1867.] 



Colouring -matters hy the Spectrum Microscope. 



449 



and to it belong most of the vegetable colours used in dyeing, and nearly 

 all yellows. 



17. 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 therefore 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 analogous 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 dis- 

 solved in a strong solution, sealed up in tubes ; and it is occasionally aa 

 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 difi'er very considerably 

 in colours which are otherwise very similar ; as, for example, the yellow 

 colouring-matters of the root of rhubarb and of turmeric. The iodine or 

 ^romine should be added in sufficient quantity, 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. 



Hypochlorite of Soda. — This reagent, with or without the addition of 

 citric acid, is sometimes useful, as for instance in detecting the adulteration 

 of rhubarb with turmeric ; but generally its action is too powerful and too 

 uniform. 



Vermanganate of Potash. — This also usually acts too powerfully on 

 colouring-matters. The excess can easily be removed by sulphite of soda, 

 which makes an alkaline solution pale yellow, but an acid solution quite 

 colourless. 



18. Grouping of Colours. 



Having now considered some of the chief peculiarities of the most useful 

 reagents, I proceed to describe what appears to me to be the most con- 

 venient method of dividing colouring-matters into groups and subgroups, 

 so as to enable us to ascertain the nature of any particular suljstance under 



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

 VOL. XV. 2 P 



