

xxvin.] METALLIC STRATA. 413 



these domes are hotter than the rest of the surface, since the 

 bright lines of hydrogen are seen to surmount them. It is 

 quite possible that the general billowy outline of the chromo- 

 sphere when most quiescent is due to this cause. 



(2) Metallic strata. The next increase in the action is evi- 

 denced by a more complicated outline of the chromosphere, 

 accompanied by a more complicated structure chemically. 



Occasionally the level of this sea over a very large region is 

 gradually, peacefully, and quietly raised, and when that happens 



FIG. 121. Well ings-up of metallic strata. (Tacchini.) 



observations show us that this apparent welling-np is due to 

 the intrusion of other vapours. There seems to be a gradual 

 evaporation from out the photosphere, or a gradual heating 

 of slowly-falling material over large regions, pushing up the 

 upper level of the sea of hydrogen, so that the portion of 

 the atmosphere near the photosphere gets richer and richer; 

 we get, in fact, layers of different substances above it. 



Above is a drawing showing two wellings-up of metallic strata 

 in the chromosphere to which I have referred. The distance 

 from the horizontal line shows the depth of the strata indicated 



