1873.] Comparative Vegetable Chromatology. 



459 



difficulty from green or yellow leaves ; but being then mixed with yellow 

 xanthophyll, it is almost impossible to obtain it pure, and it therefore 

 turns green when treated with hydrochloric acid and turpentine. If no 

 sensitizer be present, such mixtures may change so slowly that the blue 

 product may fade as fast as it is formed, and never be very visible. 



Yellow Xanthophyll. 



This may be obtained in its most pure state from different kinds of 

 bright yellow flowers, as, for instance, the yellow Chrysanthemum. 

 When dissolved in bisulphide of carbon it is of a clear lemon-yellow 

 colour, differing most decidedly from that of xanthophyll properly so 

 called. The spectrum shows two very distinct absorption-bands in the 

 green end of the blue: these lie still further within the blue when 

 absolute alcohol is used as the solvent, and, on adding a little hydro- 

 chloric acid, the yellow xanthophyll is decomposed, first into a new and 

 paler yellow substance*, with the absorption-bands at a different interval 

 and more towards the blue end, and afterwards into a fine deep blue 

 colouring-matter, which will be more fully described further on. Yellow 

 xanthophyll may be obtained in an impure state from ordinary green 

 leaves, by first removing the chlorophyll and then adding successive 

 small quantities of water, agitating with bisulphide of carbon after each 

 addition, and keeping separate each quantity thus carried down. The 

 first contains much xanthophyll and a little yellow xanthophyll, and each 

 subsequent quantity contains less of one and more of the other, as may 

 be proved by both chemical and photochemical methods, or by the posi- 

 tion of the absorption-bands, until, towards the last, the yellow xantho- 

 phyll greatly preponderates, mixed, however, with so much lichno- 

 xanthine that the absorption-bands seen in its spectrum are far less 

 distinct than when it is prepared from the petals of such flowers as I 

 have named. On treating the solution in absolute alcohol with hydro- 

 chloric acid, though it turns to a blue-green, a residual yellow colour 

 (lichnoxan thine) remains when the blue has faded, which is not the case 

 when pure. If the leaves have an acid juice, the last residue consists, 

 not of yellow xanthophyll, but of the above-named yellow product of the 

 action upon it of weak acids. 



General relations of the Xanthophyll Group. 



These five different kinds of colouring -matters belonging to the 

 xanthophyll group are related to one another in a very interesting 

 manner. Making use of same scale of measurement and notation as 

 described in my former paper f, the centres of the absorption-bands of 



* This fact has also been noticed by Stokes (Journal of Chem. Soc. vol. ii. 1864, 

 p. 309). 



t Proceedings of Eoy. Soc. 1867, toI. xv. p. 434. 



