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



matter, so that in some cases] it is slight, and sometimes neither 

 acids nor alkalies produce any effect. Their relative action on the 

 central and upper absorption also varies very greatly in different 

 colours. If there is no general absorption in the centre of the spec- 

 trum when the colour is neutral, but only an absorption at the blue 

 end, acids and alkalies act on it in precisely the same manner as on 

 the absorption at the blue end in the case just described, raising or 

 lowering it to an extent varying greatly according to the substance ; 

 and the same may be said of any general absorption at the red end. 

 The reverse certainly occurs when an acid is added to chromate of 

 potash, or excess of ammonia to a salt of copper ; and, according 

 to Stokes (Phil. Trans. 1862, p. 609), alkaloid bases usually show 

 this reverse action. It may depend on the different properties of 

 two distinct compounds, which does not appear to be the cause 

 of the phenomena now under consideration. In the case of all the 

 vegetable colouring- matters which I have examined, the tendency of 

 acids is to raise, and of alkalies to depress, the general absorption in 

 each part of the spectrum — the extent of this action depending on 

 the strength and quantity of the reagents, and on the nature of each 

 colouring-matter ; and thus we have a general rule, and not seve- 

 ral, as commonly adopted by chemists, each of very limited appli- 

 cation : — for instance, that vegetable blues are turned red by acids, 

 and green by alkalies ; and that vegetable yellows are reddened by 

 alkalies. I may here remark that some colours 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 

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

 much lowered that the blues are transmitted — this clear space cor- 

 responding 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 decomposition. 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 



* Phil. Trans. 1862, p. 599. 



