218 DR. EDWARD SCHUNCK ON INDIGO- BLUE. 



XXVIII. On Indigo-Blue from Polygonum tinctorium and 

 other Plants. By Edward Schunck^ Ph.D., F.R.S. 



Eead April i6th, 1878. 



Some papers read before tlie Society many years ago, and 

 subsequently published in its ' Memoirs ' "^^ contain an 

 account of my experiments with the leaves of the Isatis 

 tinctoria, or common woad, the well-known plant employed 

 in Europe for dyeing blue before the introduction of indigo 

 from the East. I showed that the leaves of this plant do 

 not, as some have supposed, contain either indigo-blue or 

 its hydride ready formed, but yield by careful treatment a 

 peculiar giucoside — indican — which, when acted on by acids 

 and other reagents, splits up into indigo-blue and indigo- 

 glucine, the latter being a body resembling glucose. My 

 experiments also show that this substance, indican, is a 

 highly unstable body, undergoing when its watery solution 

 is heated for some time, or, more rapidly, by the action of 

 caustic alkalies, an entire change, on the completion of 

 which it no longer yields indigo-blue by decomposition 

 with acids, but in place of the latter gives indigo-red, 

 indifulvine, leucine, and other products. Though I suc- 

 ceeded in ascertaining the composition of indican and the 

 relation in which it stands to indigo-blue, the difficulty of 

 obtaining large quantities of it in consequence of its ex- 

 cessive liability to change, prevented my proceeding further 

 with the investigation. It seemed to me, however, that it 

 might be of some interest to ascertain whether other indigo- 



* 'Memoirs,' ser. 2, vol. xii. p. 177, and vol. xiv. p. 181. 



