60 ABSORPTION AND EMISSION. 



denominated thallium, from the beautiful green ray which 

 . characterizes its spectrum. 



(77) The position of the bright lines in the spectra of the 

 gases is not affected by the temperature of incandescence, but 

 their brightness is ; and it happens thus, that lines which 

 are invisible at the lower temperature, become visible when 

 the temperature is much raised ; and this process continues 

 until, at extreme temperatures, the spectrum becomes con- 

 tinuous. Thus lithium, whose spectrum consists of a single 

 red line at ordinary temperatures, exhibits an orange and a 

 blue band when the temperature is highly raised. 



\ 



(78) The rays of light emitted by an incandescent gas 

 are those which it absorbs at lower temperatures. This 

 property, which has been denominated the reversal of the 

 spectra of the gases, was first discovered in the spectrum of 

 sodium vapour. When a little common salt is introduced 

 into the wick of a spirit-lamp, the spectrum exhibits the 

 bright yellow double line which is characteristic of sodium ; 

 and it is found that the vapour of sodium, at the ordinary 

 temperature, stops or absorbs precisely the same ray, when it 

 is interposed in the path of the continuous light emanating 

 from an oil lamp, the light of which gives with the prism a 

 continuous spectrum. A more exact process is to introduce 

 a small piece of metallic sodium into a glass tube closed at 

 both ends, and from which the air has been expelled by in- 

 troducing hydrogen. On heating the tube by means of a 

 spirit-lamp, the metal is vaporized. By such means Bunsen 

 and Kirchoff found that the vapours of the alkaline metals, 

 lithium, potassium, strontium, and barium, when interposed 

 in the light of a continuous spectrum, absorb precisely the 

 same rays which they emit when self-luminous. 



(79) The general law, of which the foregoing is a par- 



