1867.] Colouring -matters hy the Spectrum Microscope. 451 



of the two sets of experiments, we may decide whether it belongs to groups 

 A, B, or C, or is a mixture of any of them. If the substance is insoluble 

 in water but soluble in alcohol, the same experiments should be made, only 

 we must add the colour dissolved in alcohol to as much water as can be used 

 without making the solution turbid, and must remember how much the 

 presence of alcohol may interfere with the action of some of the reagents. 



Another portion of the neutral colour should then be dissolved in as 

 strong alcohol as will give a clear solution, and ammonia, benzoic acid, a 

 little citric acid, and much of it added one after the other ; and all the 

 spectra carefully observed, as well as any other facts which may present 

 themselves. 



By thus using three separate quantities of colour, and adding reagents 

 one after the other, vv^e may obtain about a dozen spectra, which may differ 

 from one another in important particulars, or in some few cases may be all 

 alike. The experiments are so easily made, that the whole series of twelve 

 spectra may be seen in the space of five minutes ; and the total quantity of 

 material need not in some cases be more than yw^~q ^ grain. The facts 

 thus learned may show that for particular practical purposes some different 

 method could be employed with advantage, and that only one or two simple 

 experiments are needed. For example, suspected blood-stains should' be 

 treated in an entirely different manner, as described in my Paper on that 

 subject * ; and in examining dark-coloured wines, in order to form some 

 opinion of their age from the relative quantity of the colour belonging to 

 group C, gradually formed by the alteration of the original colouring- 

 matter of the grape belonging to group B, it is only requisite to observe 

 the effect of sulphite of soda after the addition of citric acid. It would, 

 however, extend this Paper beyond the limits I have prescribed to myself, 

 if I were to enter into practical applications, and I shall therefore merely 

 give a description of a convenient method of grouping the various colours. 



20. Subgroups. 



Since the narrow absorption-bands are decidedly the most important 

 means of identification, it appears to me that we cannot do better than 

 adopt subdivisions founded on their number. We may thus divide each 

 group A, B, and C into subgroups, in which the neutral aqueous solu- 

 tions exhibit 0, 1, 2, 3, &c. decided absorption- bands. Sometimes one of 

 them may be so obscure that we may hesitate whether it should be counted 

 or not ; but practically this is no very serious objection, if we decide to 

 reckon only distinct bands, and to look on the fainter as important merely 

 in identifying individual colours. If no absorption-band can be seen in 

 the neutral solution, we may take into account those seen when more or 

 less ammonia is added ; and if none occur in either case, we may make use 

 of those seen in the alcoholic solution when neutral, and after the addition 

 of ammonia. Whenever in this order of experiments the solution gives 



* Quarterly Journal of Science, vol. ii. p. 205. 



2 p 2 



