of rest,
two of which have a much greater mass than the third, would concentrate
into a single mass only in certain exceptional cases. In general, the
two most considerable bodies would unite together, while the third would
revolve around their common centre of gravity. Attraction would thus
become the cause of a sort of movement which would seem to be explicable
solely by an impulsive force.

It might be supposed, indeed, that in explaining this part of his system
Laplace had before his eyes the words which Rousseau has placed in the
mouth of the vicar of Savoy, and that he wished to refute them: "Newton
has discovered the law of attraction," says the author of _Emile_, "but
attraction alone would soon reduce the universe to an immovable mass:
with this law we must combine a projectile force in order to make the
celestial bodies describe curve lines. Let Descartes reveal to us the
physical law which causes his vortices to revolve; and let Newton show
us the hand which launched the planets along the tangents of their
orbits."

According to the cosmogonic ideas of Laplace, comets did not originally
form part of the solar system; they are not formed at the expense of the
matter of the immense solar nebula; we must consider them as small
wandering nebulae which the attractive force of the sun has caused to
deviate from their original route. Such of those comets as penetrated
into the great nebula at the epoch of condensation and of the formation
of planets fell into the sun, describing spiral curves, and must by
their action have caused the planetary orbits to deviate more or less
from the plane of the solar equator, with which they would otherwise
have exactly coincided.

With respect to the zodiacal light, that rock against which so many
reveries have been wrecked, it consists of the most volatile parts of
the primitive nebula. These molecules not having united with the
equatorial zones successively abandoned in the plane of the solar
equator, continued to revolve at their original distances, and with
their original velocities. The circumstance of this extremely rare
substance being included wholly within the earth's orbit, and even
within that of Venus, seemed irreconcilable with the principles of
mechanics; but this difficulty occurred only when the zodiacal substance
being conceived to be in a state of direct and intimate dependence on
the solar photosphere properly so called, an angular movement of
rotation was i