1867.] Colouring -matters by the Spectrum Microscope. 455 



25. Fading of Group C. 



Sometimes in examining colours of group C, advantage may be taken of 

 the different rate at which their acid solutions decompose and fade, when 

 a considerable quantity of sulphite of soda has been added to an acid solu- 

 tion,, The two solutions should be made as nearly equal as possible in all 

 respects, and then the rate of fading may prove that they are very different, 

 or may show that one is a mixture. After fading, the addition of excess 

 of ammonia may show valuable facts. For example, the colour of the root 

 of the red beet {Beta vulgaris) is pink, but that of the leaves is red ; the 

 spectrum differing from that of the root merely in having the blue end 

 much absorbed. On keeping acid solutions of both to which sulphite of 

 soda has been added, that of the root becomes colourless, and that of the 

 leaves yellow ; and thus, considering that acid solutions of colours be- 

 longing to group C are very rarely pink, it is almost certain that the 

 colour of the leaves is the same as that of the root, but modified by the 

 yellow colour so common in leaves. 



26. Conclusion. 



Such, then, is a general outline of the method which I have hitherto found 

 the most convenient in studying different colouring-matters, and for de- 

 termining to what individual species any particular colour may belong. I 

 need hardly say that it is just the sort of qualitative analysis to employ 

 in detecting adulterations in many substances met with in commerce, as well 

 as in inquiries where very small quantities of material are at command. 

 By this method we might be able in a few minutes to form a very satisfac- 

 tory opinion, or at least one that might meet all practical requirements, and 

 even under unfavourable circumstances we might narrow the inquiry to a 

 surprising extent ; and if this can be said even now, surely further research 

 cannot fail to make it most useful in cases where ordinary chemical 

 analysis would be of little or no use. 



The Society then adjourned over the Easter Recess to Thursday, May 2. 



