FORMATION OF INDIGO-BLUE. 185 



logwood and archil. Indeed, in this case the colouring 

 matter, if this name be applied merely to substances 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 colour- 

 ing 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 fermentation or putrefaction, not re- 

 quiring the intervention of oxygen or of alkalies, a con- 

 clusion, 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 experi- 

 ments 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 pre- 

 viously 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 

 2 A 



