FORMATION OF INDIGO-BLUE. 201 



is carefully conducted only a trace of carbonaceous 

 t. It dissolves completely in concentrated sul- 

 phuric acid in the cold, forming a solution of a beautiful 

 de colour. This solution when heated does not become 

 black, but on the contrary rather paler and evolves only a 

 trace of sulphurous acid. When mixed with water it gives 

 bo precipitate and retains its fine purple colour, which does 

 not disappear or become weakened when the acid is neutralised 



carbonate of soda, but soon vanishes entirely when an 

 excess of caustic soda or ammonia is added. The solution in 

 sulphuric acid after dilution with water imparts a fine purple 

 colour to cotton, wool, and silk. When treated with nitric 

 acid of ordinary strength indirubine begins to dissolve 

 even in the cold and to a greater extent on the application 

 of heat, forming a purple solution, which on being further 

 heated becomes red and on boiling yellow. The whole of 

 the substance is dissolved without leaving any resinous resi- 

 due, such is always left when indigo-blue is treated with 

 nitric acid, forming a clear yellow solution. This solu- 

 tion leaves on evaporation a residue which dissolves only 

 partially in hot water. A brown resinous substance is left 

 ^solved by the latter, and the liquid filtered from this 

 is bright yellow and very bitter and yields when mixed 



carbonate of potash and evaporated, crystals, apparently 



i crate of potash, which detonate when heated. Very 

 dilut acid also decomposes and dissolves it on boil- 



ing, but its decomposition is effected with far more difficulty 

 that of indigo-blue by the same means. In like 

 manner a boiling solution of bichromate of potash, mixed 



sulphuric acid, which easily decomposes indigo-blue, 

 seems to have very little effect on it even when the boiling 

 is continued for a considerable time. When suspended 

 in water and exposed to the action of a stream of chlorine 

 gas it loses its colour very slowly and is changed into a 

 sinous substance containing chlorine which i 



