276 Experiments on Indigo, 



very possible that the other ingredients may also absorb oxygen, 

 especially when the putrid fermentation commences. 



It is evident, however, that a further analysis of pure indigo 

 in the dry way is a very desirable object. It seems to have 

 escaped the attention of Gay-Lussac and Thenard, and latterly 

 of Marcet and of Dr. Prout, in their Researches upon the 

 Constituents of Vegetable Compounds. 



It is worth while in this place to mention a fact observed in 

 the course of these experiments, namely, that where a slight 

 excess of potash is mixed with the vat liquid, the indigo formed 

 remains in solution, and passes through the filter with ease, 

 leaving the precipitated extractive matter behind. This blue 

 solution will keep for any time, and does not deposit its indigo 

 even in the open air ; but as soon as the alkali is saturated by 

 an acid, it immediately precipitates, leaving the liquid colour- 

 less. If the dose of potash be not sufficient to throw down all 

 the yellow matter, the solution retains a green colour, and cloth 

 dipped into it takes a green dye. When hung up to dry, how- 

 ever, the mixed action becomes evident; for the dye remains 

 fixed in the lower part of the cloth, while the yellow, more 

 perfectly dissolved, spreads with the liquid to the upper part 

 which was not dyed. There is no reason to believe, in this case, 

 that the air changed the colour from green to blue, although 

 such may be true of the dyer's vat-liquid. 



The quantity of indigo deposited per se from 1000 parts of 

 yellow liquid of the specific gravity 1003.1, varied but little, 

 and may be estimated at 0.75 parts. In practice, a vat of 637 

 cubic feet is considered to yield a good produce at 16 seers, 

 which is as nearly as possible 0.75 per 1000 of liquid. The 

 common produce of the vats in this part of the country does not 

 exceed 0.5, or one five thousandth of their weight of indigo. 



But when potash, lime, or other precipitants are employed, 

 the weight of the indigo is much increased ; not, as experi- 

 ment proves, from an increase of the actual produce, nor from 

 any union of the precipitant with the indigo, but from its caus- 

 ing the deposition of another matter, to which I have given the 

 name of yellow or brown extract. 



To obtain this yellow extract in an insulated state, 10,000 

 grains of mother liquid were evaporated to dryness : a solid 



