4Si Dr. Schunck on the Formation of Indigo-blue. 



tion on cooling in long crystalline needles, which were red by 

 transmitted light. The alcoholic solution has a fine purple 

 colour. It is perfectly insoluble in alkaline liquids; but if it be 

 treated with a boiling solution of caustic soda, to which some 

 deoxidizing substance, such as protochloridc of tin or grape-sugar, 

 is added, it dissolves with ease, just as indigo-blue does under 

 the same circumstances, forming a solution from which it is 

 again deposited in purple flakes by the action of the atmospheric 

 oxygen. It dissolves in concentrated sulphuric acid in the cold, 

 forming a purple solution, which on the addition of water gives 

 a dark precipitate, the supernatant liquid remaining of a fine 

 jjurple colour. It is decomposed by boiling nitric acid. On 

 being heated between two watch-glasses, it produces on the 

 upper glass a sublimate consisting of beautiful purple needles 

 which dissolve in boiling alcohol, forming a fine purple solution, 

 which on cooling deposits crystalline needles. This sublimate 

 seems to consist, not of any product of decomposition formed by 

 heat, but of the substance itself, which when freed from all im- 

 purities possesses the property of crystallizing. 



The quantity of indirubine which I obtained, even when ope- 

 rating on large quantities of indican, was so exceedingly small, 

 that I was unable to apply any means for effecting its further 

 purification. 



I was, however, enabled by chance to procure from another 

 source a sufficient quantity of the substance for an examination 

 of its properties and composition. Some time before commen- 

 cing my investigation of the woad plant, I had obtained from 

 India a quantity of the (bied leaves of the Indigofera tinctoria, 

 for the purpose of ascertaining the state in which the colouring 

 matter is contained in them. Though the leaves reached me as 

 soon as possible after having been gathered and dried, their ex- 

 amination led to no definite results, the process of fermentation 

 by which the colouring matter is formed having probably been 

 already completed, and I therefore laid them aside. Their pecu- 

 liar greenish-purple colour and the glaucous appearance of their 

 surface, which resembled that of glazed green tea, showed how- 

 ever that they must contain, ready formed, some peculiar species 

 of colouring matter. I was therefore induced to examine them 

 again, and this examination led to the conclusion that their 

 colour was caused Ijy a thin coating of a substance which was, 

 there could be little doubt, identical with indirubine. This sub- 

 stance was isolated by the following process. 



Having prepared a liquid containing pi-otochloride of tin dis- 

 solved in an excess of caustic soda, tlie leaves wore immersed in 

 it while boiling. The boiling was continued until the leaves had 

 lost their purple tinge and become pale green. The green muddy 



