284 THE REVOLUTIONS OF THE CRUST OF THE EARTH. 



ment, diflfering only in the time occupied in going throngh these 

 changes. 



What are these phases, and what is the order of their succession ? 



The hypothesis of M. Faye in regard to the development of celestial 

 bodies has the advantage of accounting for a large number of cosmical 

 phenomena, and at the same time of giving an exact idea of the activity 

 of matter, from its extreme dispersion, to the formation of planets. As 

 it is founded upon the physical constitution of the sun, it may be well 

 to give a glance at some of the theories which have been formed in re- 

 gard to this luminary. 



According to the Scotch scientist, Wilson, the sun is an opaque, solid 

 or liquid body, surrounded with a dense atmosphere, the exterior strata 

 of which are luminous in themselves and constitute the photosphere. 

 Arago proved the existence of an incandescent gas, by means of the 

 polariscopic telescope ; for from the entire surface of the solar disk he 

 obtained no polarized light, which would not have been the case if the 

 sun were a solid body or an incandescent liquid. In order to explain 

 the penumbra, W. Herschel supposed between the sun and the photo- 

 sphere the existence of a cloudy envelope, which reflects and absorbs 

 all the luminous and calorific rays. The production of spots was attrib- 

 uted to gaseous emanations of a volcanic nature, which rend the cloudy 

 envelope and the photosphere and allow us to perceive the opaque body. 



Moreover the appearance of the spots toward the edge of the sun, 

 according to all the laws of perspective, indicate that they are actual 

 depressions and not protuberances, as Leland considers them, nor a 

 superficial phenomenon, as La Hire has supposed, nor yet clouds, as M. 

 Kirchofif has declared them to be in an hypothesis generally accepted 

 in Germany. As the latter hypothesis is connected with a discovery 

 of great importance to spectral analysis, we will consider it more in 

 detail. 



It is evident from the investigations of Angstrom that gases and 

 vapors always absorb the rays of refrangibility, corresponding to those 

 they would themselves emit, if they were carried to incandescence. 

 Starting with this fact, MM. Bunsen and Kirchhoff have shown that 

 we may produce certain lines of the solar spectrum, by interposing 

 vapors of different metals in the path of the light of a continuous spec- 

 trum, proceeding from an incandescent body. In order to apply to the 

 sun the results of this experiment, M. Kirchhoff supposed the sun to 

 be a solid or an incandescent liquid, surrounded with an atmosphere 

 containing vapors of different metals of which we perceive the dark 

 rays in the solar spectrum. The spots, according to this hypothesis, are 

 only clouds of conical form, which are produced by a sudden cooling of 

 certain regions of the solar atmosphere. The protuberances, having 

 the appearance of floating clouds, which are observed during total 

 eclipses of the sun, appear to confirm this view. But when we consider, 

 what is well known, that the spots are seldom seen beyond 50© of lati- 



