504 HISTORY OF SCIENCE. 



by a lime salt, to which is due the intense red line indicated by the 

 thick band in the lowest spectrum of the diagram (marked "flame"), 

 is supposed to undergo dissociation in the stronger heat of the electric 

 arc, so that one of its .constituents gives rise to the intense blue line 



*;| 



ft^^S 



SIRIUS 



SUN 

 ARC 



FLAME 



a 



FIG. 228. 



which is prominent in the arc spectrum of calcium. At the still more 

 elevated temperature of the sun this last constituent is in its turn resolved 

 into elements which give the intense lines H and K towards the violet 

 end of the spectrum, and finally at the temperature of Sirius, which 

 there is reason to believe is a hotter sun than our own, the chief ele- 

 ment of calcium gives the marked line H, while the blue and red lines 

 that are conspicuous at lower temperatures almost die out, because a 

 very small proportion of the compound which gives these lines can 

 exist at the very intense temperature of Sirius. According to this view 

 the molecular grouping of the atoms which constitute the particles of 

 calcium is resolved by elevation of temperature into other groupings, 

 which have their principal lines in the violet part of the spectrum. 



Plate XII. will serve to give the reader some notion of the several 

 bright line spectra which have been referred to in the foregoing pages. 

 The upper one (i) is intended to show merely the positions of some 

 of the Fraunhofer lines. The spectra below are ranged thus : 2 so- 

 dium ; 3 potassium ; 4 lithium ; 5 calcium ; 6 strontium ; 7 barium ; 

 8 thallium; 9 indium; 10 Sirius; n nebula; 12 hydrogen; 13 

 nitrogen ; 14 coal-gas. 



