1886.] 



A Theory of Voltaic A ction. 



301 



ammonise in a flask connected by glass tubing to the stopcock T. In 

 each case the potential was reversed, and the difference of potential 

 appeared rather less in the ammonia atmosphere than in air, bnt 

 owing to the action of the gas on the vulcanite piece M, causing it to 

 twist and so alter the position of the quadrants, I was unable to get 

 reliable quantitative comparisons with the standard cell. 



15. In a cell composed of copper and nickel in distilled water, 

 nickel is the positive metal, and the electromotive force is about 

 0'24)D. On adding a few drops of a solution of ammonia a reversal 

 takes place and copper becomes the positive metal. The electro- 

 motive force appears at first rather greater than that of the water 

 cell but falls off quickly. 



The analogous experiment with gaseous ammonia was made in a 

 qualitative way only. The quadrants of the electrometer were re- 

 placed by a flat ring half copper and half nickel, the two metals 

 soldered together. (This had been used in a former experiment with 

 hydrochloric acid gas.) 



The metals of the ring having been rubbed bright with emery- 

 paper, the potential in air only near the nickel was positive to that 

 near the copper, the index passing over about divisions, when 

 the electrification of the needle was reversed. 



After admitting ammonia gas in the same way as before described 

 the copper side became positive, at first slightly, but after two hours 

 the index moved over six divisions for a reversal of the needle. The 

 two kinds of experiments therefore agree here also fairly well. 



16. I now sum up the results (regarded qualitatively only) ob- 

 tained in this kind of experiment, where a change in the constituents 

 of the atmosphere surrounding a pair of metals in contact reverses 

 the difference of potential near them in correspondence with the 

 reversal of electromotive force, which takes place after a similar 

 change in the corresponding liquid electrolyte used with the same 

 metals as a voltaic cell. 



Pairs of metals. 



Compound whose addition 

 in solution causes a reversal 

 of electromotive force in a 

 hydro-element or cell. 



Compound whose addition 

 as gas to the atmosphere 

 surrounding the metals in 

 contact causes a reversal of 

 the difference of potential 

 observed near them. 



Copper-Iron 



Copper- Nickel . . . 

 Copper-Nickel . . . 





Hydrogen sulphide. 

 Ammonia. 

 Hydrogen sulphide. 

 Ammonia. 

 Hydrochloric acid. 



