284. FRAGMENTS OF SCIENCE. 
enormous force of the collision. The burning of charcoal 
in oxygen is an old experiment, but it has now a signifi- 
cance beyond what it used to have; we now regard the act 
of combination on the part of the atoms of oxygen and 
coal as we regard the clashing of a falling weight against 
the earth. The heat produced in both cases is referable to 
a common cause. A diamond, which burns in oxygen as a 
star of white light, glows and burns in consequence of the 
falling of the atoms of oxygen against it. And could we 
measure the velocity of the atoms when they clash, and 
could we find their number and weights, multiplying the 
weight of each atom by the square of its velocity, and add- 
ing 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 ex- 
pended 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 Ibs. to a height of 20 miles above the earth's surface. 
Conversely, 100 Ibs. 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. 
