of
tensions being 'consumed' and 'generated,' they do not mean thereby
that old attractions have been annihilated, and new ones brought into
existence, but that, in the one case, the power of the attraction to
produce motion has been diminished by the shortening of the distance
between the attracting bodies, while, in the other case, the power of
producing motion has been augmented by the increase of the distance.
These remarks apply to all bodies, whether they be sensible masses or
molecules.

Of the inner quality that enables matter to attract matter we know
nothing; and the law of conservation makes no statement regarding that
quality. It takes the facts of attraction as they stand, and affirms
only the constancy of working-power. That power may exist in the form
of MOTION; or it may exist in the form of FORCE, _with distance to act
through_. The former is dynamic energy, the latter is potential
energy, the constancy of the sum of both being affirmed by the law of
conservation. The convertibility of natural forces consists solely in
transformations of dynamic into potential, and of potential into
dynamic energy. In no other sense has the convertibility of force any
scientific meaning.

Grave errors have been entertained as to what is really intended to be
conserved by the doctrine of conservation. This exposition I hope
will tend to remove them.

********************

II. RADIATION.

[Footnote: The Rede Lecture delivered in the Senate House before the
University of Cambridge, May 16, 1865.]

*****

1. Visible and Invisible Radiation.

BETWEEN the mind of man and the outer world are interposed the nerves
of the human body, which translate, or enable the mind to translate,
the impressions of that world into facts of consciousness and thought.

Different nerves are suited to the perception of different
impressions. We do not see with the ear, nor hear with the eye, nor
are we rendered sensible of sound by the nerves of the tongue. Out of
the general assemblage of physical actions, each nerve, or group of
nerves, selects and responds to those for the perception of which it
is specially organised.

The optic nerve passes from the brain to the back of the eyeball and
there spreads out, to form the retina, a web of nerve filaments, on
which the images of external objects are projected by the optical
portion of the eye. This nerve is limited to the apprehension of the
phenomena of radiation, and, not