190 THE CHEMISTRY OF THE SUN. [CHAP. 



visible in the star, which, be it remembered, has been inde- 

 pendently determined to be hotter than our sun, are precisely 

 those lines, and none other, which we often see bright on the 

 disk of the sun itself. I have emphasised the fact that we 

 have independent evidence that the star with very few lines is 

 hotter than our sun. It is also presumable that the stars with 

 the fluted spectra are much cooler than our sun, because they are 

 red stars the light of which is exceedingly feeble ; on grounds 

 independent altogether of spectroscopic evidence, they are 

 supposed to be stars in the last visible stage of cooling. 



3. The Question of Multiple Spectra. 



The fact that some elementary bodies have double spectra, 

 that is, that under changed conditions of temperature or elec- 

 tric tension they gave us now a fluted spectrum and now one 

 composed of lines, was independently discovered by Pliicker and 

 the younger Mitscherlich. 1 Mitscherlich, in the clearest manner, 

 at once pointed out that this fact might be taken as evidence 

 that the elements on which he experimented were in reality 

 compound bodies. 



Pliicker, afterwards joined by Hittorf, called the different 

 spectra seen under different conditions spectra of the first and 

 second orders. On this point they wrote seventeen years ago : 2 



" The first fact which we discovered in operating with our 

 tubes . . . was the following one : 



" There is a certain number of elementary substances which, when 

 differently Jieated, furnish two kinds of spectra of quite a different 

 character, not having any line or band in common. 



" The fact is important, as well with regard to theoretical con- 

 ceptions as to practical applications the more so as the passage 

 from one kind of spectrum to the other is by no means a con- 

 tinuous one, but takes place abruptly. By regulating the temper- 

 ature you may repeat the two spectra in any succession ad libitum." 



1 See Phil. Mag. 1864, vol. xxvii. p. 46, and vol. xxviii. p. 1878. 



2 Pliicker and Hittorf on "The Spectra of Ignited Gases and Vapours," Phil. 

 Trans. Royal Society, 1865, part i. p. 6. 



