126 On the Examination of 



crimson Cineraria, but the bands are so placed tbat on mixing the 

 two colours a band of one covers up the clear space between the 

 bands of the other, and thus the mixture may show merely a broad 

 absorption, very hke that in the colouring matter of many plants. 

 However, on adding a httle sulphite of soda to a very slightly acid 

 mixed solution, and examining it at once, the bands of the Cineraria 

 may be seen for a short time, since those of the Lobelia are more 

 rapidly destroyed. Even in the case of those colouring matters 

 which are not immediately changed in an acid solution by the 

 addition of sulphite, one may slowly fade, whilst the other is scarcely 

 changed, and thus evidence may be obtained of the existence of a 

 mixture. Numberless illustrations might be given of the application 

 of such a method, but these will I trust explain sufficiently well the 

 general principle. In particular instances we may of course make 

 use of other reagents to destroy one colour and leave another, but 

 in tho case of closely-aUied substances the difficulty is that few or 

 no reagents will act sufficiently on one without changing both, and 

 we are compelled to rely on the comparison of the spectra of the 

 colours partially separated artificially, or met with naturally in 

 varying proportion. 



For effecting a partial separation of colouring matters soluble in 

 water no reagent is more convenient than ether. On agitating the 

 aqueous solution with more of this than can be dissolved, the excess 

 rises to the top, and the aqueous solution subsides to the bottom. 

 In the case of some colouring matters the greater part remains dis- 

 solved in the water ; whereas, in the case of others, the greater part 

 rises to the top, dissolved in the ether ; whilst occasionally nearly 

 the same amount is dissolved in both. It will thus be seen that, if 

 a mixture were thus treated, a partial separation might often be 

 effected, 'and on evaporating to dryness, redissolving in water, and 

 comparing the spectra, either in the natural state or after reagents 

 had been added, the differences might clearly prove that two or more 

 colouring matters were present. In order to make this more intel- 

 ligible, it will be well to enter into a few general principles involved 

 in the method. For example, suppose that in some particular state, 

 acid, neutral or alkaline, whichever it might be, the original solution 

 gave a spectrum with two absorption-bands, A and B, and that when 

 separated artificially into two portions one gave the band A and the 

 other B, in precisely the same conditions as before, it would be quite 

 certain that the original was a mixture of two substances. But 

 since such a complete separation would only occur in a few instances, 

 it might happen that both the products showed both the bands, only 

 in a veiy different degi'ee. Thus, for example, on making the solutions 

 of such a strength that the band A was equal in both, the band B might 

 be so much darker in one, that in experiment tubes of the same 

 depth the solution might have to be diluted to four times the volume 



