on at a distance, made the
acceptance of a plenum throughout space a necessity of thought--so, at
any rate, it has seemed to most physicists of recent decades. The proof
that all known forms of radiant energy move through space at the same
rate of speed is regarded as practically a demonstration that but one
plenum--one ether--is concerned in their transmission. It has, indeed,
been tentatively suggested, by Professor J. Oliver Lodge, that there may
be two ethers, representing the two opposite kinds of electricity, but
even the author of this hypothesis would hardly claim for it a high
degree of probability.

The most recent speculations regarding the properties of the ether have
departed but little from the early ideas of Young and Fresnel. It is
assumed on all sides that the ether is a continuous, incompressible
body, possessing rigidity and elasticity. Lord Kelvin has even
calculated the probable density of this ether, and its coefficient of
rigidity. As might be supposed, it is all but infinitely tenuous as
compared with any tangible solid, and its rigidity is but infinitesimal
as compared with that of steel. In a word, it combines properties of
tangible matter in a way not known in any tangible substance. Therefore
we cannot possibly conceive its true condition correctly. The nearest
approximation, according to Lord Kelvin, is furnished by a mould of
transparent jelly. It is a crude, inaccurate analogy, of course, the
density and resistance of jelly in particular being utterly different
from those of the ether; but the quivers that run through the jelly when
it is shaken, and the elastic tension under which it is placed when its
mass is twisted about, furnish some analogy to the quivers and strains
in the ether, which are held to constitute radiant energy, magnetism,
and electricity.

The great physicists of the day being at one regarding the existence of
this all-pervading ether, it would be a manifest presumption for any one
standing without the pale to challenge so firmly rooted a belief. And,
indeed, in any event, there seems little ground on which to base such
a challenge. Yet it may not be altogether amiss to reflect that the
physicist of to-day is no more certain of his ether than was his
predecessor of the eighteenth century of the existence of certain
alleged substances which he called phlogiston, caloric, corpuscles of
light, and magnetic and electric fluids. It would be but the repetition
of history sh