Dr. Schunck on the Formation of Indigo-blue. 75 



Roxburgh, who is the earliest authority I have had an oppor- 

 tunity of consulting on this subject, states that his predecessor, 

 De Cossigny, whose work ©n the manufacture of indigo, pub- 

 lished in the Mauritius, is very rare, was of opinion that volatile 

 alkali was the agent by which the colouring matter was extracted 

 from the plant and held in solution until volatilized by the agi- 

 tation process. Roxburgh, who, like De Cossigny, was one of 

 the few possessing special chemical information who have ex- 

 amined the process of manufacturing indigo from the Indigofera 

 on the spot, concluded, from his experiments, " that the indigo 

 plants contain only the base of the colour, which is naturally 

 green; that much carbonic acid is disengaged during its extri- 

 cation from the leaves; that the carbonic acid is the agent 

 whereby it is probably extracted and kept dissolved; that am- 

 monia is not formed during the process ; that the use of alkalies 

 is to destroy the attraction between the base and the carbonic 

 acid ; and that the vegetable base being thereby set at liberty, 

 combines with some colouring principle from the atmosphere, 

 forming therewith a coloured insoluble fecula, which falls to the 

 bottom and constitutes indigo*." Roxburgh first directed atten- 

 tion to the fact, that it is possible to obtain indigo by merely 

 treating the plant with hot water, and then agitating the infu- 

 sion with ail', from which it follows that fermentation is not an 

 absolutely essential condition of the formation of indigo-blue. 



Chevreulf, who was the first chemist of any eminence who 

 examined the indigo-bearing plants and their constituents, in- 

 ferred from his analyses of the Isatis tinctoria and Indigofera 

 anil, that these plants contain indigo in the white or reduced 

 state, in the same state in which it exists in the indigo vat ; that 

 in this state it is held in solution by the vegetable juices ; and 

 that when this solution is removed from the plant, it is converted 

 by the action of the atmospheric oxygen into indigo-blue. The 

 authority of so distinguished an investigator as Chevreul has had 

 great weight with chemists, and most persons have adopted his 

 view without question, though it is founded chiefly on the fact 

 of the colouring matter being deposited from a watery extract of 

 the plant, and of the only form in which it is known to be soluble 

 in water being that of reduced indigo. 



According to MichelottiJ, the extraction of indigo consists 

 simply in dissolving a compound of malic acid and indigo, which 

 is afterwards decomposed by the precipitants employed. 



A few years after the appearance of Chevreul's memoirs, 

 Giobert, Professor of Chemistry at Turin, published a work on 



* Transactions of the Society of Arts, vol. xxviii. 



t Ann. de Chim. ser. 1. vol. Ixvi. p. 5; vol. Ixviii. p. 284, 



X Journal de Physique, Avril 1B12. 



G2 



