150 PROCEEDINGS OF THE AMERICAN ACADEMY 



because of the relatively great density of the substance in the liquid 

 or solid state ; thus the apparent brilliancy of the faculae is readily 

 understood. 



If there is any disturbing cause which would tend to direct currents 

 of gas, over a considerable area of the solar surface, toward a point, 

 this smoke, instead of quietly settling down to lower levels between 

 the granules, would concentrate about this point, there exercising a 

 marked general absorption which would betray itself as a spot. At 

 this place the suspended particles would sink to lower levels with 

 constantly increasing temperature, until finally, heated to intense in- 

 candescence, they would revolatilize. Thus the floor or substratum of 

 every spot must be a portion, dejiressed it is true, of the photosphere. 

 All the spectroscopic phenomena of spots, which have proved so per- 

 plexing, are thus naturally and easily explained. 



In the immediate neighborhood of a spot, the centripetal currents 

 bend down the ordinary convection or granule-producnig currents, so 

 that they are approximately level. Before, the latter cooled suddenly 

 by rarefaction in their upward course ; now, they cool mainly by the 

 much slower process of radiation ; thus, while before the locus of pre- 

 cipitation was restricted, it is now greatly extended. This is the cause 

 of the great elongation of the granules in the penumbra, — a real 

 elongation, I imagine, and not merely an apparent one. 



Finally, concerning the close duplicity of certain lines, we may rea- 

 son thus: — If we could surround the sun by a stratum of gas hotter 

 than the photosphere and much rarer than that producing the corre- 

 sponding Fraunhofer lines, we should, as is shown by a course of rea- 

 soning which I have given in another place,* see each dark line 

 divided by a sharp bright line in its centre, that is, doubled. But as a 

 consequence of the theory this supposed condition must be practically 

 met in the case of certain vapors in the sun. The gases just over the 

 granules, in the vertical currents, are at a very high temperature, 

 essentially that of the condensing material itself, consequently much 

 hotter and I'arer than the relatively low-lying vapors which, as we 

 have seen, produce the F'rauuhofer lines. 



There are, however, certain evident limitations to these conditions ; 

 in other words, we cannot expect to see all the dark lines doubled by 

 any increase of dispersive power. For instance, a line must have a 

 marked tendency to broaden with increased pressure, otherwise the 

 duplication cannot be pronounced. Again, the layer of rare vapor must 



* On Lockyer'e Hypothesis. Am. Jour. Clicni., i. 16. 



