108 Mr. Marty n Roberts in reply to Prof. Poggendorff. 



I think this explanation is proved to be the true one, both 

 by Prof. Poggendorff' s experiments and by my own. I found, — 

 1st, When comparing a ten-pair iron-zinc with a ten-pair 

 copper-zinc battery, that although in a given time of con- 

 siderable duration a much greater quantity of water was de- 

 composed by the iron than by the copper batter}', yet that at 

 the first immersion of the plates in acid a much more rapid 

 decomposition was effected by the copper than by the iron 

 combination, although in a very short time this superiority 

 disappeared, and the iron became the strongest. 



2ndly. When a single pair, copper-zinc, was tried against 

 a similar pair, iron-zinc, and their powers tested by a differ- 

 ential galvanometer ; at the first immersion the deviation was 

 in favour of the copper, but in a short time the iron overcame 

 it by a deviation of fifty degrees. 



Srdly. When these pairs were inspected at intervals, it was 

 seen that the iron preserved a bright metallic surface, while 

 the copper was covered by a dark coat of oxide. 



4thly. A copper-zinc pair was tested against an iron-zinc 

 pair of equal surface, to ascertain the comparative loss of 

 metal by oxidation and solution during the working of these 

 little batteries, which were allowed to remain in action for 

 about twelve hours : result as follows : — 



showing that the copper loses none of its weight when in com- 

 munication with zinc ; but that iron lost six grains under si- 

 milar circumstances. 



From these experiments we may conclude, that as at the 

 first immersion the copper-zinc is superior to the iron- zinc, 

 but afterwards becomes inferior to it, some change must have 

 taken place during this interval which reduces the electric 

 current of the copper-zinc battery; this reduction is proved 

 by the first and second experiments ; what the change is was 

 seen by inspection in the 3rd experiment, viz. a clean surface 

 of iron, and an oxidated surface of copper. Lastly, that this 

 bright iron surface is caused by the solution of the oxide of 

 the metal, is proved by the 4th experiment, where the iron 

 lost six grains of its weight. 



The cause, then, of the anomalous electric condition of iron 

 is not that this metal possesses a less power of resistance to 

 the transition of electric fluid than does copper, although in 



