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 

 nebulas 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- 



