, such as the earth
and the bodies upon the earth.

Another reason for supposing the ether to be a _plenum_, filling
absolutely all space, is that it must be perfectly frictionless; and for
this reason it cannot be composed of particles with spaces between them.
It must be frictionless, for otherwise the planets would be retarded in
their motions through space. The earth, for instance, is moving along
its orbit at the rate of eighteen miles a second; and yet the ether does
not pile up in front of it, nor is it made rarer in the wake of the
earth. Moreover, during the thousands of years during which astronomers
have been making observations absolutely no retardation has been
detected in the motions of the earth or of any of the heavenly bodies,
even to the smallest fraction of a second.

It is necessary to make the ether absolutely elastic and absolutely
rigid. We are acquainted with many materials that are elastic, and with
some that are comparatively rigid. But the elastic substances that we
are acquainted with are not rigid, and the rigid substances are not
elastic; and to assume such contradictory qualities in the ether
transports us far beyond the bounds of experimental science.

These are but a few of the difficulties raised by the assumption of the
ether as a real entity; but as there is no means of demonstrating its
existence, except by arguing the _necessity_ of having such a medium to
transmit radiant energy, it follows that no multiplication of objections
to the theory is likely to refute it in the minds of those who feel this
necessity. Those who refuse to admit the possibility of "action at a
distance," who insist on inventing a connecting material medium between
every observed effect and some material object with which it seems to be
in causal connection, will, I suppose, have to be allowed to exercise
their ingenuity in any way to satisfy their minds, even though they may
have to revise their theory with every fresh discovery in optics or
radioactivity.

There are many other ingenious mental devices, like this of the ether,
which seem to me only materialistic efforts to postpone or to dodge the
real vital lessons to be read from natural phenomena,--efforts to push
the real Cause back one step farther into the shadow,--a last desperate
effort, in the face of the constantly accumulating evidence of modern
knowledge that the great First Cause is far more intimately connected
with life and motion than many