Dr, Schunck on the Fonnation of Indigo-blue. 79 



endowed with a striking and positive colour, is only one of a 

 long chain of bodies succeeding one another, and is generally 

 not the last product of decomposition. The other process, of 

 which the formation of alizarine is an example, yields colouring 

 matters of a fixed and stable character, which are not further 

 changed by a continuance of the process to which they owe their 

 formation. Now if indigo-blue be a body which is formed from 

 some colourless substance existing in the plant, we should infer 

 a priori that the process by which it is formed is one of fermen- 

 tation or putrefaction, not requiring the intervention of oxygen 

 or of alkalies ; a conclusion, however, so much at variance with 

 the generally received ideas on the subject of the formation of 

 indigo-blue, as to require the aid of very decisive experiments 

 for its establishment. 



I shall now proceed to give an account of the experiments 

 which I have undertaken with a view to elucidate this subject. 



The only plant cultivated in this country which is known to 

 yield indigo in any quantity is woad, Isatis tinctoria ; and as it 

 was necessary to examine the indigo-bearing plant in a fresh 

 state and in considerable quantities, I had recourse to this one 

 for the purpose. Having procured 10 lbs. of good French woad 

 seed, I sowed it at the commencement of the spring of last year 

 on about half an acre of land. It was sown in drills about two 

 feet apart, each drill being previously well supplied with farm- 

 yard manure. In a short time the young plants appeared^ and 

 grew vigorously during the summer months. Some of the plants 

 bore flowers, and ripened their seeds in the course of the autumn. 



At no time during the whole progress of the growth were 

 there any visible indications of the presence of blue colouring 

 matter on the leaves or stems. Some of the ripe seeds only 

 were tinged with a dark purple colour, forming a thin coating 

 on the exterior. The greater proportion of the plants, however, 

 bore leaves which did not exhibit the glaucous appearance nor 

 the fleshy consistency, which, according to authors, are charac- 

 teristic of the cultivated variety of woad. They were, on the 

 contrary, of a bright grass-green, and possessed but little succu- 

 lence — characters which belong rather to the wild variety. 



As soon as I could collect a small quantity of leaves, I com- 

 menced my experiments. Having taken some leaves, I chopped 

 them fine and then extracted them with boiling water. The filtered 

 liquid was light brown and transparent, it had a bitter taste and 

 an acid reaction. It deposited no indigo-blue, however long it 

 might be left exposed to the atmosphere, and hence it might 

 have been inferred that it contained no indigo-blue. Neverthe- 

 less, a very simple experiment sufficed to show that it was capable 

 of yielding an appreciable quantity of that colouring matter. On 



