148 PROCEEDINGS OF THE AMERICAN ACADEMY 



reversed, i. e. dark. As we approached the edge, however, oiving to 

 the opacity of the inner shell, the conditions would approximate to what 

 they would be if the layers A and B existed alone, the central body 

 being removed, and the lines would fade ; if faint, they would vanish. 

 This is our explanation of the phenomena of Class IV. 



Every theory involves certain conditions. We finally judge of the 

 Soundness or unsoundness of any theory largely from the consideration 

 of these implied conditions, and of the extent to which they are ful- 

 filled by it. For instance, our explanation of the fact that certain 

 very fine lines are stronger at the centre (IV.) demands that the sub- 

 stances giving such lines should be found in the chromosphere, indeed 

 mainly restricted to the chromosphere. Fortunately I can say that 

 one of them (6371), which I first discovered and measured carefully, 

 is identical with the fourteenth line of Young's second Catalogue of 

 Chromosphere Lines. The other one, the wave-length of which I 

 took from Angstrom's chart without correction, may correspond with 

 Young's ninth (6429.9) hne of the same Catalogue, which differs in 

 place by only one sixth the distance between the D lines. This I 

 shall test at the earliest opportunity. 



If the theory I have jjroposed is correct, it affords the first definite 

 evidence of the existence of chemical compounds in the sun, for in 

 accordance with it the lines of Class III. and Class IV. belong to sub- 

 stances which are not found in the lower photosphere. We know 

 however that all gases must increase in density in passing from theii 

 outer limit towards the centre of the sun ; and we have seen a proof 

 of this in the case of hydrogen and certain other vapors in the discus- 

 sion of our observations, which showed that the characteristic lines 

 indicated greater density when they originated at greater depths. 

 The only escape from the contradiction is in the assumption that the 

 lines of the last two cases (III., IV.) are due to compound vapors hav- 

 ing a dissociation temperature below that of the lower photosphere. 

 Of course, the substances of Class IV. have a lower dissociation tem- 

 perature than those of Class III. 



A naturally suggested and legitimate subject of speculation is as to 

 the nature of the substance which, by precipitation, forms the cloud- 

 masses of the photosphere. We may predicate three properties with 

 greater or less positiveness, viz. : — 



1st. The substance has a boiling-point above that of iron, for iron 

 vapor at a lower temperature exists in the immediate neighborhood. 



2d. The molecular weight is probably not great ; for, though pre- 

 cipitated below the upper natural limit of its vapor, there are few ele- 



