the Constitution of the Sun. 99 



of the soundness or unsoundness of any theory largely from 

 the consideration of these implied conditions and of the extent 

 to which they are fulfilled by it. For instance, our explana- 

 tion of the fact that certain very fine lines are stronger at the 

 centre (IV.) demands that the substances 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 14th line of Young's second Catalogue 

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



o 



which I took from Angstrom's chart without correction, may 

 correspond with Young's 9th (6429*9) line of the same 

 catalogue, which differs in place by only one sixth the dis- 

 tance between the D lines. This I shall test at the earliest 

 opportunity. 



If the theory I have proposed 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 substances which are not found in the lower 

 photosphere. We know, however, that all gases must increase 

 in density in passing from their outer limit toward the centre 

 of the sun ; and we have seen a proof of this in the case of 

 hydrogen and certain other vapours in the discussion of our 

 observations, which showed that the characteristic lines indi- 

 cated 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 com- 

 pound vapours having a dissociation-temperature below that of 

 the lower photosphere. Of course the substances of Class IV. 

 have a lower 'dissociation-temperature 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 predi- 

 cate three properties with greater or less positiveness, viz.: — 



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

 for iron-vapour at a lower temperature exists in its immediate 

 neighbourhood. 



2nd. The molecular weight is probably not great; for though 

 precipitated below the upper natural limit of its vapour, there 

 are few elements found in abundance above it, and those in 

 general of low vapour-density. 



3rd. The element is not a rare one. Of these guides the last 

 is of the least value. 



The substances which apparently meet all these conditions 

 are carbon and silicon; nor is it easy to name any other which 

 will. Accepting for a moment as an hypothesis that the light 



H 2 



