362 LAPLACE. 
_ 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 
nebulz 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 ac- 
tion 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 ex- 
tremely 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 depend- 
ence on the solar photosphere properly so called, an an- 
gular movement of rotation was impressed on it equal to 
that of the photosphere, a movement in virtue of which 
it effected an_entire revolution in twenty-five days and a 
half. Laplace presented his conjectures on the formation 
of the solar system with the diffidence inspired by a re- 
