Dr. Schunck on the Formation of Indigo-blue. 77 



a very decided conviction that indigo-blue pre-exists in the Poly- 

 gonum iinctorium, but not in a free state ; that it is combined 

 with some organic substance or substances, which render it 

 soluble in water, ?ether and alcohol; and that it requires the 

 operation of potent agencies in order to destroy this combination 

 and set the indigo at liberty. Osmin Hervey*, in a memoir on 

 the Polygonum tinctorium, which in some parts is rather obscure, 

 inferred from his experiments: — 1. That indigotine exists in 

 the leaves of this plant in a state of combination with a resin. 

 3. That this natural compound of indigo and resin contains both 

 white indigo and blue indigo, and of the latter a larger propor- 

 tion the older the leaves are. 3. That by the influence of cer- 

 tain organic substances, the indigo-blue is again reduced to the 

 colourless state, if the solution be effected by means of water, 

 without any destruction of the natural compound taking place. 

 Girardin and Preisserf again returned to Chevreul's view, that 

 the colouring matter is contained in the leaves of this plant in 

 the form of reduced indigo. Since the publication of these trea- 

 tises, no new ideas have, as far as I know, been promulgated by 

 chemists in reference to this subject J. 



It will be seen that the opinions of the chemists which I have 

 just shortly reviewed are of three kinds, and may be stated as 

 follows : — 1. Indigo-blue exists ready formed in the plants from 

 which it is derived. 2. It is contained in these plants in the 

 form of reduced indigo. 3. It does not pre-exist in the vege- 

 table, but is formed subsequently to the extraction of the latter 

 by means of a process of fermentation, a process which manifests 

 itself by the evolution of gases of various kinds. To each of these 

 viewsvery strong objections maybe I'aised. If the colouring matter 

 is formed at once in the plant, it is difficult to conceive by what 

 means it comes to be dissolved by water, for no combination of 

 indigo-blue with any organic substance can be produced which 

 is soluble in vrater. If to this it be objected, that a compound 

 of this nature is produced by the plant, and cannot after decom- 

 position be reproduced, then it is at once admitted that indigo- 



* Journal de Pharmacie, vol. xxvi. p. 290. f I'jid- P- 344. 



X I cfiimot refrain from expressin"; on the present occasion my regret, 

 wliich is probably shared by many others, at the want of a general chemical 

 bibliography, comprising references to all the known works, treatises, pajjcrs, 

 &c. on chemical subjects written since the commencement of the modern 

 a^ra in chemistry. In searching for the authorities referred to in this pajier, 

 1 have felt this want verj' sensibly. It is with some diificulty that the 

 mere names of all the works and memoirs relating to any special branch of 

 the science, jiarticularly such as have fallen into oblivion, arc discovered. 

 The only attempt to supjdy this deficiency, and that only in regard to one 

 department of chemistry, is 'WoWsQuelleiilileratur der Organiscken C/iemie. 

 Halle, 1845. 



