162 Royal Society :— Mr. H. C. Sorby on Analysis of Animal 



difference ; for this is very often the case, and very important facts 

 may be overlooked if too great an excess be added at first. The ad- 

 dition of a small bit of sulphite of soda then shows at once whether 

 or not a colour belonging to group A is present ; and on adding 

 excess of citric acid we may also determine whether it chiefly be- 

 longs to groups B or C. Ammonia should then be added in excess, 

 which may or may not restore it to the same state as before the ad- 

 dition of the acid. To another portion of colour carbonate of soda 

 should be added, and then excess of citric acid, both spectra being 

 carefully observed ; and finally sulphite of soda, which definitely 

 shows whether or not there is any other colour than one belonging to 

 group C. Combining the results 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 with- 

 out making the solution turbid, and must remember how much the 

 presence of alcohol may interfere with the action of some of the re- 

 agents. 



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 re- 

 agents one after the other, we 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 10 1 Qo of a grain. The facts thus learned may show that 

 for particular practical purposes some different method could be em- 

 ployed 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 ad- 

 dition of citric acid. It would, however, extend this Paper beyond 

 the limits I have prescribed to myself, if I were to enter into prac- 

 tical 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 im- 

 portant 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 

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



