chap, xxvi.i SPECTRUM ANALYSIS. 323 



sodium be rendered incandescent in the name of a 

 bunsen gas lamp, and the rays be transmitted through 

 a prism, the bright yellow lines constituting the 

 spectrum of sodium will be obtained, but if between 

 the bunsen lamp and the prism an ordinary spirit 

 lamp, burning with a salted wick, be interposed, the 

 bright yellow disappears. That is to say, the vapour 

 of sodium produced by the spirit lamp has absorbed 

 the light proceeding from the vapour of higher tem- 

 perature and of the same quality behind it. The 

 vapour of sodium will absorb and retain light whose 

 period of vibration is identical with its own. If light 

 proceeding from a source pass through an atmosphere, 

 the atmosphere will prevent the passage of such rays 

 as correspond to those which it would itself produce. 

 In the case of the solar spectrum, therefore, the dark 

 lines are due to the absorption of certain rays in 

 passing through the atmosphere surrounding the sun. 

 To take again the D lines, this implies that there is 

 incandescent sodium vapour in the sun's atmosphere, 

 and that it separates out and retains the vibrations of 

 its own period. It is evident that this affords a means 

 of information as to the chemical constitution of the 

 sun and other luminaries. 



Spectrum analysis. Since it has been found 

 that certain substances give definite coloured lines 

 when the rays from their incandescent vapour are 

 passed through a prism, and since the same bands will 

 not be produced by two different substances, it is 

 evident that there is afforded a method for analysing 

 compound bodies, and detecting the presence of cer- 

 tain constituents. The spectra of gases can be 

 obtained by the use of tubes exhausted of air and 

 containing a small quantity of the particular gas. An 

 electric spark is passed through the tube from an 

 induction coil, and the spectrum of this obtained. 



The spectroscope (Fig. 150) is an arrangement of 



