xactly
counterbalancing the stress required to penetrate it in front.

To a person unaccustomed to think of seemingly solid matter as really
composed of particles relatively wide apart, it is hard to understand
the claim that ether penetrates the substance of solids--of glass,
for example--and, to use Young's expression, which we have previously
quoted, moves among them as freely as the wind moves through a grove
of trees. This thought, however, presents few difficulties to the mind
accustomed to philosophical speculation. But the question early arose
in the mind of Fresnel whether the ether is not considerably affected by
contact with the particles of solids. Some of his experiments led him to
believe that a portion of the ether which penetrates among the molecules
of tangible matter is held captive, so to speak, and made to move along
with these particles. He spoke of such portions of the ether as "bound"
ether, in contradistinction to the great mass of "free" ether. Half a
century after Fresnel's death, when the ether hypothesis had become
an accepted tenet of science, experiments were undertaken by Fizeau
in France, and by Clerk-Maxwell in England, to ascertain whether any
portion of ether is really thus bound to particles of matter; but the
results of the experiments were negative, and the question is still
undetermined.

While the undulatory theory of light was still fighting its way, another
kind of evidence favoring the existence of an ether was put forward by
Michael Faraday, who, in the course of his experiments in electrical and
magnetic induction, was led more and more to perceive definite lines or
channels of force in the medium subject to electro-magnetic influence.
Faraday's mind, like that of Newton and many other philosophers,
rejected the idea of action at a distance, and he felt convinced that
the phenomena of magnetism and of electric induction told strongly for
the existence of an invisible plenum everywhere in space, which might
very probably be the same plenum that carries the undulations of light
and radiant heat.

Then, about the middle of the century, came that final revolution of
thought regarding the nature of energy which we have already outlined in
the preceding chapter, and with that the case for ether was considered
to be fully established. The idea that energy is merely a "mode
of motion" (to adopt Tyndall's familiar phrase), combined with the
universal rejection of the notion of acti