S'i'l" Prof. Connel on the Voltaic Decomposition 



ought to have been liberated, and the bhie colour produced 

 in one or other of the tubes before reversed ; but as this change 

 did not occur till after reversal, the effect was due to nascent 

 hydrogen at the negative pole. This hydrogen must have 

 combined with bromine if the bromide is dissolved as such, 

 as is usually held, and is most probable; or with oxygen if 

 the whole or a part of it decomposes water, forming hydro- 

 bromic and iodic acids *. The appearances of effervescence 

 lead to the same conclusion. The oxygen liberated at the 

 positive pole in B before reversal must have come from water, 

 and implies an equivalent quantity of hydrogen passing into 

 A, and uniting with the oxygen of water in the course of de- 

 composition in A. Now as this hydrogen did not appear on 

 reversal of the battery, it must have been employed in li- 

 berating iodine. 



The next case to be considered is that of the oxyacids. 

 When we observe the radical of such an a(;'-i\- solution sepa- 

 rating at the negative pole and oxygen at tli^ positive, we are 

 at first sight inclined to suppose that the acid itself has been 

 the subject of direct voltaic decomposition ; and accordingly 

 it has been often thought that sulphur and phosphorus, when 

 they are observed in such circumstances, are the result of 

 direct decomposition of the corresponding acids. In like 

 manner we might conclude that the iodine which instantly 

 separates at the negative pole when a solution of iodic acid is 

 acted on, is the result of primary decomposition; but a 

 simple experiment at once shows the action to be secondary. 

 A mixed solution of iodic acid and of starch was placed in 

 the tube B, fig. 1, and a solution of starch in A, the con- 

 nexion being by asbestus moistened with the latter solution, 

 and B made positive and A negative by fifty pairs of two-inch 

 plates. Effervescence ensued from both poles, but after half 

 an hour's action no formation of blue was observed anywhere 

 in either tube. The battery was then reversed in the manner 

 above described. In two minutes blue matter was formed on 

 the now negative foil in the acid solution with little or no ef- 

 fervescence at that pole, but effervescence from the positive 

 foil. Thus in the first position of the battery, as there was 

 no direct decomposition of the acid, iodine did not pass to- 

 wards the positive pole ; whilst after reversal, iodine imme- 

 diately appeared by the reducing action of hydrogen derived 

 from directly decomposed water. The analogy of this ex- 



* When both poles are plunged into the solution, and the battery is in 

 fresh action, there is effervescence at both poles, a part of the hydrogen 

 only being engaged in producing the secondary action. 



