228 SPECIAL RESULTS IN THE UEANOLOGICAL 



space, disperse the to us unknown matter of which they consist 

 among the planetary orbits of the solar system which they 

 traverse ; but when separated from the head or nucleus of 

 the comet, the matter of which the tails are formed ceases 

 to be sensibly luminous to our eyes. Newton considered it 

 possible that "vapores ex Sole et Stellis fixis et caudis 

 Cometarum" might become mingled with the atmosphere 

 which surrounds the Earth ( 388 ). No telescope has yet 

 discovered anything resembling stars in the vaporous flattened 

 revolving ring of the zodiacal light. Whether the particles 

 of which this ring consists, and which in accordance with 

 dynamic conditions are imagined by some to have rotations 

 independent of the sun, and by others to revolve simply round 

 that body, shine by reflected light, or whether, like many ter- 

 restrial fogs and vapours, ( 389 ) they are self-luminous, remains 

 undecided. Dominique Cassini believed them to be small 

 planetary bodies ( 39 ) . We feel as it were involuntarily im- 

 pelled to look in all fluids for detached ( 391 ) molecular parts, 

 like the full or hollow vesicles in clouds j and the gradations 

 of increasing density in our solar system from Mercury to 

 Saturn and Neptune, (from 1*12 to 0*14 : the Earth being 

 taken as =. 1,) conduct us to comets, through the outer- 

 most strata of whose nuclei faint stars are visible : they 

 even conduct us gradually to detached particles so rare that 

 the forms of their aggregation can scarcely be said to possess 

 definite outlines. 



It was these very considerations on the constitution of 

 the apparently nebulous zodiacal light, which, long previous 

 to the discovery of the small planets between Mars and 

 Jupiter, and before the formation of conjectures respecting 

 meteoric asteroids, led Cassini to entertain the idea of cos- 



