Experiments on Indigo. 207 



assumed the nature of a peroxide, for it is very unalterable, 

 quite insoluble in water, alcohol, ether, saline infusions, alkalies, 

 and dilute acids. Concentrated sulphuric acid alone acts as a 

 solvent, without changing its nature. Nitric acid converts it 

 into a yellow bitter principle. Acetic acid by degrees deoxy- 

 dizes it. Chlorine also destroys its colour immediately. 



In saying that concentrated sulphuric acid effects no change 

 upon indigo, I am not strictly correct. It does not destroy the 

 colouring matter, or cause a decomposition, as would be the 

 case with most other vegetable compounds ; but the researches 

 of Crum and Berzelius have rendered it. probable, that three 

 different modifications of indigo may be brought about by the 

 agency of this acid, differing from one another in the quality of 

 oxygen or of water in their composition. 



The three modifications are thus distinguished : — 



1. The Pure Indigo, which is obtained by sublimation from 

 the crude cakes at a temperature of 550°. This is crystal- 

 lized in long flat prismatic needles ; has a copper colour by 

 reflected, and a fine blue by transmitted light. It sublimes 

 entirely at the temperature stated, without residue: its specific 

 gravity is 1.35. 



2. Cerulhiy or Saxon Blue, is obtained by digesting the 

 former substance in sulphuric acid : it is supposed by Crum to 

 have lost its water of composition thereby. It is soluble in 

 water, but is precipitated by most of the neutral salts. 



3. Phenicin, or Purple Indigo, is obtained by suddenly 

 diluting the sulphuric acid which has begun to dissolve indigo. 

 It separates as an insoluble powder, which when filtered and 

 washed, is soluble in pure boiling water, and may be procured 

 as a precipitate again by the addition of any neutral salt. 

 Phenicin is supposed by Berzelius to be an intermediate state 

 between soluble and insoluble indigo ; but Crum asserts that 

 100 parts of indigo will yield 120 of phenicin. 



The indigo of commerce most probably ranks under one of 

 the two latter denominations, or, perhaps, both of them ; for 

 there can be little doubt that some new combination of elements 

 takes place in the sublimation of the (i pure" or •' crystallized 

 indigo," since with the utmost care, not more than one-fifth of 

 the weight of crystals can be procured ; and, during their forma- 



oct.— dec 1829. T 



