FOBMATION OF INDIGO-BLUE. 181 



or successors on the same field of investigation. The chief 

 conclusions at which he arrived, partly by experiment and 

 partly by reasoning, are contained in the following propo- 

 sitions: 1. Indigo-blue does not pre-exist in the plant, but 

 is formed by the operations, 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 plants, and which has a tendency 

 to be converted into indigo ; this principle may be called 

 indigogene, 3. This principle differs from indigo in con- 

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

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

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

 portion of carbon is caused by the latter undergoing combus- 

 tion, and being converted into carbonic acid. 5. It differs 

 in its properties 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 atmosphere. 6. Its combusti- 

 bility 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 sub- 

 ject of a series of investigations by several French chemists, 

 chiefly in order to ascertain whether this plant, if grown in 

 France, could be advantageously employed for the prepara- 

 tion 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 Chevreul, that the indigo is contained 

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

 of this view an experiment, which consisted in treating fresh 



