of Bright Lines in the Solar Speetrum. 61 



the light of the star has to traverse before reaching our spec- 

 troscopes. When, however, the region of combustion had 

 encroached sufficiently to reverse the metallic lines, these 

 would shine out with much greater brilliancy than the non- 

 metallic lines, and we should have a background of continuous 

 spectrum crossed by the bright lines of the metals of smallest 

 vapour-density. Such stars would only be expected among 

 those which are, so to speak, in the latest phase of their " che- 

 mical period." It is significant that y Cassiopeia, /3 Lyrse, 

 and ^Argo, three stars which show bright lines in their 

 spectra, all have sufficiently complex spectra to warrant the 

 belief that they have entered upon a late phase of their exist- 

 ence. Before the actual reversal of the metallic lines there 

 must exist a period in the life-history of many stars when the 

 temperature and extent of the zone of combustion is such as 

 to obliterate the dark lines of those metals which will ulti- 

 mately appear as bright lines. Such appears to be the case 

 with the hydrogen in a Orionis ; and according to the present 

 views it might perhaps be predicted that this star will sooner 

 or later show a permanent hydrogen-spectrum of bright lines. 

 It is conceivable that in certain cases the composition of a 

 star's atmosphere may be such as to permit a considerable 

 amount of cooling before any combination took place among 

 its constituents ; under such circumstances a sudden catas- 

 trophe might mark the period of combination, and a star of 

 feeble light would blaze forth suddenly, as occurred in 1866 

 to t Coronas Borealis. In other cases, again, it is possible 

 that the composition of a star's atmosphere may be of such a 

 nature as to lead to a state of periodically unstable chemical 

 equilibrium ; that is to say, during a certain period combina- 

 tion may be going on with the accompanying evolution of 

 heat, till at length dissociation again begins to take place. In 

 this manner the phenomena of many variable stars may per- 

 haps be accounted for. On the whole, the possibility of actual 

 combustion taking place in the atmosphere of a slowly cooling- 

 star previously at a temperature of dissociation does not seem 

 to me to have had sufficient weight attached to it ; and in 

 concluding, I would point out the important factor which is 

 thus introduced into calculations bearing upon the age of the 

 sun's heat in relation to evolution. 



London, June G, 1878. 



