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beds of coal, composed chiefly of carbon, which has not yet
closed in chemical union with oxygen. Distance still inter-
venes between the atoms of carbon and those of oxygen,
across which the atoms may be impelled by their mutual
attractions, and we can do nothing more than utilize the
motion produced by this attraction. Once the carbon and
the oxygen have rushed together, so as to form carbonic
acid, their mutual attractions are satisfied, and, while they
continue in this condition, as dynamic agents they are dead.
A pound of coal produces by its combination with oxygen
an amount of heat which, if mechanically applied, would
raise a weight of 100 lbs. to a height of twenty miles above
the earth’s surface. Conversely, 100 lbs. falling from" a
height of twenty miles, and striking against the earth,
would generate an amount of heat equal to that devel-
oped by the combustion of a pound of coal. Wherever
work is done by heat, heat disappears. A gun which fires
a ball is less heated than one which fires blank cartridge.
The quantity of heat communicated to the boiler of a
working steam-engine is greater than that which could be
obtained from the recondensation of the steam after it had
done its work; and the amount of work performed is the
exact equivalent of the amount of heat missing. We dig
annually nearly 100 millions of tons of coal from our pits.
The amount of mechanical force represented by this quantity
of coal seems perfectly fabulous. The combustion of a
single pound of coal, supposing it to take place in a minute,
would be equivalent to the work of 300 horses; and if we
suppose 120 millions of horses working day and night with
unimpaired strength, fora year, their united energies would
enable them to perform an amount of work just equivalent
to the heat to be derived from the annual produce of our
coal-fields. Our woods and forests are also sources of
mechanical energy, because they also have the power of
uniting with the atmospheric oxygen, and the molecular

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