thus produced. Once the carbon and the oxygen have
rushed together, so as to form carbonic acid, their mutual attractions
are satisfied; and, while they continue in this condition, as dynamic
agents they are dead. Our woods and forests are also sources of
mechanical energy, because they have the power of uniting with the
atmospheric oxygen. Passing from plants to animals, we find that the
source of motive power just referred to is also the source of muscular
power. A horse can perform work, and so can a man; but this work is
at bottom the molecular work of the transmuted food and the oxygen of
the air. We inhale this vital gas, and bring it into sufficiently
close proximity with the carbon and the hydrogen of the body. These
unite in obedience to their mutual, attractions; and their motion
towards each other, properly turned to account by the wonderful
mechanism of the body, becomes muscular motion.

One fundamental thought pervades all these statements: there is one
tap root from which they all spring. This is the ancient maxim that
out of nothing nothing comes; that neither in the organic world nor in
the inorganic is power produced without the expenditure of power; that
neither in the plant nor in the animal is there a creation of force or
motion. Trees grow, and so do men and horses; and here we have new
power incessantly introduced upon the earth. But its source, as I
have already stated, is the sun. It is the sun that separates the
carbon from the oxygen of the carbonic acid, and thus enables them to
recombine. Whether they recombine in the furnace of the steam-engine
or in the animal body, the origin of the power they produce is the
same. In this sense we are all 'souls of fire and children of the
sun.' But, as remarked by Helmholtz, we must be content to share our
celestial pedigree with the meanest of living things.

Some estimable persons, here present, very possibly shrink from
accepting these statements; they may be frightened by their apparent
tendency towards what is called materialism--a word which, to many
minds, expresses something very dreadful. But it ought to be known
and avowed that the physical philosopher, as such, must be a pure
materialist. His enquiries deal with matter and force, and with them
alone. And whatever be the forms which matter and force assume,
whether in the organic world or the inorganic, whether in the
coal-beds and forests of the earth, or in the brains and musc