460 



Mr. H. C. Sorby on 



[June 19, 



these various substances, wben dissolved in bisulphide of carbon, are very 

 closely as follows (D=3|, P=7|) : — 



Phycoxanthine 5 



Peziza xanthine 



Orange xanthophyll 



Xanthophyll 



Yellow xanthophyll 



These relations will be better understood by means of the following 

 figure : — 



Pig. 2. Spectra of the xanthophyll group compared. 



Red end. - Blue end. 



Phycoxanthine 



Peziza xanthine 



Orange xanthophyll. 



Xanthophyll 



Yellow xanthophyll. 



When these various substances are dissolved in benzole, their absorp- 

 tion-bands are all equally raised towards the blue end, so that we appear 

 to have a remarkable series of very closely related substances. 



Action of Light on the Xanthophyll Group. 



The behaviour of these substances when exposed to light is very in- 

 teresting. Taking a mixture of any two in bisulphide of carbon, and 

 exposing to the open sun, that which has its absorption extending over 

 the wider space, and has its absorption-bands the nearer to the red end, 

 is destroyed more rapidly than the one of which the absorption extends 

 over a less space, and the absorption-bands lie nearer to the blue end. 

 If any contiguous two species be mixed, the rate at which they are de- 

 composed differs so little that very much of both must be destroyed 

 before the one whose absorption lies the nearer to the blue end is left 



