442 Mr. H. C. Sorby on Analysis of Animal and Vegetable [Apr. 11, 



These facts will be better understood by means of the following draw- 

 ing :— 



Eed end. Fig. 2. Blue end. 



1. Citric acid. 



2. Neutral. 



3. Ammonia. 



Whence it will be seen that citric acid raises and greatly increases the cen- 

 tral absorption, and ammonia lowers and also increases it. At the same 

 time the absorption at the extreme blue end of the spectrum is raised by 

 the acid almost to beyond the range of vision, but lowered to the centre 

 of the spectrum by ammonia. Acids and alkalies of intermediate character, 

 as, for example, boracic acid and bicarbonate of ammonia, produce inter- 

 mediate effects. These well-known phenomena may be looked upon as 

 typical of acids and alkalies, but the extent of their action varies for each 

 particular colouring-matter, so that in some cases^it is slight, and some- 

 times 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 spectrum, 

 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 several, as commonly adopted by 

 chemists, each of very limited application ; 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 



