1868.] 



Constitution of the Sun and Stan 



33 



But it is very much to be wished that a comparison should be made of the 

 spectra of boron, fluorine, sulphur, chlorine, titanium, and phosphorus, 

 with the sun's spectrum, and especially of chlorine, if any weight is to be 

 attached to the suspicion, founded on very insufficient grounds, that the 

 solar lines 43-40— , 43*55—, 66-38, 66*50, 66*68, and 70*00, the group of 

 three lines at 45*1, and several others, are to referred to this element. 



51. The absence from the sun's atmosphere of such gases as nitrogen 

 and oxygen, and of hydrogen from the atmospheres of some other stars, 

 and the fact that while some active chemical agents lose, like sulphuric 

 acid, their energy under such increasing temperatures as our laboratories 

 can provide, others, like boracic acid, become practically more powerful, 

 give a considerable amount of colour to the presumption that compound bo- 

 dies exist in the sun. The masses of the molecules of these compound 

 bodies will in most cases be too high to permit them, however volatile, to 

 reach the cool parts of the sun's atmosphere, so as to reveal themselves in 

 conspicuous solar lines. But the probability of their so appearing is very 

 much greater in the class of ruddy stars, as we shall find in the sequel ; and, 

 perhaps it is not impossible that the line B of the solar spectrum, or some 

 of the lines less refrangible than B, may result from some compound of 

 low vapour-density, such as hydrochloric acid*. It is certainly very re- 

 markable that neither B nor any line less refrangible has up to the pre- 

 sent been identified with a ray of any simple substance. 



52. Upon a general view of all the lines of the solar spectrum it appears 

 that their intensity continuously diminishes from the violet end of the 

 spectrum up to the line B. At this point, owing to the sudden introduc- 

 tion of an entirely new set of lines, their intensity abruptly and very much 

 increases. These new lines either have a terrestrial origin or come from 

 substances which stand high in the solar atmosphere. The lines, however, 

 which originate further down, do not attain their minimum of intensity 

 until they reach a point further to the right than B. This appears both 

 from the progressive diminution of their intensity up to B, and from the 

 total, or almost total, absence of lines further on, wherever a vacuity is left 

 between the lines which we must attribute to a different origin, as at wave- 

 lengths 71*1, 73*8, and in the wide spaces between the prominent lines from 

 this situation up to the line A. 



53. When this is considered in connexion with the cause to which the 

 diminution of intensity is to be referred, it indicates that if two perfectly 

 radiating bodies were gradually heated while the difference of their tempe- 

 ratures was kept constantly the same, the point of the spectrum at which 

 the difference of their brightness is least would advance with increasing 

 temperatures towards the red end of the spectrum. When the body of 



* If there be chlorine in the sun's atmosphere, the presumption upon chemical 

 grounds is very strong that there must be hydrochloric acid in the upper regions ; and 

 from its vapour density (18 - 25) the lines of hydrochloric acid would be black in 

 whatever part of the spectrum they might occur. 



VOL. XVII. I) 



