a condition of rest; they look upon a state of equilibrium as that to
which all bodies normally tend.

The mathematician does not suppose that a quantity will alter, or that a
given point in space will change its direction with regard to another
point, spontaneously. And it is the same with the physicist. When Newton
saw the apple fall, he concluded at once that the act of falling was not
the result of any power inherent in the apple, but that it was the
result of the action of something else on the apple. In a similar
manner, all physical force is regarded as the disturbance of an
equilibrium to which things tended before its exertion,--to which they
will tend again after its cessation.

The chemist equally regards chemical change in a as the effect of the
action of something external to the body changed. A chemical compound
once formed would persist for ever, if no alteration took place in
surrounding conditions.

But to the student of Life the aspect of nature is reversed. Here,
incessant, and, so far as we know, spontaneous change is the rule, rest
the exception--the anomaly to be accounted for. Living things have no
inertia, and tend to no equilibrium.

Permit me, however, to give more force and clearness to these somewhat
abstract considerations, by an illustration or two.

Imagine a vessel full of water, at the ordinary temperature, in an
atmosphere saturated with vapour. The _quantity_ and the _figure_ of
that water will not change, so far as we know, for ever.

Suppose a lump of gold be thrown into the vessel--motion and disturbance
of figure exactly proportional to the momentum of the gold will take
place. But after a time the effects of this disturbance will
subside--equilibrium will be restored, and the water will return to its
passive state.

Expose the water to cold--it will solidify--and in so doing its
particles will arrange themselves in definite crystalline shapes. But
once formed, these crystals change no further.

Again, substitute for the lump of gold some substance capable of
entering into chemical relations with the water:--say, a mass of that
substance which is called "protein"--the substance of flesh:--a very
considerable disturbance of equilibrium will take place--all sorts of
chemical compositions and decompositions will occur; but in the end, as
before, the result will be the resumption of a condition of rest.

Instead of such a mass of _dead_ protein, however, take a particle of
_