184 MR. E. SCHUNCK ON THE 



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 ob- 

 scure department of chemical science, I resolved, though 

 without anticipating 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 vege- 

 table 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 with- 

 in the plant until decay has commenced, or at least until the 

 vital energy has begun to decline. The processes of decom- 

 position by which colouring matters are formed from other 

 substances are of two kinds. The first consists in the ab- 

 sorption of oxygen and the elimination of hydrogen in the 

 form of water; it is a process of decay {Verwesung. Liebig) 

 and requires the presence not only of oxygen but of some 

 alkali or other base. The second process is one which con- 

 sists in the splitting up of the original compound into two 

 or more simpler bodies, of which one or more are colouring 

 matters ; it is a process of fermentation, and may in general 

 be effected 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 colouring matters of 



• Gehlen is the only chemist who, as far as T am aware, has asserted that the 

 a{?itation with air in the manufacture of indigo may have for its object, not so 

 mwch the oxidation as the aggregation or separation of the particles of indigo 

 from the solution. Vide Schweigger's Journal, B. VI. 1812. 



