Experiments on Indigo. 277 



residue was in one case collected, weighing 47 grains. With 

 another liquor it was 20 grains heavier; and in the experiment 

 made on a small scale with the leaves, where the fermentation 

 continued for 24 hours, the same weight of mother liquor 

 yielded 246 grains of residue. As this element of the plant 

 seems to be so variable, and as it must evidently produce much 

 influence in the manufacture, it is probably one of the chief 

 causes of the nicety usual in timing the fermentation, and of 

 the variable tendency of the indigo to precipitate in the beating 

 vat. The dried extract has a dark-brown colour and vitreous 

 surface, similar to that of dried gluten, or the brown extract of 

 toast-water : it has a peculiar, not unpleasant smell, and rather 

 a bitter taste : it deliquesces in a damp atmosphere, and dis- 

 solves in water, which it dyes of a deep brown or bistre. 

 Although the original mother liquor is exceedingly liable to 

 ferment and become putrid, the brown matter undergoes no 

 change, either when kept dry, or in a moistened state. 



It is precipitated from its aqueous solution by potash, soda, 

 ammonia, lime, and their carbonates ; by infusion of galls, 

 acetate of lead, and nitrate of silver. The acids, and prussiate 

 of potash, did not affect it ; but the action of re-agents was not 

 investigated in detail. 



Sufficient has been adduced to prove, that whenever lime or 

 the alkalies are used in the vat, the indigo must be adulterated, 

 more or less, with this substance ; and I suspect that the 

 brown and green matters, separated by Chevreul and others, 

 in their analysis of indigo, are attributable to this source. In 

 an experimental way, indigo of a true dark green colour was 

 easily collected, which weighed more than twice as much as 

 the blue colour alone : it was also harder and more compact, 

 and more liable to shrink and crack than the pure indigo cake; 

 for besides the impurity just described, there was always found 

 a large proportion of earthy residue, on burning indigo, where 

 a precipitant was used, than where it was not ; the quantity 

 was even as much as double or triple in amount. 



Carbonate of lime, alumine, and oxide of iron, are the chief 

 ingredients of the earthy residue, and I have seen them vary 

 from five to nearly fifty per cent, of the indigo : the last, as 

 may be supposed, was merely refuse, and quite unsaleable. 



