y, and
adding all together, we should get a number representing the exact
amount of heat developed by the union of the oxygen and carbon.

Thus far we have regarded the heat developed by the clashing of
sensible masses and of atoms. Work is expended in giving motion to
these atoms or masses, and heat is developed. But we reverse this
process daily, and by the expenditure of heat execute work. We can
raise a weight by heat; and in this agent we possess an enormous store
of mechanical power. A pound of coal produces by its combination with
oxygen an amount of heat which, if mechanically applied, would suffice
to raise a weight of 100 lbs. to a height of 20 miles above the
earth's surface. Conversely, 100 lbs. falling from a height of 20
miles, and striking against 'the earth, would generate an amount of
heat equal to that developed 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 re-condensation
of the steam, after it had done its work; and the amount of work
performed is the exact equivalent of the amount of heat lost. Mr.
Smyth informed us in his interesting discourse, that we dig annually
84 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 108 millions of horses working day and night with unimpaired
strength, for a year, their united energies would enable them to
perform an amount of work just equivalent to that which the annual
produce of our coal-fields would be able to accomplish.

Comparing with ordinary gravity the force with which oxygen and carbon
unite together, chemical affinity seems almost infinite. But let us
give gravity fair play by permitting it to act throughout its entire
range. Place a body at such a distance from the earth that the
attraction of our planet is barely sensible, and let it fall to the
earth from this distance. It would reach the earth with a final
velocity of 36,747 feet a second; and on collision with the earth the
body would generate about twice the amount of heat generated by the
combustion of an equal we