52 Mr. 11. Meldola on a Cause for the Appearance 



great that there is perfect dissociation throughout his whole 

 atmosphere. Under these circumstances free oxygen would 

 exist in the presence of electro-positive elements; and, in ac- 

 cordance with Stoney's hypothesis, both this element and 

 nitrogen (if present) would extend to a considerable height in 

 the sun's atmosphere, rising as a necessary consequence, into 

 regions which are cooler than that stratum which is cool 

 enough to reverse the spectral lines of those metals having the 

 smallest molecular mass, viz. Na, Ca, and Mg*. Professor 

 Draper's suggestion that the enormous thickness of incan- 

 descent oxygen may overpower the light of the photosphere, 

 can only hold good, when considered in connexion with this 

 hypothesis, if the temperature of the upper portions of the 

 oxygen atmosphere does not differ to any great extent from 

 that of the lower and hotter portions. When, however, we 

 bear in mind the comparatively low vapour- density of oxygen, 

 and consider at the same time to what an enormous height the 

 hydrogen atmosphere extends, it appears probable that the 

 height reached by oxygen would be such that the temperature 

 of the upper portions of this gas would be considerably lower 

 than that of the subjacent layers ; so that any excess of radia- 

 tion over that of the photosphere given out by the hottest 

 portions of the incandescent oxygen would be obliterated by 

 the absorption of the cooler portions above. 



[The same reasoning can be applied if we suppose that the 

 temperature of the oxygen falls off at some particular level ; 

 so that above this boundary the state of molecular aggregation 

 of the gas corresponds to Dr. Schuster's "compound-line" 

 spectrum, while below this boundary the greater heat of the 

 gas resolves its molecules into the atoms giving the ordinary 

 line-spectrum. The eifect of this state of affairs is practically 

 the same as would be brought about by annihilating a certain 

 portion of the upper oxygen layers, since the two different 

 molecular states of the gas give totally dissimilar spectra. We 

 are thus reduced to an oxygen atmosphere of smaller extent, 

 and the foregoing reasoning obtains.] 



Angstrom suggested f that the non-appearance of the lines 



* Stoney lias shown (Proc. Roy. Soc. xvii. p. 14) that a gas or vapour, 

 even when present in only small quantity, will nevertheless extend to 

 nearly its full height in the solar atmosphere. 



t He remarks (liechercltes sur le spectre Solaire, Upsal, 1869, p. 37) that 

 it is " tres-probable que la temperature elevee du soleil ne suffit pas pour 

 produire les raies brillantes de Toxygene et de Tazote, et que par conse- 

 quent, meme en supposant que ces corps existent actuellement dans le 

 soleil, its ne doivent pourtant pas occasioner de raies obscures dans le 

 spectre solaire." He further suggests that oxygen and nitrogen may exist 

 in the corona. 



