the Constitution of the Sim. 101 



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 incandescence, they 

 would revolatilize, Thus the floor or substratum of every spot 

 must be a portion, depressed it is true, of the photosphere, 

 All the spectroscopic phenomena of spots, which have proved 

 so perplexing, are thus naturally and easily explained. 



In the immediate neighbourhood of a spot the centripetal 

 currents bend down the ordinary convection or granule-pro- 

 ducing 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 precipitation 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 reason thus: — If we could surround the sun by a stratum 

 of gas hotter than the photosphere and much rarer than that 

 producing the corresponding Fraunhofer lines, we should, as is 

 shown by a course of reasoning 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 vapours in the sun. The gases just over the granules, 

 in the vertical currents, are at a very high temperature, essen- 

 tially that of the condensing material itself, consequently much 

 hotter and rarer than the relatively low-lying vapours which, 

 as we have seen, produce the Fraunhofer lines. 



There are, however, certain evident limitations to these con- 

 ditions; in other words, we cannot expect to see all the dark 

 lines doubled by any increase of dispersive power. For in- 

 stance, a line must have a marked tendency to broaden with 

 increased pressure ; otherwise the duplication cannot be pro- 

 nounced. Again, the layer of rare vapour must be thin, or its 

 temperature cannot be relatively high throughout, as demanded 

 by the theory. This evident condition doubtless gives the 

 reason why the hydrogen-lines, though the broadest in the 

 solar spectrum, are not sensibly double. 



The theory of the constitution of the sun above proposed may 

 be briefly recapitulated thus : — 



Convection currents, directed generally from the centre of 

 the sun, start from a lower level, where the temperature is pro- 

 bably above the vaporizing temperature of every substance. 



* "On Lockyer's Hypothesis," Amer, Journ. Chem. vol. i. p. 15. 



