330 GREAT MEN OF SCIENCE 



the sun. According to the law, which is assured by so many 

 other general experimental results linked to it, this cannot 

 be a matter of chance; it is necessary to assume that some- 

 where in the path of the ray of light from its origin in the sun 

 to our spectroscopes on the earth, sodium vapour exists, 

 consisting of free sodium atoms as in the case of the flame. 

 But this can only be in a gaseous atmosphere at a high tem- 

 perature on the sun itself, an atmosphere which must be 

 traversed by the white light radiated from an incandescent 

 liquid or solid core of the sun. In this way a well-founded 

 explanation of the hitherto unexplained Fraunhofer lines in 

 the sun's spectrum was given, and furthermore, by the aid 

 of Bunsen's results, the possibility of a chemical investiga- 

 tion of the sun's atmosphere was opened up. 



Few investigators have ever been able to rejoice at one 

 time in so many important results as Bunsen and Kirchhoff 

 in this case.^ 



Kirchhoff now measured the dark lines of the sun's spec- 

 trum still more elaborately than Fraunhofer had already 

 done, and he likewise measured the emission lines of ele- 

 ments, particularly iron, in order to be able to determine 

 coincidences in full detail. In this way, besides sodium and 

 iron, hydrogen, magnesium, calcium, and several other 

 elements known on the earth were proved to exist in the 

 sun's atmosphere. In the case of one of the dark lines in the 

 sun's spectrum, the element to which it belonged was only 

 subsequently found upon the earth, as giving a bright line in 

 the same position; this element was therefore called helium 

 (one of the noble gases). At total eclipses of the sun, when 

 the centre of the sun is obscured by the moon, the bright 

 spectrum lines are also seen - for the first time in the year 



^The place in which this discovery was made is indicated on the house 

 called 'Im Riesen' in the Hauptstrasse in Heidelbeigby means of a tablet. 

 On the opposite side of the road is the Friedrichsbau, built three years 

 later, into which Kirchhoff then moved; the chemical laboratory on the 

 Wredeplatz had already (1855) been previously rebuilt for Bunsen. 



