270 Experiments on Indigo. 



an excellent eudiometrical liquid, as it absorbs the oxygen of 

 the air with great avidity, without giving out any gas to com- 

 plicate the result. I had occasion to remark this circumstance 

 in the course of the experiments hereafter described ; and as 

 the solution may be prepared direct from the manufacturer's 

 vats in any quantity, it may prove a valuable discovery in the 

 laboratory as a useful substitute for Sir Humphry Davy's 

 eudiometrical liquid, composed of green sulphate of iron, satu- 

 rated with nitrous gas, which is difficult to preserve, and may 

 give out a little nitrogen in its operation. It remains, however, 

 to ascertain how long the alkaline solution of indigogene will 

 keep unaltered. 



Having thus briefly enumerated some of the principal pro- 

 perties of indigo, as a substance sui generis, (and there appears 

 to be no other vegetable product which resembles it in contain- 

 ing so much oxygen without being acid, and in the absence of 

 hydrogen, and the presence of azote,) I shall proceed to the 

 experiments on the process of manufacture, which form the 

 immediate subject of this paper. 



MANUFACTURE. 



" The plant, after being cut and carried to the factory, is 

 thrown into the steeper or superior vat, where it is pressed with 

 timbers adapted to the walls of the vat, to prevent its rising in 

 the water, which is then filled in from a reservoir, so as com- 

 pletely to cover the plant." 



During the fermentation which follows, bubbles of gas rise to 

 the surface, to ascertain the nature of which our first attention 

 was directed. 



The bubbles collected from the vats were found to contain 

 merely 7 or 8 per cent, of carbonic acid ; the remainder being- 

 common air, with from 12 to 18, instead of 21, per cent, of 

 oxygen. Earthen vessels were inverted, and left with their 

 mouth immersed all night in the vats ; but the air in them w 7 as 

 found unchanged. When bottles were partially filled with the 

 liquor of the vat, and well closed, the air, after a day, was 

 always found in them contaminated with about 18 per cent, of 

 carbonic acid, — the rest being common air, without diminution 

 of oxygen, excepting that portion due to the original air now 

 replaced by the carbonic acid gas. 



