280 THE CHEMISTRY OF THE SUN. [CHAP. 



Again, it is known that by compression we can turn green 

 chlorine vapour green because it absorbs the blue into yellow 

 liquid chlorine yellow because an absorption in the red is 

 added ; and the changes in colour of sulphur and phosphorus 

 are also in accordance with the hypothesis. There is also 

 another proof behind of a novel character, to which a slight 

 reference has already been made on p. 211. In my work on 

 spectrum photography I had been struck with the possible con- 

 nection between the continuous absorption in the blue recorded 

 in the above experiments, and the continuous action in the blue 

 of the salts of silver, and it seemed probable that further 

 study would provide us with continuous action in the red, if we 

 could either complicate the molecular structure of the salt of 

 silver or use some other metal altogether. I invited my friend 

 Dr. Russell to join me in this research, but other work made it 

 impossible. Still, however, we had not long to wait, and I have 

 already recorded with what success Captain Abney has worked 

 in this direction, having discovered a method of obtaining the 

 silver salt of the same chemical nature but more complex in its 

 molecular arrangement. 



It may be argued that because we observed more absorption 

 in the red with the oxyhydrogen flame than we did in the 

 cooler tube, that the red absorption may have been produced 

 by the breaking up of the blue-absorbing molecule. But this is 

 not so, because the blue was produced at last at the expense of 

 the red, and the fact that the red-absorbing vapour is produced 

 more richly at higher temperatures is in accordance with what 

 we know of hydrocarbon vaporizing, if we assume it to be more 

 complex. In organic work the higher the temperature used to 

 break up a compound the richer in complex forms is the vapour 

 first given off. 



So much then for the independent confirmation of the exist- 

 ence of red and blue molecules, and of the fact that the one 



