254 THE CHEMISTKY OF THE SUN. [CHAP. 



of the sun, which practically gives us the summation of all the 

 absorptions of all the regions from the top of the atmosphere to 

 the bottom. 



Now as a matter of fact we have the opportunity, when we 

 observe the spectrum of a sun-spot or a prominence, of deter- 

 mining the spectrum of a practically isolated mass of vapours 

 in the hottest region open to OUT inquiries, and seeing whether 

 it is like or unlike the general spectrum of the sun. What 

 then are the facts ? 



It is as unlike as possible : the intensities of the lines, as we 

 have seen, are inverted to a wonderful extent. More than this : 

 there is a constant difference between the spectra of sun-spots 

 and the spectra of metallic prominences, though we see these 

 phenomena generally at about the same niveau in the sun's 

 atmosphere. 



To get an idea of this inversion, maps were prepared of the 

 spectra of the chief chemical substances showing the behaviour 

 of the various lines under the various conditions. The result 

 is very striking ; indeed, it is striking to quite an unexpected 

 degree. The whole character of the spectrum of iron, for 

 instance, is changed when we pass from the iron lines seen 

 among the Fraunhofer lines to those seen among the spot and 

 prominence lines ; a complex spectrum is turned into a simple 

 one, the feeble lines are exalted, the stronger ones suppressed 

 almost altogether. 



Since then the spectra of spots and prominences are confes- 

 sedly the spectra of the hottest regions of the sun available for 

 our inquiries, we can test the nature of the common lines by 

 seeing how they behave when we pass from the general solar to 

 these special solar spectra. 



With special reference to this point the various observations 

 recorded of the lines visible in solar disturbances at the sun's 

 limb, and of those observed to be widened, brightened or other- 

 wise modified in the spectra of solar spots were confronted. 



