178 THE CHEMISTRY OF THE SUN. [CHAP. 



Angstrom, in that exceedingly important memoir which 

 accompanies his Atlas, states : 1 



" In increasing successively the temperature I have found that 

 the lines of the spectra vary in intensity in an exceedingly compli- 

 cated way, and consequently new lines even may present themselves 

 if the temperature is raised sufficiently high." 



Kirchhoff and Bunsen, indeed, as early as 1860, seem to have 

 got a glimpse of the same thing. They wrote : 2 



" If the intensity of the light ... be increased new lines 

 appear . . . and the relation of the brightness of the old ones 

 becomes altered. In general an indistinct line becomes brighter, 

 upon increasing the illumination, more rapidly than does a brighter 

 line, but not to such an extent that the indistinct line ever 

 overtakes in intensity the brighter one. A good example of this 

 is seen in the two lithium lines. We have only observed one 

 exception to this rule, namely in the line Ba y, which by light of 

 small intensity is scarcely visible, whilst Ba y appears plainly, but 

 by light of greater intensity becomes more visible than the latter." 



Touching these variations I may remark that Kirchhoff did 

 not agree that the temperature upon which Angstrom laid such 

 strong stress was really the cause at work. 3 He attributed those 

 variations rather to the mass and the thickness of the vapours ex- 

 perimented upon nay, he went further : at a time when scarcely 

 any facts were at his command he broached a theorem which 

 went to prove this ; arid yet what had Kirchhoff himself done ? 

 He had traversed his own theorem. He states that his observa- 

 tions (those referred to in Chap. VI.), were made by means of a 

 coil, using iron poles one millimetre in thickness. Now the 

 thickness of a short spark taken from iron poles one millimetre in 

 thickness would probably be two millimetres. Next Kirchhoff 

 located the region where the absorption which produces the 



1 Recherches sur le Spectre Solaire, pp. 38, 39. 



2 Kirchhoff and Bunsen, Phil. Mag. vol. xx. s. 4, p. 94 Augtist. 1860. 



3 Recherches sur le Spectre Solaire, pp. 38, 39. 



