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



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 deoxidized 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 fermenta- 

 tion to which the extract of the plant is subjected, as it is the 

 oldest, so it is the most probable view. Nevertheless, the fact 

 that indigo may be procured from plants by mere infusion with 

 hot water and precipitation with lime-water, without any of the 

 usual signs of fermentation being manifested, appears to militate 

 against this view. On one point all authorities seem to agree, 

 viz. that the contact with oxygen is a necessary condition of the 

 formation, or at least precipitation of the indigo from the watery 

 extract*. 



Such being the state of our knowledge on this rather obscure 

 department of chemical science, I resolved, though without anti- 

 cipating any very decided success, to endeavour to throw a little 

 more light on it. I was induced to do so chiefly by the following 

 consideration. The principal vegetable colouring matters have 

 now been discovered to be not direct products of the vital energy 

 of plants, but products of decomposition of substances contained 

 in the vegetable, which are themselves mostly colourless. The 

 formation of these colouring matters takes place equally well out 

 of the plant as within it. Indeed, it is probable that it never 

 happens within the plant until decay has commenced, or at least 

 until the vital energy has begun to decline. The processes of 

 decomposition by which colouring matters are formed from other 

 substances are of two kinds. The first consists in the absorption 

 of oxygen and the elimination of hydrogen in the form of water ; 

 it is a process of decay ( Verweswiff, Licbig), and requires the 

 presence, not only of oxygen, but of some alkali or other base. 

 The second process is one which consists in the splitting up of 

 the original compound into two or more simpler bodies, of which 

 one or moi'e are colouring matters ; it is a process of fermenta- 

 tion, and may in general be eSected as well by the action of 

 strong acids as by that of ferments. The first process gives rise 

 to colouring matters of a very fugitive nature, such as the colour- 

 ing matters of logwood and archil. Indeed, in this case the 

 colouring matter, if this name be applied merely to substances 



* Gehlen is tlie only chemist who, as far as I am aware, has asserted that 

 the agitation with air in the manufacture of indigo maj' have for its object, 

 not so much the oxidation as the aggregation or sepai-ation of the particles 

 of indigo from the solution. Vide Schweigger's Journal, vol. vi. 1812. 



