exists in coals is proved by
the fact that the ordinary solvents for bituminous substances, such as
bisulphide of carbon and benzol, have no effect upon them, as would be
the case if they contained bitumen soluble in these re-agents. The term
is, however, a convenient one, and one whose use is almost a necessity,
from its having an almost universal currency among coal miners. The
proportion of carbon in bituminous coals may vary from 80 to 90%--the
amount being highest as they approach the character of anthracite, and
least in those which are nearest to lignites. The amount of hydrogen is
from 4-1/2 to 6%, while the oxygen may vary within much wider limits, or
from about 3 to 14%. These variations in composition are attended with
corresponding differences in qualities, which are distinguished by
special names. Thus the semi-anthracitic coals of South Wales are known
as "dry" or "steam coals," being especially valuable for use in marine
steam-boilers, as they burn more readily than anthracite and with a
larger amount of flame, while giving out a great amount of heat, and
practically without producing smoke. Coals richer in hydrogen, on the
other hand, are more useful for burning in open fires--smiths' forges
and furnaces--where a long flame is required.

Gas coal.

The excess of hydrogen in a coal, above the amount necessary to combine
with its oxygen to form water, is known as "disposable" hydrogen, and is
a measure of the fitness of the coal for use in gas-making. This excess
is greatest in what is known as cannel coal, the Lancashire kennel or
candle coal, so named from the bright light it gives out when burning.
This, although of very small value as fuel, commands a specially high
price for gas-making. Cannel is more compact and duller than ordinary
coal, and can be wrought in the lathe and polished.

TABLE I.--_Elementary Composition of Coal_ (the figures denote the amounts per cent).

+----------------------------------------------------------------------------------------+----------------------+
| | Composition |
| | exclusive of Water, |
| | Sulphur and Ash. |
+----------------------------+--------+-------+------+-------+------