Mixed Colouring! Matters with the Spectrum Microscope. 125 



to prove that the colouring matter itself differs essentially. The 

 spectra seem to show that occasionally the presence of some other 

 substance, insoluble in water, which has a strong affinity for the 

 colouring matter, is the true cause of this variation. I shall there- 

 fore presume that we have to deal with colouring matters separated 

 from any others that differ materially in their solubility. 



There are few cases in which the mixed nature of a coloured 

 aqueous solution can be more easily ascertained than when the con- 

 stituents differ so much in character that the addition of some 

 reagent will more or less completely destroy the spectrum of one 

 without having any effect on that of the other. For this purpose 

 no substance is superior to sulphite of soda. Without producing 

 any real decomposition, this almost entirely removes the detached 

 absorption at the red end of the spectrum- of certain colours, but has 

 no effect whatever on that of others. In the case of some colours 

 it thus acts when the solution contains excess of ammonia, but in 

 the case of others it has then little or no action, but removes the 

 absorption when the solution contains excess of such a moderately 

 weak acid as citric. The application of this principle to the exami- 

 nation of the colouring matters of plants is extremely simple and 

 satisfactory. If, for example, we obtain the red colouring matter 

 from the leaf-stalks of the common red rhubarb, or from the petals 

 of the crimson Calceolaria, as described in my paper " On the 

 Colouring Matter of Leaves," * and to its solution in water add a 

 little citric acid, the colour is clear, deep pink, made almost or quite 

 colourless by the addition of a little sulphite of soda, which change 

 in colour is due to the complete removal of the broad absorption- 

 band in the green. If, however, this same red colouring matter is 

 mixed with some of the yellow substances found in many leaves, 

 the spectrum of the said solution, in addition to the same broad 

 absorption-band in the green, shows absorption at the blue end ; 

 and on adding the sulphite, this yeUow colour, which absorbs the 

 blue end, may be distinctly seen alone. A similar plan may be 

 adopted in the case of some substances which are changed when the 

 solution contains sHght excess of ammonia. I could not give a 

 more striking example than that of a solution of blood mixed with 

 so much magenta that the absorption-bands of the blood are com- 

 pletely hidden. On adding a small quantity of sulphite, they are 

 seen as well as if no impurity had been present. It may, however, 

 happen that there is a mixture of two colours, both of which are 

 acted upon in the same manner by the sulphite, but one more 

 rapidly than the other. Thus, for example, the colouring matter of 

 the petals of the blue Lobelia gives a spectrum with two remarkably 

 distinct absorption-bands. So also does that of the petals of the 



* ' Quarterly Juurntil of Microscopical Science' (New Ser.), xi., 1871, pp. 215- 

 234. 



