THE SUN ITS CHEMICAL ANALYSIS. 189 



of the sun should be increased by one second — forty thousand years 

 that it should be enlarged by one minute. At this rate, at the end 

 of two thousand years a terrestrial observer who should have lived 

 all that time would not see the sun differ more from itself than we see 

 it diff"er every year from season to season, in proportion as the earth 

 approaches or recedes from it. If the mass of the sun were aug- 

 mented by the addition of meteoric substances, we should not be in 

 a position to perceive it, while, in return, the constancy of the sun's 

 diameter affords no argument for rejecting this hypothesis, which has 

 the advantage of assigning a rational cause for the development of 

 the solar heat. 



It will be asked, perhaps, whence come those meteors whose fiery 

 torrent falls incessantly on the surface of the orb which lights us. 

 Have you never seen, on a clear evening of March or September, at 

 the time of the equinoxes, a whitish gleam in the western part of the 

 heavens? It occupies from twenty to thirty degrees in extent and 

 projects itself above the horizon, following nearly the direction of the 

 ecliptic. This has been called the zodiacal light, because those who 

 first observed it conceived it to be limited to the zodiac. Under the 

 hazy sky of our climates this pale glimmering is perceived but rarely 

 after the evening twilight or before the rising of the sun at the beginning 

 of spring or autumn, and it is easily confounded with the gleams of 

 the receding or approaching day ; but under the tropics the phenome- 

 non displays itself in all its magnificence. On the summits of the 

 Cordilleras, in the prairies of Mexico, under the transparent skies of 

 Cumana, upon the coasts of the Sea of the South, the zodiacal light 

 appeared to Alexander von Humboldt more lustrous than the milky:- 

 way. At the equinox, at the moment when the solar disc sinks be- 

 neath the horizon, total obscurity succeeds almost immediately to 

 day; and at once the zodiacal light is seen stretching up to half the 

 height of the heavens and only vanishing at the approach of mid- 

 night. 



Domenico Cassini, in 1683, was the first who observed the zodiacal 

 light, and he considered it as a sort of luminous ring connected with 

 the solar equator ; he recognized in effect that this light follows the 

 solar equator in proportion as the latter withdraws itself from the 

 ecliptic. Thomson thinks that this vast luminous ring is the reser- 

 voir of the meteors by which the central sun is maintained. Such a 

 theory adapts itself readily to the great cosmogcnic conception of 

 Laplace : The zodiacal ring extended between the sun and the orbit 

 of the earth would, under that view, be a residue of the cosmic mat- 

 ter which, in the beginning, composed the entire nebula from whence 

 emerged by degrees our complex system of planets. 

 4. However this may be, in proportion as astronomy studies with more 

 care the phenomena of the heavens, does it meet there with more sur- 

 prising marvels. What admirable discoveries have been recently 

 made in our own solar system which had been supposed to be entirely 

 explored ! Among these discoveries, that of MM. Kirchoff" and 

 Bunsen must be regarded as one of the most important. The spectral 

 analysis of the solar atmosphere has furnished the proof of the chemi- 



