142 COSMOS. 



yet we certainly know nothing definite regarding its actual 

 material dimensions ; its augmentation* by emanations from 

 the tails of myriads of comets that come within the Sun's 

 vicinity ; the singular changes affecting its expansion, since it 

 sometimes does not appear to extend beyond our Earth's orbit ; 

 or, lastly, regarding its conjectural intimate connection with 

 the more condensed cosmical vapor in the vicinity of the Sun. 

 The nebulous particles composing this ring, and revolving 

 round the Sun in accordance with planetary laws, may either 

 be self-luminous or receive light from that luminary. Even 

 in the case of a terrestrial mist (and this fact is very remark- 

 able), which occurred at the time of the new moon at mid- 

 night in 1743, the phosphorescence was so intense that ob- 

 jects could be distinctly recognized at a distance of more than 

 600 feet. 



I have occasionally been astonished, in the tropical climates 

 of South America, to observe the variable intensity of the 

 zodiacal light. As I passed the nights, during many months, 

 in the open air, on the shores of rivers and on llanos, I enjoy- 

 ed ample opportunities of carefully examining this phenome- 

 non. When the zodiacal light had been most intense, I have 

 observed that it would be perceptibly weakened for a few 

 minutes, until it again suddenly shone forth in full brilliancy 

 In some few instances I have thought that I could perceive — 

 not exactly a reddish coloration, nor the lower portion darkened 

 in an arc-like form, nor even a scintillation, as Mairan affirms 

 he has observed — but a kind of flickering and wavering of 

 the light.f Must we suppose that changes are actually in 

 progress in the nebulous ring 1 or is it not more probable that, 

 although I could not, by my meteorological instruments, de- 

 tect any change of heat or moisture near the ground, and 

 small stars of the fifth and sixth magnitudes appeared to shine 

 with equally undiminished intensity of light, processes of con- 

 densation may be going on in the uppermost strata of the air, 

 by means of which the transparency, or, rather, the reflection 

 of light, may be modified in some peculiar and unknown man* 



* Sir John Herschel, Astron., § 487. 



t Arago, in the Annuaire, 1832, p. 246. Several physical facts ap 

 pear to indicate that, in a mechanical separation of matter into its small- 

 est particles, if the mass be very small in relation to the surface, the 

 electrical tension may increase sufficiently for the production of light 

 .and heat. Experiments with a large concave miiTor have not hitherto 

 given any positive evidence of the presence of radiant heat in the zo- 

 diacal light. (Lettre de M. Matthiessen k M. Arago, in the Comptet 

 Rendus, t. xvi., 184.3, Avril, p. 687.) 



