was demonstrated that any one could be obtained at the
expense of any other; and apparatus was devised which exhibited the
evolution of all these kinds of action from one source of energy.
Hence the idea of the 'correlation of forces' which was the immediate
forerunner of the doctrine of the conservation of energy.

It is a remarkable evidence of the greatness of the progress in this
direction which has been effected in our time, that even the second
edition of the 'History of the Inductive Sciences,' which was
published in 1846, contains no allusion either to the general view of
the 'Correlation of Forces' published in England in 1842, or to the
publication in 1843 of the first of the series of experiments by which
the mechanical equivalent of heat was correctly ascertained.[I] Such a
failure on the part of a contemporary, of great acquirements and
remarkable intellectual powers, to read the signs of the times, is a
lesson and a warning worthy of being deeply pondered by anyone who
attempts to prognosticate the course of scientific progress.

[Sidenote: What this doctrine is.]

I have pointed out that the growth of clear and definite views
respecting the constitution of matter has led to the conclusion that,
so far as natural agencies are concerned, it is ingenerable and
indestructible. In so far as matter may be conceived to exist in a
purely passive state, it is, imaginably, older than motion. But, as it
must be assumed to be susceptible of motion, a particle of bare matter
at rest must be endowed with the potentiality of motion. Such a
particle, however, by the supposition, can have no energy, for there
is no cause why it should move. Suppose now that it receives an
impulse, it will begin to move with a velocity inversely proportional
to its mass, on the one hand, and directly proportional to the
strength of the impulse, on the other, and will possess _kinetic
energy_, in virtue of which it will not only continue to move for ever
if unimpeded, but if it impinges on another such particle, it will
impart more or less of its motion, to the latter. Let it be conceived
that the particle acquires a tendency to move, and that nevertheless
it does not move. It is then in a condition totally different from
that in which it was at first. A cause competent to produce motion is
operating upon it, but, for some reason or other, is unable to give
rise to motion. If the obstacle is removed, the energy which was
there, but coul