314
XX
LIGHT AND ELECTRICITY
have made upon this
And it is of the chain,
I
foundations lie. Such researches as I
subject form but a link in a long chain.
and not only of the single link, that I would speak to you.
must confess that it is not easy to speak of these matters in a
way at once intelligible and accurate. It is in empty space,
in the free ether, that the processes which we have to
describe take place. They cannot be felt with the hand,
heard by the ear, or seen by the eye. They appeal to our
intuition and conception, scarcely to our senses. Hence we
shall try to make use, as far as possible, of the intuitions and
conceptions which we already possess. Let us, therefore, stop
to inquire what we do with certainty know about light and
electricity before we proceed to connect the one with the
other.
What, then, is light? Since the time of Young and
Fresnel we know that it is a wave-motion. We know the
velocity of the waves, we know their length, we know that
they are transversal waves; in short, we know completely
the geometrical relations of the motion. To the physicist
it is inconceivable that this view should be refuted; we can
no longer entertain any doubt about the matter. It is
morally certain that the wave theory of light is true, and
the conclusions that necessarily follow from it are equally
certain. It is therefore certain that all space known to us
is not empty, but is filled with a substance, the ether, which
can be thrown into vibration. But whereas our knowledge
of the geometrical relations of the processes in this substance
is clear and definite, our conceptions of the physical nature
of these processes is vague, and the assumptions made as to
the properties of the substance itself are not altogether con-
sistent. At first, following the analogy of sound, waves of
light were freely regarded as elastic waves, and treated as
such. But elastic waves in fluids are only known in the
form of longitudinal waves. Transversal elastic waves in
fluids are unknown. They are not even possible; they con-
tradict the nature of the fluid state. Hence men were
forced to assert that the ether which fills space behaves like
a solid body. But when they considered and tried to explain
the unhindered course of the stars in the heavens, they found
themselves forced to admit that the ether behaves like a