76 Dr. Schuuck on the Formation of Indigo-blue. 



Woacl*, wliicli enunciates ideas on this subject far more nearly 

 approaching the truth than those either of his predecessors or 

 successors on the same fiekl of investigation. The chief conclu- 

 sions at which he arrived, partly by experiment and partly by 

 reasoning, are contained in the following propositions: — 1. Indigo- 

 blue does not pre-exist in the plant, but is formed by the opera- 

 tions by means of which we believe it to be extracted. 2. There 

 exists in a small number of plants a peculiar principle, different 

 from all the known proximate constituents of ])lants, and which 

 has a tendeucy to be converted into indigo ; this principle may 

 be called indigogene. 3. This principle differs from indigo in 

 containing an excess of carbon, of which it loses a portion in 

 passing into the state of indigo-blue, by means of a small 

 quantity of oxygen which it takes up. 4. The loss of this por- 

 tion of carbon is caused by the latter undergoing combustion, 

 and being converted into carbonic acid. 5. It differs in its pro- 

 perties from common indigo in being colourless, in being soluble 

 in water, and by its greater combustibility, which causes it to 

 undergo spontaneous combustion at the ordinary temperature of 

 the atmos])here. 6. Its combustibility is enhanced by heat and 

 by combination with alkalies, especially lime; it is diminished 

 by the action of all acids, even carbonic acid. 



About the year 1839, the Polygonum tinctorium, an indigo- 

 bearing plant indigenous to China, became the subject of a scries 

 of investigations by several French chemists, chieliy in order to 

 ascertain whether this plant, if grown in France, could be advan- 

 tageously employed for the preparation of a dye to substitute 

 foreign indigo, so as to obviate the necessity of paying such 

 large sums to foreign nations for this article, a necessity which 

 seems at all times to have been a subject for extreme regret in 

 France. Baudrimont and Pelletier, after an examination of this 

 plant, concurred in the opinion of Chcvrcul, that the indigo is 

 contained in it as reduced indigo; and the latter adduced in 

 support of this view an experiment, Avhich consisted in treating 

 fresh leaves of the Polygonum with aether, taking care to exclude 

 the air, until the green colour had changed to white, when, on 

 exposure to the atmosphere, they speedily became blue. Robi- 

 quctf; Colin J, Turpin§, and Joly||, on the other hand, expressed 



* Traits sur le Pastel et V Extraction de son Indiyo. Paris, 1813. 



t Journal de Phurmacie, vol. xxv. p. ()2. 



J Me'moire sur la Renouee des Teinturiers. Paris, 183!). 



§ Etudes microscopiques sur leyisement de la matii-rebleue dans lesfeuilles 

 du Polygonum tiuctorium, Memoir e In a r Academe des Sciences le 12 

 Noveinbre, 1838. 



II Observations gen^rdles sur les plantes qui peuvent fournir des couleurs 

 bleuis ci la teinture. Montpellier, 1839. 



