le universe is tending to some uniform temperature,
in which case, although all molecular motion would not have ceased, it
would have become unavailable. In this sense it may be said that the
universe is running down.

[Illustration: NIAGARA FALLS

The energy of this falling water is prodigious. It is used to generate
thousands of horse-power in great electrical installations. The power is
used to drive electric trams in cities 150 to 250 miles away.]

[Illustration: _Photo: Stephen Cribb._

TRANSFORMATION OF ENERGY

An illustration of Energy. The chemical energy brought into existence by
firing the explosive manifesting itself as mechanical energy, sufficient
to impart violent motion to tons of water.]

[Illustration: _Photo: Underwood & Underwood._

"BOILING" A KETTLE ON ICE

When a kettle containing liquid air is placed on ice it "boils" because
the ice is intensely hot _when compared with the very low temperature of
the liquid air_.]

If all the molecules of a substance were brought to a standstill, that
substance would be at the absolute zero of temperature. There could be
nothing colder. The temperature at which all molecular motions would
cease is known: it is -273 deg. C. No body could possibly attain a lower
temperature than this: a lower temperature could not exist. Unless there
exists in nature some process, of which we know nothing at present,
whereby energy is renewed, our solar system must one day sink to this
absolute zero of temperature. The sun, the earth, and every other body
in the universe is steadily radiating heat, and this radiation cannot go
on for ever, because heat continually tends to diffuse and to equalise
temperatures.

But we can see, theoretically, that there is a way of evading this law.
If the chaotic molecular motions which constitute heat could be
_regulated_, then the heat energy of a body could be utilised directly.
Some authorities think that some of the processes which go on in the
living body do not involve any waste energy, that the chemical energy of
food is transformed directly into work without any of it being
dissipated as useless heat energy. It may be, therefore, that man will
finally discover some way of escape from the natural law that, while
energy cannot be destroyed, it has a tendency to become unavailable.

The primary reservoir of energy is the atom; it is the energy of the
atom, the atom of elements in the sun, the stars, the earth, from which
nature