CH. XXXIII. BUNSEN AND KIRCHIIOFF. 323 



most minute quantities of any metal or earth which they con- 

 tained, and Mr. Fox Talbot carried out this suggestion in 

 1834. By this means in the course of time spectroscopists, 

 or men who made the spectrum their study, were able to 

 map out accurately the coloured lines of every known sub- 

 stance ; and what is still more wonderful, new metals were 

 actually discovered by the new lines they threw on the dark 

 band. The first of these two new metals, called Ccesium 

 and Rubidium, were discovered by Bunsen and Kirchhoff ; 

 the third, called Thallimn, which throws a beautiful green 

 line, was found by Mr. Crookes ; and the fourth, called 

 Iridium, by Richter and Reich. Thus you see spectrum 

 analysis gives us an entirely new and sure way of analysing, 

 or discovering the different elements in any substance. 



Bunsen and Kirchhoff explain the Dark Lines in the 

 Sun Spectrum, 1861. — But all this time no one could solve 

 the question of the black lines in the solar spectrum. Sir 

 David Brewster came very near to it once, but just failed to 

 hit upon the truth. ^ At last, in i86r, only fourteen years 

 ago, Bunsen and Kirchhoff, two celebrated professors of 

 chemistry and physics at Heidelberg, discovered the secret. 



These two men had been making a long set of careful 

 experiments upon all the different substances of our globe, 

 burning them and examining their gases one by one, and 

 marking the bright lines of each upon the spectrum. In 

 doing this they did not use one prism only as Fraunhofer 

 had done, but four (see Fig. 52, p. 324), so arranged that 

 the light coming in through a slit at the beginning of the 



' Sir William Thomson states in his address to the British Asso- 

 ciation in 1871, that Professor Stokes gave the true explanation of 

 these lines in his lectures at Cambridge in 185 1, although he did not 

 publish anything about it, and his idea was not generally known. 



