1867.] Colouring -matters- hy the spectrum Microscope. 441 



this list, 1 feel that I could not avoid mentioning some well-known facts 

 without breaking the thread of my argument ; and therefore 1 trust it may 

 not be thought out of place if I give a general account of the whole from the 

 particular point of view required by the subject more especially before us. 



The action of many reagents is so intimately related to different parts of 

 the spectrum, as to show that there must be some connexion between so- 

 called chemical reactions and optical phenomena. Not that their effect is 

 absolutely the same in the case of all coloured substances, but generally 

 only the extent differs, whilst the character of the change is uniform ; un- 

 less, indeed, decomposition take place ; and even then it has a tendency to 

 conform to a general law. 



10. Solvents. 



Water and alcohol are the most useful solvents, and the spectra of the 

 two solutions of the same substance often differ most strikingly ; in fact 

 they often behave in other respects as if they were solutions of different 

 substances. Sometimes the spectra are absolutely identical, but often well- 

 marked narrow absorption-bands are seen in the alcoholic solution, where 

 they are almost, or quite, invisible in the aqueous. Very commonly the 

 same bands are seen in both, but not exactly in the same place, alcohol some- 

 times raising them to a higher part of the spectrum, and sometimes de- 

 pressing them. Occasionally the spectrum of the dry material is like that 

 of the alcoholic solution, and unlike that of the aqueous, as if the dif- 

 ference were due to the presence of water, but in other cases it is unlike 

 both. At all events the facts clearly show that a solvent has a most im- 

 portant action on the ultimate particles of the substance 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 divi- 

 sion 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, per- 

 haps, 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 crim- 

 son colouring-matter of the common Wallflower (CAezV^w^Aw* 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 .... 1| — 41-.. 6 7. . 8-- 9 — 

 With citric acid, deep pink ... . 3|.-4^ — 6-. .81 11... 



