and Vegetable Colouring-matters by the Spectrum Microscope. 153 



is unlike both. At all events the facts clearly show that a solvent 

 has a most important action on the ultimate particles of the sub- 

 stance in solution, since it may produce a greater change in optical 

 phenomena than even chemical combination. Undistilled hard water 

 may act like a weak alkali. 



1 1 . Acids and Alkalies. 



As far as optical phenomena are concerned, there is no absolute 

 division between acids and alkalies ; for we have every connecting 

 link from the strongest acids to the strongest alkalies. In order to 

 understand their action, it is most essential to distinguish between 

 what may be called " general absorption" and " local absorption- 

 bands." There may, perhaps, be no absolute line of division ; but 

 when seen to advantage they are affected in such a different manner 

 that it is desirable to treat of each separately. 



12. General Absorption. 



As a good example of simple general absorption, we may take the 

 crimson colouring-matter of the common Wallflower (Cheiranthus 

 Cheiri) , which is soluble in water, and, along with a yellow only 

 soluble in alcohol, gives rise to the varied colours of the flowers. 



When neutral, it is crimson. . . . 2|.... 7 10 . . 1 1 



With ammonia, fine green .... If — \\-. .6 7. . 8-- 9 — 

 With citric acid, deep pink .... 3^.-4^- — 6-..8| 11... 



These facts will be better understood by means of the following 

 drawing : — 



Eed end. Fig. 2. Blue end- 



1. Citric acid. 



2. Neutral. 



3. Ammonia. 







... 

 % 



A 



; 1 



jiill 



itl'jijij . 



Ml 



1101 1 



1 



i'i'i'i pi ' 1 n nUnHiH^H 



!i!|iifj:|l||!l|||MH 



whence it will be seen that citric acid raises and greatly increases the 

 central absorption, and ammonia lowers and also increases it. At 

 the same time the absorption at the extreme blue end of the spec- 

 trum is raised by the acid almost to beyond the range of vision, but 

 lowered to the centre of the spectrum by ammonia. Acids and alka- 

 lies of intermediate character, as, for example, boracic acid and bicar- 

 bonate of ammonia, produce intermediate 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- 



