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



14. Absorption-hands. 



Though acids and alkaUes thus, to a greater or less extent, alter the 

 position of the general absorption, they act very differently on the special, 

 local absorption to which it is very convenient to restrict the term absorp- 

 tion-bands." Since I shall often have to speak of their being at equal in- 

 tervals, it would be well to say that I have found it convenient to construct 

 a wedge-shaped piece of quartz, cut parallel to the axis of the crystal, and 

 to use it along with two Nicol's prisms in such a manner that the spectrum 

 may be divided into any requisite number of equal portions, by inter- 

 ference-bands situated in any requisite position. This of course avoids 

 the errors which so often happen when we compare together measure- 

 ments that cannot be made with very great accuracy. 



As an excellent illustration I select the colouring-matter of Alkanet-root 

 {Anchusa tinctoria). It is insoluble in water, but is easily dissolved by 

 alcohol, even when much diluted with water, and gives a clear pink solu- 

 tion. The spectrum is nearly the same when the colour is dissolved in 

 absolute alcohol, as w^hen much water is present, only each of the absorp- 

 tion-bands is situated rather higher. Thus, taking the centres of the bands, 

 we have — 



b. c. d. 



Absolute alcohol 4| 5|- 7f 



Very dilute alcohol 4^ 5| 7^ 



The general spectrum of the solution in dilute alcohol will be best un- 

 derstood from the following figure. No. 1 : — 



Fig. 3. 



Red end. a b c d Blue end. 



1. Neutral or somewhat acid. 



2. A little carbonate of soda. 



3. More carbonate of soda. 



a b 



Acids produce no important change, and the effect of alkalies is best 

 seen by gradually adding carbonate of soda. This alters the colour to a 

 more and more blue-purple, and the spectrum is changed in the manner 

 shown in fig. 3. The three bands seen in the neutral solution may be 

 referred to as b, c, and and their centres occur at equal intervals of 

 about Ig. When enough carbonate of soda has been added to make it 

 slightly purple, a fourth band, a, makes its appearance, separated from b by 

 the same equal interval of 1|, whilst the other bands remain in the same 



