Dr. Schunck on the Formation of Indigo-blue. 81 



sisted of indigo-blue in a state of great puritj"-, as they dissolved 

 in boiling alcohol with a beautiful blue colour^ the alcohol depo- 

 siting en cooling crystalline scales, which were blue by trans- 

 mitted, and copper- coloured by reflected light. If, instead of 

 adding acid to any of the solutions yielding indigo-blue, caustic 

 soda was first adcled in excess and the solution was left for a few 

 moments, and then boiled with an excess of acid, it merely 

 became brown without depositing any indigo-blue. Having 

 taken some finely-choi)ped woad leaves, I pounded them in a 

 mortar with cold alcohol. On filtering, I obtained a clear green 

 solution, leaving on evaporation at a gentle heat a green syrup, 

 from which, on the addition of water, a quantity of chlorophyll 

 and fatty matter separated in drops. The watery solution, which 

 after filtration had only a yellowish tinge, on being boiled with 

 the addition of sulphuric acid, deposited a quantity of purple 

 flocks, which were treated, after filtration and washing with 

 water, with successive portions of boiling alcohol. The first 

 portions of the alcohol with which they were treated acquired a 

 beautiful purple colour, and the last portions a pure blue, each 

 portion depositing, on standing, some flocks of a fine blue colour. 

 The green mass insoluble in water contained no indigo-blue. 

 Having carefully dried a few woad leaves, I reduced them to 

 powder and then treated them in a bottle with cold aether. I 

 obtained a dark green solution, which after being filtered and 

 evaporated, spontaneously left a green syrupy residue, from 

 which water extracted, as in the preceding case, a substance 

 which, by the action of boiling sulphuric acid, yielded an abun- 

 dance of very pure indigo-blue. 



By these and similar simple and easily-performed experiments, 

 I was enabled to infer, with positive certainty, that the Isatis 

 tinctoria contains a substance easily soluble in hot and cold 

 water, alcohol and jether, which, by the action of strong mineral 

 acids, yields indigo-blue; that the formation of the colouring 

 matter from it can be efi'ected without the intervention of oxygen 

 or of alkalies ; and that the latter, indeed, if allowed to act on it 

 before the application of acid, entirely prevent the formation of 

 colouring matter, and it now only remained to separate it from 

 the other constituents of the plant and ascertain its properties 

 and composition. But, though I arrived at the conclusion just 

 stated without any great difficulty, I found that the isolation 

 and jn'eparation, in a state of purity, of tlie sitbstance whose 

 existence had been indicated by these experiments, constituted a 

 problem of no easy solution. I soon discovered that this body 

 is extremely liable to decomposition; so much so, as comj)letely 

 to justify the assertion of an author, who, in speaking of the 

 difficulties of the manufacture of indigo, says that " nothing is 



