in the furnace of the steam engine and is broken to pieces so that it
can no longer hold its store of energy, which is at once liberated in
its active form as heat. The engine then takes the energy thus
liberated, and as a result of its peculiar mechanism converts it into
the motion of its great fly-wheel. With this notion clearly in mind the
question forces itself to the front whether the same facts are not true
of the living animal organism. It, too, is fed with food containing a
store of energy; and should we not regard it, like the steam engine,
simply a machine for converting this potential energy into motion, heat,
or some other active form? This problem of the correlation of vital and
physical forces is inevitably forced upon us with the doctrine of the
correlation of forces. Plainly, however, such questions were
inconceivable before about the middle of the nineteenth century.

This mechanical conception of living activity was carried even farther.
Under the lead of Huxley there arose in the seventh decade of the
century a view of life which reduced it to a pure mechanism. The
microscope had, at that time, just disclosed the universal presence in
living things of that wonderful substance, _protoplasm._ This material
appeared to be a homogeneous substance, and a chemical study showed it
to be made of chemical elements united in such a way as to show close
relation to albumens. It appeared to be somewhat more complex than
ordinary albumen, but it was looked upon as a definite chemical
compound, or, perhaps, as a simple mixture of compounds. Chemists had
shown that the properties of compounds vary with their composition, and
that the more complex the compound the more varied its properties. It
was a natural conception, therefore, that protoplasm was a complex
chemical compound, and that its vital properties were simply the
chemical properties resulting from its composition. Just as water
possesses the power of becoming solid at certain temperatures, so