and that there was any measurable relation between the powers of the living organism and the forces of heat and chemical affinity was of course unthinkable before the formulation of the doctrine of the correlation of forces.  But as soon as that doctrine was understood it began to appear at once that, to a certain extent at least, the living body might be compared to a machine whose function is simply to convert one kind of energy into another.  A steam engine is fed with fuel.  In that fuel is a store of energy deposited there perhaps centuries ago.  The rays of the sun, shining on the world in earlier ages, were seized upon by the growing plants and stored away in a potential form in the wood which later became coal.  This coal is placed 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 protoplasm possesses the power of assimilating food and growing; and, since we do not doubt that the properties of water are the result of its chemical composition, so we may also assume that the vital properties of protoplasm are the result of its chemical composition.  It followed from this conclusion that if chemists ever succeeded in manufacturing the chemical compound, protoplasm, it would be alive.  Vital phenomena were thus reduced to chemical and mechanical problems.