FORMATION OF INDIGO-BLUE. 183 



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 vegetable, but is formed subsequently to the ex- 

 traction 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 views very strong objec- 

 tions may be raised. 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 water. If to this it be objected, that a compound 

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

 composition be reproduced, then it is at once admitted that 

 indigo-blue is not contained as such within the vegetable. 

 That it cannot exist as reduced indigo is evident, since the 

 latter requires the presence of some alkali for its solution in 

 water, and the juice of most, if not all, indigo-bearing plants 

 is acid. It is difficult, moreover, to conceive how deoxidised 

 indigo, a body having so great an affinity for oxygen, can 

 exist in the interior of plants which we know are constantly 

 evolving that element. That the colouring matter is formed 

 by the process of fermentation to which the extract of the 

 plant is subjected, as it is the oldest, so it is the most pro- 

 bable view. Nevertheless, the fact that indigo may be pro- 

 cured from plants by mere infusion with hot water and 



ography, comprising references to all the known works, treatises, papers, &c. 

 on chemical subjects written since the commencement of the modern era in 

 chemistry. In searching for the authorities referred to in this paper, I have 

 felt this want very sensibly. It is with some difficulty that the mere names of 

 all the works and memoirs relating to any special branch of the science, par- 

 ticularly such as have fallen into oblivion, are discovered. The only attempt to 

 supply this deficiency, and that only in regard to one department of chemistry, 

 is WolflF's Quellenliteratur der Organischen Chemie. Halle, 1846. 



