6 THE POPULAR SCIENCE MONTHLY. 



Herschel, that the visible surface of the Sun is a luminous envelope, 

 within which there are cloudy envelopes covering a dark central body ; 

 and that, when by some disturbance the luminous envelope is broken 

 through, portions of the cloudy envelope and of the dark central body 

 become visible as the penumbra and umbra respectively. This hy- 

 pothesis, at one time received with favor mainly because it seemed to 

 permit that teleological interpretation which required that the Sun 

 should be habitable, accounted tolerably well for certain of the appear- 

 ances more especially the appearance of concavity which the spots 

 have when near the limb of the Sun. But, though Sir John Herschel 

 supported his father's hypothesis, pointing out that cyclonic action 

 would account for local dispersions of the photosphere, there has of 

 late years become more and more manifest the fatal objection that the 

 genesis of light and heat remained unexplained, and that no supposition 

 of auroral dischai-ges did more than remove the difficulty a step back ; 

 since, \inless light and heat could be perpetually generated out of noth- 

 ing, there must be a store of force perpetually being expended in pro- 

 ducing them. 



A counter-hypothesis, following naturally from the hypothesis of 

 nebular origin, is that the mass of the Sun must be incandescent ; that 

 its incandescence has been produced, and is maintained, by progressing 

 aggregation of its once widely-diffused matter ; and that surrounding its 

 molten surface there is an atmosphere of metallic gases continually 

 rising, condensing to form the visible photosphere, and thence precipi- 

 tating. What, in this case, are the solar spots ? Kirchhoff, proceed- 

 ing upon the hypothesis just indicated, which had been set forth before 

 he made his discoveries by the aid of the spectroscope, contended that 

 the solar spots are simply clouds, formed of these condensed metallic 

 gases, so large as to be relatively opaque ; and he endeavored to ac- 

 count for their changing forms as the Sun's rotation carries them away, 

 in correspondence with this view. But the appearances as known to 

 observers are quite irreconcilable with the belief that the spots are 

 simply drifting clouds. Do these appearances, then, conform to the 

 supposition of M. Faye, that the photosphere encloses matter which is 

 wholly gaseous and non-luminous ; and that the spots are produced 

 when occasional up-rushes from the interior burst through the photo- 

 sphere ? This supposition, while it may be held to account for certain 

 traits of the spots, and to be justified by the observed fact that there 

 are up-rushes of gas, presents difficulties not readily disposed of. It 

 does not explain the manifest rotation of many spots ; and, indeed, it 

 does not seem really to account for that darkness which constitutes 

 them spots ; since a non-luminous gaseous nucleus would be permeable 

 by light from the remoter side of the photosphere, and hence holes 

 through the near side of the photosphere would not look dark. 



There is, however, another hypothesis which more nearly reconciles 

 the facts. Assuming: the incandescent molten surface, the ascending 



