PRODUCTS DERIVED FROM INDIGO-BLUE. 67 



planation I should have been inclined to rest contented, 

 had I not acquired a knowledge of some other facts relating 

 to indigo-blue, to which the same explanation cannot be 

 applied, but which evidently belong to the same class. 



It is well known to those dyers who employ the so-called 

 woad-vat, in which the reduction of the indigo-blue is 

 effected by the action of various organic matters, such as 

 woad, madder, and bran, together with lime, that if the 

 process be not carefidly managed it may change its cha- 

 racter entirely, the contents of the vat entering into a state 

 of complete putrefaction — a change which results in the 

 total destruction, or at least disappearance, of the colouring 

 matter. Now this phenomenon, the reality of which cannot 

 be doubted, though its nature has never been subjected to 

 scientific scrutiny, cannot be explained in accordance with 

 what is at present known regarding indigo-blue, which is 

 considered by chemists to be a body of such a stable cha- 

 racter as not to be decomposed by any except very potent 

 agents, such as chlorine, bromine, and nitric acid. In no 

 work on scientific chemistry is it stated that indigo-blue 

 may be decomposed by any process of fermentation or putre- 

 faction, in the same way as sugar or albumen. 



In my experiments on indigo-blue I have generally em- 

 ployed for its reduction and purification the process of 

 Fritzsche, which consists in acting on it with a mixture of 

 alcohol, grape-sugar, and caustic soda. The colouring 

 matter dissolves when the mixture is heated, and is again 

 deposited on exposure to the atmosphere in crystalline 

 needles. Now in performing this operation with very 

 small quantities of indigo-blue and an excess of alcohol 

 and grape-sugar, I found that the colouring matter did 

 not make its appearance again on agitating the solution 

 with air. The yellow colour of the liquid passed as usual 

 through red to green ; but, instead of the indigo-blue being 

 precipitated, the whole became yellow or brownish-yellow, 



f2 



