452 Mr. H. C. Sorby on Analysis of Animal and Vegetable [Apr. IJ, 



any decided absorption-band the subgroup is determined ; and it is only 

 when none has been produced that the process must be carried further. 

 The general connexion of the subgroups will be best seen from the 



The same system is applicable to each division, I, 2, and 3, and to each 

 group A, B, and C. We can easily express the subgroups by using one 

 or more of the signs aq, am, al, am, with a figure to indicate the number 

 of bands in the first term that contains any ; those before it being given to 

 show the facts more clearly. 



Each colour can be indicated by writing after the subgroup the charac- 

 teristic spectrum, or, for the sake of simplicity, merely the position of the 

 centres of the bands, when they are seen as independent as possible ot 

 general absorption. If the centres of the bands are in different positions 

 the colours cannot be the same ; but if they agree it does not necessarily 

 follow that they are the same. It is probable, but must be further proved 

 by the correspondence of other spectra. As examples I give a few well- 

 marked cases. 



Purple Pansy I, A, aq^ am^ (4). 



Crimson Rose I, A, aq^ am^ al^ am^ (^i)* 



Blue Lobelia {L. speciosa) 1, B, e?^2 "^i)' 



Pink Stock {Matthiola annua) \,^, aq^ (3J, 65). 



Several blue species of Campanula 1, B, ag^ (2|, 4, 5§, 7i). 



Brazil-wood (Ccesalpinia crista) \, C, aq^ (S^). 



ljOgwood.(Hcematoxylum campechianum) 1, C, aq-^ 

 Sandalwood (^Pterocarpus santalinus) . .3, C, aq^ (6, 7|). 

 Alkanet-root {Anchusa tinctoria) 3, C, aq^ (4|, 5|, 71). 



21. Individual Colours. 

 Having then ascertained to which subgroup any particular colour be- 

 longs, it is in the next place requisite to determine what particular sub- 

 stance it is. When it gives rise to well-marked absorption-bands, this may 

 be more or less definitely decided by their exact position and character ; 

 since they may of course occur in different situations, or vary much in ab- 

 solute and relative breadth and in intensity. Thus, choosing closely re- 

 lated spectra, we have, for example, — 



1, B, aq^ * * 



Blue Lobelia speciosa 2\ — 3^ . . . 3| — 4f 11... 



Ym\i Matthiola annua 2|— 4|...4i— 5|...8 10.. 11 — 



1, C, 05', , ^ 



Logwood (Hcematoxylum campechianum) 3| — 51... 7—8 — 



^x?iz\[-vioodL {CcBsalpinia crista) 4| — b^..7—S — 



