paratus to stir water vigorously, that apparatus being driven by
falling weights or a rotating flywheel or by any other mechanical means,
the water became heated. A certain amount of mechanical energy had been
used up and a certain amount of heat had appeared. The relation between
these two things was found to be invariable. Every physical change in
nature involves a transformation of energy, but the total quantity of
energy in the universe remains unaltered. This is the great doctrine of
the Conservation of Energy.

Sec. 13

Substitutes for Coal

Consider the source of nearly all the energy which is used in modern
civilisation--coal. The great forests of the Carboniferous epoch now
exists as beds of coal. By the burning of coal--a chemical
transformation--the heat energy is produced on which at present our
whole civilisation depends. Whence is the energy locked up in the coal
derived? From the sun. For millions of years the energy of the sun's
rays had gone to form the vast vegetation of the Carboniferous era and
had been transformed, by various subtle processes, into the potential
energy that slumbers in those immense fossilized forests.

The exhaustion of our coal deposits would mean, so far as our knowledge
extends at present, the end of the world's civilisation. There are other
known sources of energy, it is true. There is the energy of falling
water; the great falls of Niagara are used to supply the energy of huge
electric power stations. Perhaps, also, something could be done to
utilise the energy of the tides--another instance of the energy of
moving water. And attempts have been made to utilise directly the energy
of the sun's rays. But all these sources of energy are small compared
with the energy of coal. A suggestion was made at a recent British
Association meeting that deep borings might be sunk in order to utilise
the internal heat of the earth, but this is not, perhaps, a very
practical proposal. By far the most effective substitutes for coal would
be found in the interior energy of the atom, a source of energy which,
as we have seen, is practically illimitable. If the immense electrical
energy in the interior of the atom can ever be liberated and controlled,
then our steadily decreasing coal supply will no longer be the bugbear
it now is to all thoughtful men.

The stored-up energy of the great coal-fields can be used up, but we
cannot replace it or create fresh supplies. As we have seen, energy