306 



the sun of the substances which emit these luminous bands, and 

 their appearance as dark lines is because "the spectrum of an 

 incandescent gas becomes reversed, when a source of light of suffi- 

 cient intensity, giving a continuous spectrum, is placed behind the 

 luminous gas." It also appears that " sodium vapour at a tempe- 

 rature much below that at which it becomes luminous, exerts its 

 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. 

 Pliicker 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 

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



