ON PYROXYLIC SPIRIT, &c. 125 



as above stated, as well as more decidedly when stronger powers were employed, — 

 that acid also appearing proportionally sooner in the positive Mquid when alcoho- 

 lic than when aqueous solutions were employed — soon shewed me the true nature 

 of the action, and that the acid was not detected in the instances alluded to, 

 merely from its having been formed in smaller quantity, and having suffered that 

 decomposition to which it is so subject, as soon as it passed to the positive side. 

 Whence the acid comes I shall explain presently. 



When a power of seventy-two pairs of 4-inch plates was employed, all the 

 other arrangements being exactly the same as in the beginning of the expeiiment 

 described in the preceding page, acid was detected in five minutes not only at the 

 positive end of the asbestus, but on the positive side of the negative alcoholic so- 

 lution, with alkah at the negative pole ; broAvning having begun to appeal* about a 

 minute before, and effervescence at both poles still earlier. The browning went 

 on increasing, without any discoloration having occurred in the negative tube in 

 a quarter of an hour. The battery was then reversed, when the usual instant dis- 

 coloration ensued at the positive pole. 



In these experiments, therefore, the appearance of iodine at the positive pole 

 before reversal of the battery, is really dependent on hydriodic acid being drawn 

 to that side, and decomposed by nascent oxygen, as in the case of aqueous solu- 

 tions of iodide of potassium. The hydriodic acid comes, I conceive, principally 

 from the point where the alcoholic solution is in contact with mater, and where it 

 becomes an aqueous one of hydriodate of potash, Avhich salt is resolved into its 

 constituent acid and alkali by the voltaic agency. 



Another source of the acid seems to be the secondary action of hydrogen at 

 the negative pole, in virtue of which acid and alkali appear to be there formed 

 as formerly stated, the acid being immediately afterwards carried towards the 

 positive pole ; and accordingly, in one of the preceding experiments, where the 

 stronger power was employed, acid was detected in the alcoholic liquid, — the af- 

 finity of potassium for the oxygen of the water of the alcohol of course forming 

 this secondary action. 



There is another arrangement which shows, I think, stiU more clearly the se- 

 condary nature of the action, in virtue of which iodine appears. 



The usual alcoholic solution of iodide of potassium was placed in a tube A, 

 fig. 2. of the same size as before, and connected on the one side with the negative 

 side of seventy-two pairs of 4-inch plates, and on the other by asbestus, with a 

 glass cup B of the capacity of |th of an ounce containing water, which, in its turn, 

 was connected by the same means with another glass cup C, also containing wa- 

 ter, which was made positive. Within the first quarter of an hour, acid was de- 

 tected at various places on the intermediate asbestuses, and at the positive pole ; 



