FORMATION OF INDIGO-BLUE. 179 



quirements. If, however, we consult the authors who have 

 written on this subject, and the chemists who have endea- 

 voured to elucidate it, we shall obtain very unsatisfactory- 

 replies to our inquiries. The views entertained on these 

 different points are either mere surmises, or they are con- 

 clusions founded on a limited number of frequently imperfect 

 experiments. The chief cause of our ignorance on these 

 questions is, probably, that the process of manufacturing 

 indigo is one carried on, not in the more highly civilized 

 regions, but in remote parts of the world, and we are con- 

 sequently obliged to rely for our knowledge concerning it 

 chiefly on the accounts of travellers, who are usually pos- 

 sessed of merely general information, or of the manufacturers 

 themselves, who are far from competent to give an opinion on 

 a complex organo-chemical process. 



Fourcroy, according to Robiquet, considered the formation 

 of indigo-blue to be a result of the process of fermentation 

 employed in its preparation. 



Roxburgh, who is the earliest authority I have had an 

 opportunity of consulting on this subject, states that his pre- 

 decessor, De Cossigny, whose work on the manufacture of 

 indigo, published 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 volatized by the agitation process. Roxburgh, 

 who like De Cossigny, was one of the few possessing special 

 chemical information who have examined the process of 

 manufacturing indigo from the indigoferae on the spot, con- 

 cluded, 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 extrication from 

 the leaves ; that the carbonic acid is the agent whereby it is 

 probably extracted and kept dissolved ; that ammonia is not 

 formed during the process; that the use of alkalies is to 

 destroy the attraction between the base and the carbonic 



