1867.] Colouring -matters by the Spectrum Microscope. 447 



In spectra showing one absorption-band, there is very commonly a general 

 absorption, extending from it towards the blue end ; whereas it so seldom 

 extends towards the red end that it is doubtful if it ever occurs in sub- 

 stances, undoubtedly not a mixture of two colours. It can, however, so 

 easily occur in mixed colours, that any substance giving rise to such a 

 spectrum is probably a mixture. Many illustrations might be given, but I 

 will select Brazil-wood, and the same artificially mixed with the colour of 

 beet-root. Adding bicarbonate of ammonia to both, we have — 



Brazil-wood alone 4| — 5| ... 7 



Brazil-wood and beet-root 3^...4f — 5| 8 



Here, then, the shading below the absorption-band from 3| to 4| is evidence 

 of the second colour, and if such a mixture had occurred naturally its 

 mixed character might easily have been overlooked. I have found many 

 cases similar to this, and had proved that they were mixtures, before I was 

 aware that the spectra indicated it. If these very common facts turn out 

 to be general laws, we might thus detect at once the presence of as many 

 as three different substances, or at all events might learn what further ex- 

 amination was desirable. 



15. Sulphite of Soda. 

 Sulphite of soda is a most valuable reagent, and its action very remark- 

 able. It enables us to divide colours into three groups, according as it 

 produces a change in an ammoniacal, or acid solution, or in neither. The 

 action is related in a very simple manner to the spectra. Having added an 

 excess of ammonia, there may be a well-marked broad absorption over more 

 or less of the red, orange, yellow, and upper green ; and above this a 

 clear transparent space, limited by a variable amount of absorption, ex- 

 tending downwards from the extreme blue. Fig. 2 will illustrate my 

 meaning. In the case of one group of colours, the addition of sulphite of 

 soda almost immediately removes the detached, broad absorption in the 

 lower part of the spectrum, but leaves that at the blue end quite unchanged, 

 or only slightly modified by the solution being made more alkaline. If, 

 then, as in the case of magenta, there is no absorption at the blue end, 

 sulphite of soda makes the solution quite colourless ; whereas if the blues 

 are absorbed, as in the case of the ammoniacal solutions of the colour of red 

 roses, and some species of Bianthus, it changes the colour from green to 

 yellow. If the absorption extends continuously down from the extreme 

 blue to the orange, as often happens when ammonia is added to yellow 

 colours, sulphite of soda produces no change. It is only when there is a 

 more or less perfect division between the upper and lower absorption, that 

 it has any effect ; and then it simply and entirely removes the lower absorp- 

 tion. Some colours are changed immediately, even when a very small 

 quantity of sulphite is added ; but others require more and change 

 gradually, though still very soon. 



