86 Dr. Schunck on the Formation of Indigo-bhe. 



a dark colour, while the surface becomes covered with a blue 

 pellicle. The deposit is found to consist principally of indigo- 

 blue with a httlc indirubine, and a trace of other products of 

 decomposition. The filtered liquid, on being boiled, becomes 

 muddy and deposits some brown fiocks, which contain no indigo- 

 blue. The quantity of indigo-blue formed by the action of 

 nitric acid seems indeed to be comparatively larger than when 

 sulphuric or muriatic acid is employed. It is hardly necessary 

 to add, that if this be really the case, it cannot be ascribed to 

 any oxidizing effect produced by the acid. A watery solution of 

 indican, on being mixed with oxalic or tartaric acid and left to 

 stand, yields a dark blue or purple deposit, consisting of indigo- 

 blue and indirubine, which, when oxalic acid is ein ployed, arc 

 remarkably free from other products of decomposition. The li- 

 quid filtered from this deposit yields in either ease, when boiled, 

 a few more fiocks, and after being filtered, mixed with sul})huric 

 acid and boiled again, it gives an additional quantity. These 

 flocks contain indu-ubine and indiretinc, but no indigo-blue. 

 Even acetic acid produces a slight effect on indican. On adding 

 that acid to a watery solution of the latter, the mixture deposits, 

 on standing, some dark flocks, consisting of indigo-blue and 

 indirubine, but their quantity is trifling. 



A more minute examination of this process of decomposition 

 showed that it was more complicated, and that the products 

 formed by it were more numerous than I had at first imagined. 

 The products of decomposition which I have observed arc of three 

 kinds. The first are insoluble in water, and are deposited in the 

 shape of powder or flocks from the acid liquid ; the second 

 remain dissolved in the latter; the third arc volatile, and are ob- 

 tained by distilling the liquid either whilst the action of the acid 

 is proceeding or after it has ceased. For the purpose of pre- 

 paring these various products, I found it to be necessary to ob- 

 tain indican in a state of absolute purity by successive solution 

 in alcohol, water and tether; for though some indican is always 

 decomposed when its watery solution is evaporated, the substances 

 into which it is thereby converted afford, by decomposition with 

 acids, products which do not differ in their nature, but only in 

 their relative proportions from those which are formed when 

 perfectly pure indican is employed. I therefore contented myself 

 with extracting the dry leaves of the woad plant with cold alcohol, 

 evaporating the extract in the apparatus above described, adding 

 water to the residue and filtering. The solution of indican thus 

 obtained was mixed with a considerable quantity of sulphm-ic 

 acid, and the green fatty matter precipitated by the acid was 

 separated by filtration. The action of the acid passed, as I in- 

 variably observed, through two distinct stages, and I found it 



