152 MR CONNELL ON THE VOLTAIC DECOMPOSITION OF 



of the battery, the iodine did not pass towards the negative pole, and the acid, 

 consequently, did not suffer voltaic decomposition ; whilst on reversal iodine very 

 soon appeared at that pole, by the reducing action of hydrogen. 



In regard to oxides, it is not easy to obtain such direct experimental evidence, 

 because the metals of such as are soluble in water react on the solvent : but we 



% 



may take the decomposition of metallic oxides, as contained in soluble salts, 

 during which the metal frequently appears at the negative pole, as illustrating the 

 action on solutions of such oxides as are dissolved by pure water. It is now pretty 

 generally admitted, that when metal appears in solutions of metallic salts at the 

 negative pole, it is due to the reducing action of nascent hydrogen ; and this opinion I 

 have verified directly by finding that, when a solution of sulphate of copper was made 

 positive, by 50 pairs of 2-inch plates, and connected by asbestos with distilled water 

 which was made negative, neither metallic copper nor oxide was carried towards 

 the negative pole during half an hour's action ; whilst, on reversal of the battery, 

 the now negative foil was found in a quarter of an hour to be coated with metal- 

 lic copper without any elastic fluid from that pole. It was also found, by a simi- 

 larly arranged experiment with an aqueous solution of chloride of zinc, that nei- 

 ther zinc nor oxide was carried in a similar time to the negative pole, when that 

 pole was placed in distilled water ; but that, after reversal, metallic zinc soon 

 appeared at that pole, from the reducing action of hydrogen ; and holding, as I 

 do, from the experiments formerly detailed,* that chloride of zinc is dissolved as 

 a muriate, the above result affords the same illustration as that with sulphate of 

 copper. A similar result was obtained when distilled water was carefully poured 

 over a concentrated solution of muriate of zinc in a bent tube, instead of connect- 

 ing the liquids by asbestos, and the poles plunged separately into the two liquids ; 

 the only difference being, that, previous to reversal, a little oxide of zinc ap- 

 peared to be formed at the boundary of the water and the metallic solution. 



I thought it likely that the employment of a negative pole of metallic tellu- 

 rium might have thrown farther light on this matter ; since, if the tellurium in 

 the solution of a common metal combines with hydrogen, as it does under ordi- 

 nary circumstances of voltaic action, no metal ought to be reduced from the 

 solution. But I found that, when so employed, copper and zinc still appear 

 at the negative pole. This will doubtless be thought by some a proof in favour 

 of the direct decomposition of the metallic oxide ; but, on due consideration, it 

 appears to me to be quite insufficient to establish it. If the oxide really suf- 

 fers direct decomposition, it seems evident that in the experiments above de- 

 tailed, in which metal appeared, on reversal, at the negative pole, that either it, 

 or at least oxide, ought also to have appeared proceeding towards that pole pre- 

 vious to reversal. It is impossible to perceive what should constitute the differ- 



* Edin. Trans, vol. xiv. p. 12?. 



