Dr. Gladstone on the Atmospheric Lines of the Solar Spectrum. 71 



absorptive power at exactly the same point of the spectrum as it 

 does at the highest temperatures which we can produce." It is, how- 

 ever, as yet an undetermined question at what temperature below 

 that at which it becomes luminous, a gas ceases to absorb ; though, 

 from the close connexion which has been shown to exist between 

 emission and absorption, there can be little doubt that such a limit 

 would be arrived at. 



This theory of the origin of Fraunhofer's lines, and the uncer- 

 tainty just alluded to, suggested the desirableness of comparing the 

 " atmospheric lines " of absorption with the luminous bands which 

 make their appearance in the linear spectra of the light emitted by 

 the different constituents of the atmosphere when sufficiently heated. 

 If all the rays emitted by these different constituents should be 

 found to coincide with the " atmospheric lines," it would show at 

 once the origin of the lines, and prove that gases at the ordinary 

 temperature will absorb rays of the same refrangibility as those they 

 emit when themselves glowing. If, on the contrary, they should be 

 found not to coincide, it would prove by an extreme case (for we 

 operate thus upon many miles of oxygen and nitrogen gases at least) 

 that this connexion between absorption and emission either does not 

 extend to these elements, or is confined within those narrower limits 

 of temperature which theory seems to require. It was also conceiv- 

 able that the atmospheric lines might coincide with the rays emitted, 

 not by all, but by a portion of the constituents of the atmosphere. 



The following data exist for this comparison. Angstrom * has 

 delineated the luminous bands due to the gas when the electric 

 spark is sent through nitrogen, oxygen, carbonic acid, hydrogen, &c. 

 Plucker -f* has described and measured those bands which appear in 

 the spectra of the same and other gases when they serve as the 

 residuary gas in Geissler's tubes. Under the influence of the elec- 

 tric discharge, the vapour of water, carbonic acid, ammonia, and 

 other compounds are decomposed ; but it occurred to me that the 

 oxyhydrogen flame must consist partly, if not entirely, of glowing 

 steam, since it is only when the two elements combine that the heat 

 is evolved ; and that similarly the flame of carbonic oxide burning in 

 air must contain the light emitted from glowing carbonic acid. I 

 therefore subjected these two flames to prismatic examination. 



The result of the comparison has been that not one of these obser- 

 vations shows any accordance between tbe luminous bands due to 

 the gas, and the dark lines that make their appearance in the solar 

 spectrum when the sun is shining through a great depth of air. 

 Hydrogen alone is inconclusive. Neither is there any accordance 

 between these luminous bands and the more prominent lines of the 

 ordinary solar spectrum. 



This shows that oxygen and nitrogen, and perhaps other gases, 

 though in enormous quantity, do not absorb at the ordinary tempe- 

 rature rays of the same refrangibility as those they emit when heated 

 by the means specified. 



It would not be legitimate to infer from this that the atmospheric 

 lines have not their origin in the absorbent power of one or more of 



* Pogg. Ann. xciv. p. 141. f Ibid. cvii. pp. 497, 638. 



