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



indigo-blue whenever indican is decomposed by acids. Never- 

 theless, the quantitj' of indigo-blue produced is always relatively 

 larger when the indican is pure than when the latter has begun 

 to change. Of the two colouring matters, the indigo-blue is 

 always the first to be formed. If sulphuric or muriatic acid be 

 added to a solution of indican in the cold, and the mixture be 

 allowed to stand in the cold for some time, a slight precipitate 

 is gradually deposited which consists almost entirely of indigo- 

 blue. It is only after boiling for some time that the formation 

 of indirubine commences, when the colour of the liquid changes 

 from blue to purple. The constant occurrence of a red colouring 

 matter both in indigo and in the indigo-bearing plants along 

 with indigo-blue — a fact which has been repeatedly observed, — 

 has led chemists to suspect that there must be some necessary con- 

 nexion between the two. From the experiments just described, 

 it follows that, in the case of woad at least, they are both pro- 

 ducts of decomposition of one substance. If nitric acid be added 

 to a wateiy solution of indican, a slight deposit of indigo-blue is 

 formed, which of course disappears immediately on heating the 

 liquid. 



There is another very remarkable property of indican which I 

 have to describe, a property, the knowledge of which will pro- 

 bably throw great light on the process of manufacturing indigo. 

 If indican in the form of syrup, as obtained by evaporation of 

 the wateiy solution, be heated for some time in the water-bath, 

 or if its watery solution be boiled, or even moderately heated, it 

 undergoes a complete metamorphosis. If the solution be now 

 evaporated, it leaves a yellow syrupy residue, not to be distin- 

 guished in appearance from indican itself. It will be found, 

 however, to have become insoluble in aether, and not easily soluble 

 in alcohol. If aether be added to its solution in alcohol, the 

 solution becomes milky, and deposits oily drops which collect at 

 the bottom of the vessel to a yellow or brown syrup, the un- 

 changed indican, if there be any present, remaining dissolved in 

 the aether. In its other outward properties it has not undergone 

 any marked change. In the next stage of the process the in- 

 dican acquires a brown colour, and becomes quite insoluble in 

 cold alcohol ; but it still dissolves, though with difficulty, in 

 boiling alcohol. By continuing the process the substance 

 acquires a dark brown colour, and its watery solution now gives 

 a copious precipitate with acetate of lead. As soon as the indican 

 has entered even on the first stage of this process of change, it 

 ceases to give the least trace of indigo-blue with acids. A short 

 period occirrs at the commencement of the process, during which 

 the watery solution, when boiled with sulphuric acid, deposits 

 purple flocks consisting of indirubine only. Afterwards, how- 



