1867.] Colouring-matters by the Spectrum Microscope. 445 



position as at first, only modified in intensity. The band a becomes 

 darker and darker as more carbonate is added, until, when the solution is 

 a fine purple, it is as dark as the others (see No. 2) ; and on adding^more 

 carbonate it becomes still darker, and the bands c and d more faint, until 

 the solution is a purple-blue ; and the spectrum has only the two well- 

 marked bands a and b, shown by No. 3. 



The bright blue colouring-matter of the flowers of Lobelia speciosa 

 gives, when neutral, almost exactly the same spectrum as that of 

 Alkanet-root when alkaline, No. 3, having two well-marked absorption- 

 bands, whose centres are at 2| and 4^ ; and on adding carbonate of soda, 

 the upper one is gradually removed, and the centre of the lower is de- 

 pressed to near 2\. More or less similar results occur in the case of many 

 other blue colouring-matters ; and on adding a slight excess of acid the 

 general absorption is raised, and other bands may be developed higher up,, 

 at equal intervals ; but when a large excess has been added, they are lost 

 in a strong general absorption. Too strong an alkali may also destroy 

 narrow bands in a similar manner, as is well seen in the case of Brazil- 

 wood. The neutral aqueous solution shows an absorption-band, made far 

 more distinct by the addition of bicarbonate of ammonia, which makes it 

 phik and very fluorescent. The spectrum is then 



4|-5| 7; 



but on adding excess of ammonia the solution ceases to be fluorescent, the 

 narrow absorption-band is lost, and the spectrum becomes 



3i.-4:i— 8-..9i. 

 Ammonia does not produce this effect when the colour is dissolved in 

 alcohol, the solution remaining fluorescent and still giving the narrow 

 band ; and, as a general rule, that solvent greatly impedes or entirely pre- 

 vents such changes, and on this account almost invariably shows absorp- 

 tion-bands to the greatest advantage. 



"We therefore see from the above examples that the absorption follows 

 the general rule, and is raised by acids, and depressed by alkalies ; but this 

 only applies to the absorption when viewed as a whole, and not to the 

 separate bands ; for those reagents change their intensity, but not their 

 position. In some cases, indeed, their position is slightly altered, so that 

 perhaps it would be more correct to say that acids and alkalies may raise 

 and depress the general absorption to an extent equal to a considerable 

 fraction of its own great breadth ; whereas they either do not change the 

 position of narrow absorption-bands, or merely raise and depress them by 

 a fraction of their own narrow width. It is their very definite position 

 that makes them so useful in this method of analysis. 



Unfortunately I have not hitherto been able to find a sufficient number 

 of colouring-matters giving rise to three or more well-marked absorption- 

 bands, to warrant a general conclusion ; and therefore it is perhaps prema- 

 ture to conclude that their centres always occur at equal intervals. At 



