224 DR. EDWARD SCHUNCK ON INDlGO-BLUE. 



filtered, it is evaporated^ when it leaves a sirupy residue 

 which may be treated with ether as before. 



The indigo-producing body thus obtained is, if it be per- 

 mitted to draw a certain conclusion from mere qualitative 

 reactions, identical with the indican of Isatis tinctoria. 

 Its appearance is that of a yellow transparent sirup, show- 

 ing no tendency to assume a crystalline form. It is solu- 

 ble in water, alcohol, and ether. The watery solution has 

 a more or less acid reaction. It becomes of a deep yellow 

 colour on the addition of caustic alkali, and gives with 

 basic lead acetate a light-yellow precipitate. When the 

 watery solution is mixed with a little sulphuric or hydrochlo- 

 ric acid and left to stand for some time, the surface of the 

 liquid becomes covered with a film of indigo-blue, a deposit 

 of the same substance being usually formed at the bottom. 

 The filtered liquid shows, when tested with a salt of copper 

 and an excess of caustic alkali, the well-known reaction of 

 glucose. If, however, the watery solution is left to stand 

 at the ordinary temperature for a considerable time, or if 

 it is simply boiled for some time, or if it is mixed with 

 caustic alkali and left for a short time, it no longer yields 

 indigo-blue on the addition of acid. This is probably due, 

 as I have shown to be the case with indican, to a molecular 

 change, resulting, when completed, in the formation of a 

 body which, when decomposed with acids, yields indirubine 

 and brown resinous substances in place of indigo-blue. 

 When a large quantity of watery solution is mixed with 

 acid and left to stand, a j^ortion of the substances under- 

 goes, it seems, the same change ; for the deposit formed 

 when operating with one litre or more of the solution 

 contains not only indigo-blue, but also indirubine, indi- 

 fulvine, and other products. The deposit, which in this 

 case is almost black, after being filtered off, washed, and 

 dried, is treated first m ith caustic alkali and then with cold 



