of Bright Lines in the Solar Spectrum. 53 



of oxygen and nitrogen in the solar spectrum might be ac- 

 counted for by supposing that, at the temperature of the sun, 

 the specific absorptive power of these gases may be insufficient 

 to reverse their spectra. This view, however, equally fails to 

 account for the brightness of the lines in question. 



4. Let us now make the not improbable assumption that the 

 temperature of the sun's nucleus, photosphere, and reversing 

 layer is so great that dissociation is perfect throughout these 

 regions, but that somewhere in the higher regions, or above 

 the chromosphere*, the temperature falls off sufficiently for 

 some kinds of chemical combination to take place — say, in the 

 present instance, for oxygen to combine with hydrogen. 

 Under these circumstances we should have, concentric with 

 and exterior to the chromosphere, a zone of combustion where 

 oxygen and hydrogen, already at a very elevated temperature, 

 enter into combination and become thereby raised to a state 

 of more vivid incandescence t- All elements which, by virtue 

 of their small vapour-density, extended into the region of com- 

 bustion, would be raised to incandescence by contact with the 

 flaming gases, if not actually taking part in the combustion. 

 Thus, according to the present hypothesis, we should not ex- 

 pect to find in the solar spectrum the bright lines of elements 

 having a high vapour-density. 



5. The possibility of combination taking place in the higher 

 regions of the sun's atmosphere is admitted by Stoney J, who 

 states that " gases in the solar atmosphere which are kept 

 asunder by the temperature of its lower strata may be able to 

 combine in the cooler regions above." Such combination, 

 although arising from the cooling-down of gases previously at 

 a temperature of dissociation, would nevertheless be attended 

 with the evolution of heat, and would possess the character of 

 true combustion. Professor Draper also remarks, in the paper 



* It is generally admitted that the true height of the chromosphere is 

 considerably greater than that seen by means of the telespectroscope, 

 since the amount of dispersion necessary to weaken the scattered light of 

 our atmosphere must weaken and shorten the hydrogen-lines by which 

 the chromosphere is revealed. 



t It is well known that the oxyhydrogen flame does not show the 

 lines of either of the burning gases. In the sun, however, the conditions 

 are probably very different. The combining gases may be largely diluted 

 with other inactive gases. Furthermore the pressure, as shown by the 

 researches of Frankland and Lockyer (Proc. Roy. Soc. xvii. p. 288), is 

 apparently far less in the upper regions of the chromosphere than in on- 

 own atmosphere. Roth these causes would conspire to raise the point of 

 ignition of the gases in question, so that a much higher temperature would 

 be necessary to bring about combination than if they were undiluted and 

 under greater pressure. 



| Proc. Roy. Soc. xvii. p. 



