NEBULA. 25 



irnong 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 ct stcUisfixis 

 H caudis cometarum, '• vapors from the sun, the stars, and 

 the tails of comets," might blend with our terrestrial atmos- 

 phere.^ No telescope has as yet indicated any sidereal char- 

 acter in the vaporous, rotating, and flattened ring of the zodi- 

 acal 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, accord- 

 ing to others, merely to revolve round the Sun, are illumined 

 or self-luminous, like many kinds of terrestrial vapors,! is a 

 question as yet undecided. Dominique Cassini believed them 

 to be small planetary bodies. $ It seems as if it were a re- 

 quirement of the human intellect to seek in all fluid bodies 

 for discrete molecular particles, § 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 layers of whose nuclei even a faint star contin- 

 ues visible, and finally to that of discrete particles, so deficient 

 in density that their solidity, either within large or small di- 

 mensions, can scarcely be characterized, except by the limits 

 which bound them. It was by such considerations as to the 

 constitution of the apparently vaporous zodiacal light that 

 Cassini, long before the discovery of the so-called smaller plan 

 ets between Mars and Jupiter, and prior to all conjectures re 

 garding meteor-asteroids, was led to the idea that there exist 

 cosmical bodies of all dimensions and all degrees of density. 

 \Ye here almost involuntarily touch upon the old metaphys- 

 ical controversy regarding -matter of primitive fluidity and 

 that composed of discrete molecular particles, and therefore 

 more amenable to mathematical treatment. From hence we 

 turn the more readily to our former consideration of the pure- 

 ly objective part of the phenomenon. 



In. the 3926 (2451 + 1475) positions which belong, a. to 

 the portion of the firmament visible at Slough, and which we 

 shall here, for the sake of brevity, term the northern heav- 

 ens, according to the three catalogues of Sir William Herschel 



* Newton, Philos. Nat. Principia Mathematica, 1760, torn, iii., p. 671 

 \ Cosmos, vol. i., p. 141. X Ibid., p. 140 



$ Observations at the Cape, <S 109-111. 



VoT.. IV— B' 



