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



tures of different groups. Even if it should be found that perfectly 

 simple colouring-matters may have, as it were, intermediate characters, 

 such appear to be so rare that practically they may be classed with 

 mixtures until some reason be found for classing them together as 

 exceptions. 



These reactions of sulphite of soda are so much interfered with 

 by the presence of alcohol, that it should never be employed as a 

 solvent, unless the substance is insoluble in water ; and then it should 

 be diluted as much as possible, since the ordinary spirit of wine 

 with an equal quantity of water is the extreme strength admissible, 

 and even that very much delays the reaction. The effect of various 

 other reagents is also sometimes very different, according to the 

 nature of the solvent. 



The three groups A, B, and C differ in other particulars. It is 

 easy to change A or B into C by various reagents which produce 

 decomposition ; but I do not know a case where C can be changed into 

 A or B. Caustic alkalies usually soon decompose colours belonging 

 to group A, when dissolved in water, but act slowly on those of 

 groups B and C. Usually colours of group C are far more perma- 

 nent than those of groups A and B ; and to it belong most of the 

 vegetable colours used in dyeing, and nearly all yellows. 



1 7. Other Reagents. 



Boracic Acid. — The chief value of this reagent is that it gives 

 nearly the same spectrum as that of a neutral solution when added 

 after the addition of a slight excess of ammonia. It should there- 

 fore be well fused in a platinum crucible and recrystallized, so as to 

 be quite free from any stronger acid. 



Sulphate of Iron. — Sulphate of the protoxide of iron is chiefly 

 useful as a deoxidizing agent, in the case of blood and a few analo- 

 gous substances, taking care to have citric acid present to prevent 

 the precipitation of the oxide by ammonia *. 



Alum. — Alum has a remarkable influence on some colours, and 

 it has the property of gradually restoring many after they have 

 passed into the faded modification. Many colours also may be kept 

 for a long time dissolved in a strong solution, sealed up in tubes ; 

 and it is occasionally an excellent solvent for substances insoluble in 

 either water or alcohol. The chief objection to it as a reagent is that 

 the spectra are so much influenced by the presence of ammonia, even 

 when neutralized by an acid, that it is almost impossible to compare 

 together different substances under exactly the same conditions. 



Iodine and Bromine. — Iodine dissolved in alcohol, and bromine 

 in water, are useful in producing decompositions which may differ 

 very considerably in colours otherwise very similar, as, for ex- 

 ample, the yellow colouring-matters of the root of rhubarb and of 

 turmeric. The iodine or bromine should be added in sufficient quan- 

 tity, and then ammonia used to remove the excess, and thus avoid 

 the effect of their own colour. The solution may then be made acid 

 with citric acid, and should in both cases be compared with another 

 tube to which no iodine or bromine has been added. 



* See Stokes's Paper, Proc. Roy. Soc. vol. xiii. (1864) p. 355. 



