( 337 ) 



On the Action of Iodic Acid and of Iodine on Vegetable Co- 

 lours. By Arthur Connell, Esq. F. R. S. E. Commu- 

 nicated by the Author. 



1 N the account which I lately gave of a method of converting 

 iodine into iodic acid, by the agency of nitric acid*, I mentioned 

 that the solution of the iodic acid, thus prepared, reddened Ht- 

 mus paper permanently, a circumstance which seemed inconsist- 

 ent with Sir H. Davy's statement, that that acid ultimately 

 bleaches vegetable blues. I have since investigated this matter 

 more particularly, and have found that iodic acid, whether pre- 

 pared by means of euchlorine, or by the process which I have 

 proposed, reddens litmus permanently, both in the state of in- 

 fusion, and in that of stained paper. This permanency, how- 

 ever, of the reddening action, appears to be peculiar to this 

 colouring matter. If the blue infusion of cabbage is treated in 

 the same manner, it will be found to be at first reddened by 

 the acid prepared in either way, and afterwards to become yel- 

 low, and a reddish-brown matter to precipitate to the bottom, 

 the effect being complete in a few days. Paper stained with 

 the same infusion is first reddened, and soon afterwards bleached. 

 It is probable, therefore, that the general rule of Davy holds 

 good, that vegetable colours are first reddened, and then bleach- 

 ed or made yellow by iodic acid, however prepared, the excep- 

 tion as to litmus being peculiar to that colouring matter, which, 

 as is well known, has the additional peculiarity of not being 

 acted on by alkalis. 



In the course of these experiments, I had occasion to examine 

 the action of iodine itself on vegetable colours, which, it is sin- 

 gular, is differently stated by different chemical writers. Most 

 English authors have been satisfied with repeating the observa- 

 tion of M. Gay-Lussac, in his memoir on iodine, that that sub- 

 stance bleaches vegetable colours, but with much less energy 

 than chlorine, and have not entered into any particulars as to 

 how this action may be shewn. On the other hand, M. Rose, 

 in his late valuable work-f, has stated that iodine does not 



• See this Journal for July 1831. 

 t Handbuch der Analytishen Chemie, i. 366. 2*6 Auf. 

 JAN.UARY MARCH 1832. Z 



