abstracted from the sun, in order to lift the matter
of the river to the elevation from which it falls.

As long as the river continues on the heights,
whether in the solid form as a glacier, or in the
liquid form as a lake, the heat expended by the
sun in lifting it has disappeared from the
universe. It has been consumed in the act of
lifting. But, at the moment when the river starts
upon its downward course, and encounters the
resistance of its bed, the heat expended in its
elevation begins to be restored.

The mental eye can follow the emission of heat
from its source, the sun, through the ether, as
vibratory motion, to the ocean, where it ceases to
be vibration, taking "the potential form" among
the molecules of aqueous vapour; and also to
the mountain-top, where the heat absorbed in
vaporisation is given out in condensation, while
that expended by the sun in lifting the water to
that elevation is still unrestored. This we find
paid back, to the last unit, by the friction along
the river's bed; at the bottom of the cascades
where the plunge of the torrent is suddenly
arrested; in the warmth of the machinery turned
by the river; in the spark from the millstone;
beneath the crusher of the miner; in the Alpine
saw-mill; in the milk-churn of the chalet; in the
supports of the cradle rocking the mountaineer's
baby to sleep by water-power. All these forms
of mechanical motion are simply the parcelling
out of an amount of calorific motion derived
originally from the sun. At each point at which
the mechanical motion is destroyed or
diminished, it is the sun's heat which is restored.

There are other motions and other energies
whose relations are not so obvious. Trees and
vegetables grow upon the earth; when burned
they give rise to heat, from which immense
quantities of mechanical energy are derived.
What is the source of this energy?

To answer the question, Professor Tyndall
shows his audience (or his readers) some iron
rust, which they can plainly see, produced by
the falling together of the atoms of iron and
oxygen, and also some transparent carbonic acid
gas, which they cannot see, formed by the union
of carbon and oxygen. The atoms, thus respectively
united, resemble a weight that has fallen
from a height and is lying on the ground. But
exactly as the weight can be wound up again
and prepared for another fall, even so those
atoms can be wound up, separated from each
other, and enabled to repeat the process of
combination. In the building up of plants, carbonic
acid is the material from which the carbon of
the plant is derived, while water is the
substance from which it obtains its hydrogen. The
solar beam winds up the weight; it is the agent
which severs the atoms, setting the oxygen free,
and allowing the carbon and the hydrogen to
aggregate in woody fibre. It is at the expense
of the solar light that the chemical decomposition
takes place. Without the sun, the reduction
of the carbonic acid and water cannot be
effected; and, in this act, an amount of solar
energy is consumed, exactly equivalent to the
molecular work done.

If the sun's rays fall upon a surface of sand,
the sand is heated, and finally radiates away as
much heat as it receives. But let the same
beams fall upon a forest; the quantity of heat
then given back is less than that received, for a
portion of the sunbeams is invested in the building
of the trees. It is not the shade alone
which renders the forest cool; heat is absorbed
and appropriated, as well as intercepted by the
leaves and branches as they grow.

Combustion is the reversal of this process;
and all the energy invested in a plant reappears
as heat when the plant is burned. Ignite a bit
of cotton; it bursts into flame. The oxygen
again unites with its carbon, and an amount of
heat is given out, equal to that originally sacrificed
by the sun to form the bit of cotton. So
also as regards the "deposits of dynamical
efficiency" laid up in our coal strata; they are
simply the sun's rays in a "potential form."
We dig from our pits, annually, eighty-four
millions of tons of coal, the mechanical equivalent
of which, is of almost fabulous vastness.
The combustion of a single pound of coal in one
minute, is equal to the work of three hundred
horses for the same time. It would require one
hundred and eight millions of horses, working
day and night with unimpaired strength for a
year, to perform an amount of work equivalent
to the energy which the sun of the Carboniferous
epoch invested in one year's produce of our
coal-pits. Dean Swift made an egregious
blunder when he ridiculed the philosopher of
the Flying Island who searched for the
sunbeams hidden in cucumbers.

The further we pursue this subject, the
Professor here remarks, the more its interest and
wonder grow upon us. He had already shown
how a sun may be produced by the mere exercise
of gravitating force; that, by the collision of
cold dark planetary masses, the light and heat of
our central orb, and also of the fixed stars, may
be obtained. But here we find the physical
powers, derived or derivable from the action of
gravity upon dead matter, introducing
themselves at the root of the question of vitality.
We find in solar light and heat, the very
mainspring of vegetable life. Nor can we halt at the
vegetable world; for the sun, mediately or
immediately, is the source of all animal life. Some
animals feed directly on plants, others feed on
their herbivorous fellow-creatures; but all in
the long run derive life and energy from the
vegetable world; all, therefore, as Helmholtz
has remarked, may trace their lineage to the
sun. In the animal body, the carbon and
hydrogen of the vegetable are again brought into
contact with the oxygen from which they had
been divorced, and which is now supplied by the
lungs. Reunion takes place, and animal heat is
the result. Save as regards intensity, there is
no difference between the combustion that thus
goes on within us, and that of an ordinary fire.
The products of combustion are in both cases
the same carbonic acid and water.

Looking then at the physics of the question,
we see that the formation of a vegetable is a