308 COSMOS. 



tion of a non-luminous ether pervading the universe, and ge- 

 nerating by its undulatory motion, the phenomena of light, 

 radiant heat, and electro-magnetism. 10 The emanations from 

 cometary nuclei, which in the form of tails frequently extend 

 over enormous tracts of space, disperse the substance of which 

 they are composed and with which we are unacquainted, 

 among the planetary orbits of our solar system, which they 

 intersect. But when separated from the controlling nucleus, 

 this substance ceases to be perceptibly luminous. Newton 

 even considered it possible that vapores ex sole et stellis fixis et 

 caudis cometarum, " vapours from the sun, the stars, and the 

 tails of comets," might blend with our terrestrial atmosphere.* 1 

 No telescope has as yet indicated any sidereal character in 

 the vaporous, rotating, and flattened ring of the zodiacal light. 

 Whether the particles of which this ring consists, and which 

 according to some are conceived to rotate upon themselves 

 in obedience to dynamic conditions, and according to others 

 merely to revolve round the Sun, are illumined or self-lumi- 

 nous, like many kinds of terrestrial vapours, 88 is a question 

 as yet undecided. Dominique Cassini believed them to be 

 small planetary bodies.* 8 It seems as if it were a requirement 

 of the human intellect to seek in all fluid bodies for discrete, 

 molecular particles, 34 similar to the full or hollow vesicles of 

 which clouds are formed ; while the gradations in the decrease 

 of density in our planetary system, from Mercury to Saturn and 

 Neptune (from 1-12 to 0-14; the Earth being =1), leads the 

 mind to the consideration of comets, through the external 



10 Cosmos, vol. iii. p. 40. 



81 Newton, Philos. Nat. Principia Mathematica, 1760, 

 torn. iii. p. 671. 



B Cosmos, vol. i. p. 131. * Ib. p. 130. 



* Observations at the- Cape, 109-111. 



