the one hand, and takes it away on the other; and
the state of potential energy means the condition of the bob during
the instant at which the energy, conferred by the hammering during the
one half-arc, has just been exhausted by the hammering during the
other half-arc. It seems safe to look forward to the time when the
conception of attractive and repulsive forces, having served its
purpose as a useful piece of scientific scaffolding, will be replaced
by the deduction of the phenomena known as attraction and repulsion,
from the general laws of motion.

The doctrine of the conservation of energy which I have endeavored to
illustrate is thus defined by the late Clerk Maxwell:

'The total energy of any body or system of bodies is a quantity which
can neither be increased nor diminished by any mutual action of such
bodies, though it may be transformed into any one of the forms of
which energy is susceptible.' It follows that energy, like matter, is
indestructible and ingenerable in nature. The phenomenal world, so far
as it is material, expresses the evolution and involution of energy,
its passage from the kinetic to the potential condition and back
again. Wherever motion of matter takes place, that motion is effected
at the expense of part of the total store of energy.

Hence, as the phenomena exhibited by living beings, in so far as they
are material, are all molar or molecular motions, these are included
under the general law. A living body is a machine by which energy is
transformed in the same sense as a steam-engine is so, and all its
movements, molar and molecular, are to be accounted for by the energy
which is supplied to it. The phenomena of consciousness which arise,
along with certain transformations of energy, cannot be interpolated
in the series of these transformations, inasmuch as they are not
motions to which the doctrine of the conservation of energy applies.
And, for the same reason, they do not necessitate the using up of
energy; a sensation has no mass and cannot be conceived to be
susceptible of movement. That a particular molecular motion does give
rise to a state of consciousness is experimentally certain; but the
how and why of the process are just as inexplicable as in the case of
the communication of kinetic energy by impact.

When dealing with the doctrine of the ultimate constitution of matter,
we found a certain resemblance between the oldest speculations and the
newest doctrines of physic