68 DR. EDWARD SCHUNCK ON SOME 



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 afterwards 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 

 atmospheric oxygen is again precipitated, unchanged and 

 undiminished in quantity. 



In order to ascertain on what the disappearance of the 

 colouring matter in this case depends, I first dissolved a 

 small quantity of indigo-blue by means of grape-sugar and 

 caustic soda, using water as a solvent instead of alcohol ; 

 but though the indigo-blue was kept for a long time in 

 solution, and heat was applied at the same time to assist 

 the action, it made its appearance again on exposure to the 

 air, apparently undiminished in quantity. In another ex- 

 periment, in which alcohol was used as the menstruum 

 and protoxide of tin as the reducing agent, the same result 

 was arrived at. It was therefore apparent that the dis- 

 appearance of the colouring matter was due to the com- 

 bined action of the alcohol and the grape-sugar, not to the 

 separate action of either. By the use of a great excess of 

 these two agents, together with caustic soda and the long- 

 continued application of heat to the solution, I succeeded 

 in causing several grammes of indigo-blue to disappear 

 entirely. I avoid the word decompose, because, as I shall 

 show, the colouring matter is not decomposed, but enters 

 into new forms of combination. 



It now occurred to me that since, by the action of caustic 

 alkalies on sugar, acetic and formic acids are formed, the 

 effect produced by the grape-sugar in this process might in 

 reality be due to the presence of one or both of these acids 



