INDIGO. <)13 



are cut before flowering, and allowed to steep for nine or twelve 

 hours in a vat, covered with water, in which fermentation 

 occurs with the evolution of carbonic acid and hydrogen gas. 

 A yellow coloured liquor is drawn off into another vat, in which 

 it is beat and stirred till it acquires a blue colour, and the indigo 

 precipitates. It is then drained on calico, placed on proper 

 frames, and strongly pressed by means 1 of screws, cut into 

 cakes of the proper size, and dried.* The plant thus appears 

 to contain the indigo in a very different state from that in 

 which it is ultimately obtained. It is not certain that it can be 

 extracted from the indigofera without fermentation ; but Che- 

 vreul has shown that it may be extracted from pastel, by treating 

 the latter with hot water free from oxygen, and that the yellow 

 solution thus obtained became blue, and deposited indigo. 



The indigo of commerce is of a deep blue, inclining to black ; 

 its fracture is earthy and dull, but becomes of a coppery red 

 when rubbed with a hard body, and the more brilliant and like 

 copper the colour developed by friction, the purer is the indigo 

 considered. It is far from being a pure substance, rarely con- 

 taining half its weight of blue colouring matter, and often much 

 less. Berzelius separated from it; 1, gluten of indigo, by 

 digesting indigo in fine powder with a dilute acid, which also 

 dissolves some salts of lime and magnesia ; 2, a principle which 

 he has named indigo brown, by means of a concentrated solution 

 of caustic potash gently heated; and 3, indigo red, by afterwards 

 boiling the indigo repeatedly with alcohol of density 0.830. 

 Indigo blue remains, but is not yet absolutely free from foreign 

 matter. 



To obtain it pure, recourse is had to the solution of indigo 

 in the ordinary indigo vat, or the indigo purified by the pre- 

 ceding processes may be dissolved by imitating on a small scale 

 the preparation of the dyer. One part of indigo in an impal- 

 pable powder and 2 parts of quick-lime, recently slaked, are 

 mixed, and introduced into a well stopt bottle with about 150 

 parts of water. To this is then added two thirds of the weight 

 of the lime of crystallized protosulphate of iron in fine powder, 

 or dissolved in a small quantity of hot water. The bottle then 

 being completely full and well closed, is agitated occasionally 



* Dr. Thomson's Organic Chemistry, Vegetables, p. 369. 



2 o o 



