180 MR. E. SCHUNCK ON THE 



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 

 attention to the fact, that it is possible to obtain indigo by 

 merely treating the plant with hot water, and then agitating 

 the infusion with air, from which it follows, that fermentation 

 is not an absolutely essential condition of the formation of 

 indigo-blue. 



Chevreul,t who was the first chemist of any eminence who 

 examined the indigo-bearing plants and their constituents, 

 inferred from his analyses of the Isatis tinctoria and Indigo- 

 fera 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 Michelotti,t 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 

 WoadjII which enunciates ideas on this subject, far more nearly 

 approaching the truth, than those either of his predecessors 



* Transactions of the Society of Arts, Vol. XXVIII. 



t Annales de Chimie, 1 Ser., T. LXVI., p. 5; T. LXVIIl., p. 284. 



X Journal de Physique. Avril, 1813. 



II Traite sur le Pastel et I'Extraction de son Indigo. Paris, 1813. 



