40 C. S. Hastings— Constitution of the Sun. 
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 center 
of the sun; and we have seen a proof of this in the case 0 
hydrogen and certain other vapors in the discussion of out 
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 yapors having a dissociation temperature below that of 
aii 
