Whatever may be the practical belief of the learned, their fundamental
principles forbid the avowal of a plenum, although the undulatory theory
of light renders a plenum necessary, and is so far virtually recognized
by them, and a correction for resistance is applied to the Comet of
Encke. Yet there has been no attempt made to reconcile these opposing
principles, other than by supposing that the celestial regions are
filled with an extremely rare and elastic fluid. That no definite view
has been agreed on, is not denied, and Sir John Herschel speculates on
the reality of a resisting medium, by suggesting questions that will
ultimately have to be considered, as: "What is the law of density of the
resisting medium which _surrounds_ the sun? Is it in rest or in motion?
If the latter, in what direction does it move?" In these queries he
still clings to the idea of Encke, that the resistance is confined to
the neighborhood of the sun and planets, like a ponderable fluid. But
the most profound analyst the world has ever boasted, speaks less
cautiously, (Poisson Rech.) "It is difficult to attribute, as is usually
done, the incandescence of aerolites to friction against the molecules
of the atmosphere, at an elevation above the earth where the density of
the air is almost null. May we not suppose that the electric fluid, in a
neutral condition, forms a kind of atmosphere, extending far beyond the
mass of our atmosphere, yet _subject to terrestrial attraction_, yet
_physically imponderable_, and, consequently, following our globe in its
motion?" The incandescence of aerolites must, therefore, be owing to
friction against the molecules of the electric fluid which forms an
atmosphere around the globe. According to this view, some force keeps it
there, yet it is not ponderable. As it is of limited extent, this is not
the medium whose undulations brings to light the existence of the stars;
neither is Encke's, nor Herschel's, nor any other resisting medium.
Where shall we find the present established principles of science? If we
grant the Newtonians a plenum, they still cling to attraction of _all
matter_ in some shape. If we confine them to a vacuum, they will
virtually deny it. Is not this solemn trifling? How much more noble
would it be to exhibit a little more tolerance, seeing that they
themselves know not what to believe? We do not offer these remarks as
argument, but merely as indications of that course of reasoning by which
we conclude that the upholders of the present systems of science are not
entitled to any other ground than the pure Newtonian basis of an
interplanetary vacuum.