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FORMATION OF INDIGO-BLUE. 1 99 



between two watch-glasses, it produces on the upper glass 

 a sublimate consisting of beautiful purple needles which dis- 

 in boiling alcohol forming a line 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 

 impurities possesses the property of crystallising. 



The quantity of indirubine which I obtained, even when 

 operating on large quantities of indican, was so exceedingly 

 I, 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 examina- 

 of its properties and composition. Some time before 

 commencing my investigation of the woad plant I had ob- 

 tained from India a quantity of the dried 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 examination led to no definite results, 

 the process of fermentation by which the colouring matter is 

 formed having probably been already completed, and I there- 

 fore laid them aside. Their peculiar greenish-purple colour 

 and the glaucous appearance of their surface which resembled 

 that of glazed green tea, showed however that they must con- 

 . ready formed, some peculiar species of colouring matter. 

 I was therefore induced to examine them again, and this ex- 

 amination led to the conclusion that their colour was caused 

 by a thin coating of a substance, which was, there could be 

 doubt, identical with indirubine. This substance was 

 isolated by the following process. 



Having prepared a liquid containing protochloride of tin 



dissolved iu an excess of caustic soda, the leaves were im- 



led in it while boiling. The boiling was continued until 



tin* leaves had lost their purple tinge and become pale green. 



