them. The dead matters, which by their action produce these
functions, are principally heat, moisture, light, and air. It
clearly follows therefore, from what I have said, that living bodies
must have some property different from dead matter, which renders
them capable of being acted upon by these external powers, so as to
produce the living functions; for if they had not, the only effects
which these powers could produce, would be mechanical, or chemical.
Though we know not exactly in what this property consists, or in what
manner it is acted on, yet we see, that when bodies are possessed of
it, they become capable of being acted upon by external powers, and
thus the living functions are produced; we shall therefore call this
property _excitability_, and in using this term it is necessary to
mention, that I mean only to express a fact, without the least
intention of pointing out the nature of that property which
distinguishes living from dead matter, and in this we have the
example of the great Newton, who called the property which causes
bodies in certain situations to approach each other, _gravitation_,
without in the least hinting at its nature; yet, though he knew not
what gravitation was, he investigated the laws by which bodies were
acted on by it, in the same manner, though we are ignorant of
excitability, or the nature of that property which distinguishes
living from dead matter, we can investigate the laws by which dead
matter acts on living bodies through this medium. We know not what
magnetic attraction is, and yet we can investigate its laws; the
same holds good with regard to electricity; if we ever should attain
a knowledge of the nature of this property, it would make no
alteration in the laws which we had before discovered.

I shall now proceed to the investigation of the laws by which the
excitability is acted on; but I must first define some terms which