ontinuous, and free from any dark lines, there we have
simply the Aether in its very first stage of condensation; and where we
have the dark lines appearing, such lines indicate a more advanced stage
to which the process has arrived.

[Footnote 43: _Philosophical Magazine_, July 1902.]

ART. 121. _Nebular Hypothesis._--The Nebular Hypothesis was first
introduced by Kant in his work on the _History of the Earth and Theory
of the Heavens_.

In that work he attempted to explain the origin of the universe on
purely mechanical lines. Laplace, a French mathematician, about the same
time came to similar conclusions as Kant had done, and published his
views in his work on _Exposition du Systeme du Monde_, and later on in
his more famous work the _Mecanique Celeste_.

A feature common to both these theories rested in the fact, that they
supposed that all material bodies which exist in the universe once
existed in a nebulous condition, and that they were formed out of this
nebulous matter. Further, that this nebulous matter gradually condensed,
and as it condensed, a rotational motion was imparted to them, which
rotation quickened as the condensation was continued.

Then, as the rotation was accelerated, portions were flung off by the
centrifugal force, and these portions of nebulous matter gradually
condensed, forming the various planets of the system. As these
condensed, they, in their turn, parted with some of their nebulous
matter through the repulsive energy of the centrifugal force, and these
secondary parts gave origin to the various satellites that exist round
the planets.

Now, while the general principle involved in the nebular hypothesis is
true, yet the conception according to Laplace is not verified by fact,
as we learn that Uranus and Neptune are still in a state of
self-luminosity, while their density is the smallest of all the planets.
From this we should infer that the two outermost planets are the
youngest planets of our solar system, but according to Laplace's theory,
they ought to be the oldest, as they would have been flung off first by
the parent body as it rotated; and therefore, being flung off first,
should be in a more advanced stage of development than any of the inner
planets. M. Faye has suggested a remedy for this defect in the theory.
He supposes that the nebulous matter out of which the planets were
formed, was not flung off by the central body the sun, but that each
planet was formed at d