IV.] 



GASEOUS ABSORPTION. 



41 



To observe the effect of increasing thickness of the medium 

 wedge-shaped cells may be employed, and it will then be found 

 that as the thickness increases the absorption lines broaden out 

 and new regions of absorption appear. 1 But we are not limited 

 to solids and liquids. Gases and vapours can be made to yield 

 absorption spectra. Put a few grains of iodine in a dry flask and 

 place it in front of the slit ; then warm the flask gently with a 



FIG. 20. Method of studying the absorption of iodine vapour. 



spirit-lamp. The iodine will soon volatilise and fill the flask 

 with a violet vapour, giving rise to a beautifully regular fluted 

 spectrum as shown in Fig. 21. 



When the gas or vapour is not distinctly coloured it is neces- 

 sary to pass the light through a great thickness of it. The 

 manner in which this is effected is shown in Fig. 22. The 



1 If we have one of those handy little pocket spectroscopes, which now, I am 

 glad to think, are becoming common, the absorption of the light of a candle by 

 the blood in the lobe of a friend's ear, or in the interval between two closed 

 fingers, can be well seen by placing it between the slit and the light. 



