metals in a state of intense heat, and which are brighter the 

 higher they ascend into the atmosphere, the brighter ones being 

 known to us as " Faculae." 



When we come to the atmosphere of the sun, however, our 

 knowledge is so exact, that we can not only tell its composition, 

 but even what its temperature is at different heights, and we find 

 with what wonderful precision its constituents are ranged according 

 to their respective densities, down from hydrogen the lightest, to 

 barium the heaviest. 



We have, overlapping all the rest, an atmosphere of hydrogen, 

 then at a profound depth sodium and magnesium, next at a great 

 distance further down, calcium, then in a group reaching nearly to 

 the same height, chromium, manganese, iron, nickel and cobalt, 

 then within a moderate distance of these, copper and zinc, and 

 lastly, after a vast intei-val, barium. These are all the elements 

 as yet known to exist in the sun's atmosphere, though there is 

 reason to believe that chlorine must also be classed with them. 



The presence of dark lines in the solar spectrum reveals to ua 

 that the surface of the atmosphere is cooler than the luminous 

 region beneath, and the lines having different intensities, is 

 evidence that the different zones of the atmosphere from which 

 they have their source, are at different temperatures. 



The lines due to hydrogen are intensely black, and those of 

 sodium and magnesium nearly as much so, consequently the heat 

 at the boundaries of these two latter metals must have fallen very 

 low, -whilst the upper region, tenanted by hydrogen alone, is a 

 feebly conducting body, of immense depth, warmed but moderately 

 beneath and exposed on the outside to a chilUng radiation towards 

 the open sky. 



The sodium lines are narrow and sharply defined, and are 

 precisely the same that sodium vapour gives at the temperature of 

 a spirit lamp ; but at the temperature of a Bunsen burner sodium 

 lines begin to expand and be ill defined ; hence we learn that in 

 those upper regions of the sodium zone of the atmosphere the 

 temperature is lower than that of the flame of a mixture of coal 

 gas and air. 



Again the fact that some of the iron lines are less dark than 



