33 8 NINETEENTH CENTURY. PT. HL 



yellow line ; nor does it matter if many incandescent gases 

 are mingled together, for the vapour of each one will give its 

 (nun lines without interfering with the others. 



It was Sir John Herschel in 1822 who first suggested that 

 by reducing substances to incandescent gases in a flame, and 

 marking the bright lines which they produced, it would be 

 possible to detect the most minute quantities of any metal 

 or earth which they contained, 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 substance ; and what is still more wonderful, 

 new metals were actually discovered by the new bright lines 

 they threw on the spectrum. The first two of these new 

 metals, cal'ed C cesium and Rubidium, were discovered by 

 Bunsen and Kirchhoff in 1860; the third, called Thallium, 

 which throws a beautiful green line, was found by Mr. 

 Crookes in 1861 ; the fourth, called Indium, which gives 

 two indigo coloured lines, was first seen by Richter and Reich 

 in 1864; and a fifth, called Gallium, was discovered by M. 

 Lecoq de Boisbaudran in 1875. Thus spectrum analysis 

 gives us an entirely new and sure way of analysing or dis- 

 covering the different elements in any substance. 



Bunsen and Kirchhoff explain the Dark Lines in 

 the Sun Spectrum, 1861. But for a long time no one 

 could solve the question of the black lines in the solar spec- 

 trum. Sir David Brewster came very near to it once, but 

 just failed to hit upon the truth. 1 At last, in 1861, only 



1 Sir William Thomson states in his address to the British Associa- 

 tion in 1871, that Professor Stokes gave the true explanation of these 

 lines in his lectures at Cambridge in 1851, although he did not publish 

 anyihing about it, and his idea was not generally known. Balfour 

 Stewart had also shown in 1858 that a body absorbs the same kind and 

 amount of light and heat rays which it radiates when heated. 



