1867.] Colouring-matters by the Spectrum Microscope. 443 



would appear to be exceptions, if we did not remember that waves of 

 light, or waves analogous to them, exist beyond the visible spectrum. 

 Thus, for example, when alkalies are added to the yellow solution of Brazil- 

 wood (CcBsalpinia crista), it is changed to pink, the absorption being so 

 much lowered that the blues are transmitted ; this clear space correspond- 

 ing to what was probably a clear space beyond the blues visible under 

 ordinary circumstances, but which would perhaps be seen, if examined in the 

 manner described by Stokes in his paper on the long spectrum of electric 

 light*. 



13. Fading of Solutions. 

 One striking peculiarity in the action of acids on the solutions of many 

 vegetable colours is, that, when they are in a particular state of acidity, 

 they fade to nearly or quite colourless, without there being any decompo- 

 sition. This is especially the case with pink colours dissolved in alcohol. 

 It occurs slightly with blue colours, and little, if at all, with yellows. The 

 aqueous solutions change much more slowly, but more and more rapidly 

 the more they are diluted ; and frequently attain a permanent depth of 

 colour, which is dark or pale according as the solution is strong or dilute. 

 Of course I here allude to the effect of the same total amount of colour, 

 and not to the different effect of the same quantity of a strong or dilute 

 solution. The alcoholic solutions obtained direct from the flowers often 

 fade so rapidly, and become so nearly colourless, that any one might easily 

 fancy that all the colour was lost by decomposition ; and an evaporating 

 dish containing it, might appear merely filled with brownish alcohol, and 

 yet on evaporation the whole dish might be covered with a fine deep colour. 

 The same change may occur over and over again, the deep-coloured solu- 

 tion first obtained soon fading, and the colour being restored by subsequent 

 evaporation. 



When such a colour is dissolved in a little water and added to alcohol in 

 an experiment tube, the colour may at first be very deep, but may fade so 

 rapidly that there is scarcely time to observe the spectrum before it passes 

 into that molecular state which does not absorb any of the rays of light. 

 The colouring-matter of the flowers of the red Salvia {S. splendens) is an 

 excellent example. Neutral solutions do not undergo this rapid change ; 

 a different condition of acidity is requisite for different colouring-mat- 

 ters ; and some do not change at all. A large excess of citric acid very 

 often restores the intensity of the colour ; and usually the absorption-bands 

 are seen to the greatest advantage when the solution is in that state which 

 rapidly fades ; and by adding too much colour and watching whilst it fades, 

 they may be seen and measured when at their best. This fading of a dark- 

 coloured solution must not be confounded with the change which takes 

 place on diluting some salts, as described by Dr. Gladstone in bis paper on 

 that subject t. 



* Phil. Trans. 1862, p. .599. 



t Quart. Journ. Chein. Soe. vol. xi. p. 36. 



