free hydrogen, free 
nitrogen, some compound or compounds of sul- 
phur, and several definite compounds of carbon. 
and hydrogen. Some of the constituents, such 
as nitrogen, are useless; some, such as carbonic 
acid and carbonic oxide, diminish the illuminat- 
ing power of the mixture; some, such as sulphu- 
retted hydrogen, are noxious to organic beings; 
and some, including all the most important, are 
greatly modified in their illuminating power by 
the proportions of their combination. The quality 
of coal-gas is controlled by the character of the 
coal distilled, by the degree and regulation of the 
heat in distillation, by the method of purifica- 
tion, and by several other circumstances ; and, 
even in any one manufactory, whose processes 
are not carefully and uniformly conducted, it 
may so exceedingly vary as to range between a 
specific gravity of -45 and a specific gravity of | 
‘70. In all public manufactories, it is more or 
less, and in some wholly, purified from its sul- 
phuretted hydrogen by being made to pass 
through some watery preparation of lime; but, 
even after such purification, it contains a pro- 
portion of some compound of sulphur, and in 
consequence generates by its combustion more 
or less of sulphurous acid. Cultivators of plants, 
then, need not wonder that the burning of coal- 
gas is injurious to vegetation ; and they may 
generally infer that any variety of coal-gas will 
damage their plants nearly in the proportion of 
the sulphuretted hydrogen or of the sulphuret of 
carbon which it contains. A singular explosion 
of coal-gas occurred, a number of years ago, in a 
hothouse, with a peculiarly constructed furnace, 
in the vicinity of Falkirk. Some fresh coals were 
put into the furnace, the door was shut, and the 
explosion immediately followed. ‘The flues were 
burst with extraordinary violence; and so great 
was the heat, that vines and other plants were 
completely singed and spoiled. 
COAL-SOOT. See Soor. 
COAT. The natural hairy covering of the do- 
mestic animals, and particularly the sleek and 
Or a a a 
