294 Dr. Schunck on some Products 



nation I should have been inclined to rest contented, had I 

 not acquired a knowledge of some other facts relating to in- 

 digo-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, mad- 

 der, and bran, together with lime, that if the process be not 

 carefully managed it may change its character entirely, the 

 contents of the vat entering into a state of complete putrefac- 

 tion — a change which results in the total destruction, or at 

 least disappearance, of the colouring-matter. Now this phe- 

 nomenon, 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 character 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 fermen- 

 tation or putrefaction, 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, and the colouring matter disappeared 

 entirely. In this way I had the mortification of losing a 

 quantity of indigo -blue which I had prepared with much 

 labour from human urine, though the loss resulted, as it after- 

 wards turned out, in some gain of information. 



This fact was also difficult to account for, since it is 

 usually supposed that by the combined action of reducing- 

 agents and alkalies indigo-blue merely takes up an atom of 

 hydrogen and then dissolves, and, by the action of the atmo- 

 spheric oxygen is again precipitated, unchanged and undimi- 

 nished in quantity. 



In order to ascertain on what the disappearance of the 

 colouring-matter in this case depends, I first dissolved a 



