INDIGO 901 



excited, in which the action of the .atmospheric air does not intervene, since an in- 

 flammable gas is evolved. There probably results from it some change in the com- 

 position of the colouring particles themselves ; but especially the separation or 

 destruction of a yellowish substance, which gave to the indigo a greenish tint, and 

 rendered it susceptible of undergoing the chemical action of other substances. This 

 species of fermentation passes into a destructive putrefaction, because the indigo has 

 a composition analogous to that of animal substances. Hitherto the colouring par- 

 ticles have preserved their liquidity. In the second operation, the action of the air is 

 brought into play, which, by combining with the colouring particles, deprives them 

 of their solubility, and gives them the blue colour. The beating serves, at the same 

 time, to dissipate the carbonic acid which is formed in the first operation, and which 

 by its action presents an obstacle to the combination of the oxygen. The separation 

 of this acid is promoted by the addition of lime ; but if an excess be introduced, it 

 counteracts the free combination of the oxygen. The third part of the process has 

 for its objects ;the deposition of the colouring matter, become insoluble by combina- 

 tion with oxygen, its separation from foreign substances, and its desiccation, which 

 gives it more or less hardness, whence its appearance varies.' De Cossigny was of 

 opinion that volatile alkali was the agent by which the colouring matter was ex- 

 tracted from the plant and held in solution until volatilised by the agitation process. 

 Roxburgh concluded 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 the alkalis employed is to destroy the attraction between the 

 base and the carbonic 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.' 



Chevreul, who was the first chemist of any eminence to examine the indigo- 

 bearing plants and their constituents, inferred from his analyses of the Isatis tinctoria 

 and the Indigofera 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 the solution is removed from the plant, 

 it is converted, by the action of the atmospheric oxygen, into indigo-blue. Giobert, 

 from an examination of the Isatis tinctoria, drew the following conclusions : 1. Indigo- 

 blue does not pre-exist in the plant, but is formed during 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 the property of being convertible 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 

 the action of a small quantity of oxygen which it takes up. 4. The loss of this 

 portion of carbon must be attributed to its undergoing combustion, and being con- 

 verted into carbonic acid. 5. It differs in its properties from common indigo in being 

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

 undergo spontaneous combustion at the ordinary temperature of the atmosphere. 

 6. Its combustibility is enhanced by heat and by combination with alkalis, 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 series of investigations by several French chemists, chiefly with a view 

 to ascertain whether this plant, if grown in France, could be advantageously employed 

 in the preparation of a dyeing material as a substitute for foreign indigo. Baudrimont 

 and Pelletier, after an examination of this plant, arrived at the conclusion that the in- 

 digo is contained in it as reduced indigo, in the same state as it is in woad, according 

 to Chevreul. Eobiquet, Colin, Turpin, and Joly, on the other hand, expressed a very 

 decided conviction that indigo-blue pre-exists in the plant, 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 the operation of potent agencies is requisite in 

 order .to destroy this combination and set the indigo at liberty. The explanation of 

 Chevreul, proceeding from an authority of such eminence, and being the simplest, 

 has been adopted by most chemists. Nevertheless, there are objections to it which 

 render it inadmissible. Reduced indigo is a body which is only soluble in alkalis, 

 and cannot, therefore, be contained as such in the juice of indigo-plants, which is 

 mostly acid. As it also takes up oxygen with the greatest avidity, and is converted 

 into indigo-blue, it is difficult to conceive how the whole of it can be preserved in a 

 colourless state in the cells of plants, in which it must occasionally come in contact 

 with the oxygen eliminated by the vegetable organism. If these plants contained 

 reduced indigo, the juice ought, moreover, to turn blue the moment it became exposed 



