eir exact affinities, consist essentially of undulatory
motions of one uniform medium.

A full century of experiment, calculation, and controversy has thus
sufficed to correlate the "imponderable fluids" of our forebears, and
reduce them all to manifestations of motion among particles of matter.
At first glimpse that seems an enormous change of view. And yet, when
closely considered, that change in thought is not so radical as the
change in phrase might seem to imply. For the nineteenth-century
physicist, in displacing the "imponderable fluids" of many kinds--one
each for light, heat, electricity, magnetism--has been obliged to
substitute for them one all-pervading fluid, whose various quivers,
waves, ripples, whirls or strains produce the manifestations which in
popular parlance are termed forms of force. This all-pervading fluid the
physicist terms the ether, and he thinks of it as having no weight. In
effect, then, the physicist has dispossessed the many imponderables in
favor of a single imponderable--though the word imponderable has been
banished from his vocabulary. In this view the ether--which, considered
as a recognized scientific verity, is essentially a nineteenth-century
discovery--is about the most interesting thing in the universe.
Something more as to its properties, real or assumed, we shall have
occasion to examine as we turn to the obverse side of physics, which
demands our attention in the next chapter.

IX. THE ETHER AND PONDERABLE MATTER

"Whatever difficulties we may have in forming a consistent idea of the
constitution of the ether, there can be no doubt that the interplanetary
and interstellar spaces are not empty, but are occupied by a material
substance or body which is certainly the largest and probably the most
uniform body of which we have any knowledge."

Such was the verdict pronounced some thirty years ago by James
Clerk-Maxwell, one of the very greatest of nineteenth-century
physicists, regarding the existence of an all-pervading plenum in the
universe, in which every particle of tangible matter is immersed.
And this verdict may be said to express the attitude of the entire
philosophical world of our day. Without exception, the authoritative
physicists of our time accept this plenum as a verity, and reason about
it with something of the same confidence they manifest in speaking of
"ponderable" matter or of, energy. It is true there are those among them
who are disposed to den