3l8 HISTORY OF CHEMISTRY. [LECTURE XV. 



a selective absorptive power, and this is not the case in general 

 with the latter. 110 The Fraunhofer lines are thus explained as 

 consequent upon absorptions by means of incandescent vapours. 

 Their existence led to the elucidation of the physical nature of 

 the sun ; while the determination of their positions (wave- 

 lengths) and the comparison of them with the emission spectra 

 of the elements in the gaseous state, led to the fixing of its 

 chemical composition. Kirchhoff thus became the founder of 

 a new branch of chemical science that of stellar chemistry. 

 Although this branch is still comparatively recent, it is already 

 in a position to show great results. By means of it astronomy 

 has met with new problems, and has been furnished with new 

 methods, which have immensely widened its sphere of activity ; 

 but it is not possible to enter into this subject more fully here. 



The discoverers of the spectroscopic method of analysis were 

 themselves able to establish its importance in chemistry, not 

 only by showing its application to analytical chemistry, but also 

 by the discovery of two new elements caesium and rubidium. 

 We are indebted to the same method for the discovery of 

 thallium by Crookes, 117 and of indium by Reich and Richter ; 118 

 as well as for that of gallium and scandium, of which mention 

 has already been made. 119 " 



Amongst the more recent investigations in this department 

 some which deserve to be specially mentioned are those of 

 A. Mitscherlich, who was able to show that not merely every 

 element, but every compound, possesses, in the gaseous state, 

 a spectrum peculiar to itself, 120 and those of Pliicker and Hittorf, 

 who showed that there are two spectra corresponding to every 

 element; i.e., that besides the line-spectrum there is also the 

 band-spectrum. 1 ' 21 Further, there are the investigations on 

 quantitative spectrum analysis, especially those of Vierordt 122 



116 An exception is furnished by the salts of didymium, which possess 

 a selective absorptive power. 117 Phil. Mag. [4] 21, 301 ; Annalen. 



124, 203. 118 J. pr. Chem. 89, 441. 119 Compare p. 313. 



120 Pogg. Ann. 116, 499; 121, 459- ]21 Phil- Trans. 1865, i. 



122 Anwendung des Spectralapparates zur Photometric der Absorptions- 

 spectren, Tubingen 1873, 



