Mr. J. Brown on the Theory of Voltaic Action. 143 



atmosphere containing a suitable sulphur compound, the ano- 

 maly should disappear, and we should obtain effects with the 

 condenser which would place the metals in the same potential 

 order as when immersed in sulphur electrolytes. In order to 

 verify this the following experiment was made. Starting with 

 the fact that iron is positive to copper in an oxidizing electrolyte 

 (as water), while copper is positive to iron in a solution contain- 

 ing potassium sulphide or other similar sulphur compound, I 

 made a condenser with disks 4 J- inches in diameter, one of 

 copper, the other iron, well ground together. The iron disk was 

 screwed on the lower end of an iron rod sliding in a brass tube 

 fixed with shellac in a wooden cover fastened on the neck of a 

 gas-jar. The jar stood on a wooden stand, through the middle 

 of which rose a similar insulated rod carrying the copper disk. 

 Means were provided for adjusting the disks parallel to one 

 another, and also for filling the jar which enclosed both disks 

 with any required gas. 



To measure the charge excited by the " contact" of the 

 plates a quadrant-electrometer was employed, which gave a 

 deflection of 5 millims. for the potential of a bichromate cell. 

 When the condenser-plates were placed together (in ordinary 

 atmosphere) connected with opposite pairs of quadrants and 

 then separated, the index light moved over 1 centim., the iron 

 being positive, as was to be expected. Hydrogen sulphide 

 was then allowed to flow into the gas-jar ; and on repeating 

 the connexion and separation of the plates the iron proved to 

 be negative, the light-spot moving over about 3 centims. in 

 the direction opposite to its first motion. This was repeated 

 several times ; and on examining the plates after the experi- 

 ment, the copper was found to be of a deep blue colour, while the 

 iron was scarcely altered. It will be observed here that the 

 only alteration in the circumstances of the experiment was the 

 change in the atmosphere surrounding the plates. The con- 

 tacts all remained the same ; and when the atmosphere con- 

 tained a sulphur compound, the plates assumed the same elec- 

 tric relation as they would in an electrolyte containing a 

 sulphur compound. Even the proportionate degree of tension 

 between the plates in air and in hydrogen sulphide is similar 

 to the ratio of their electromotive force in water and in potas- 

 sium sulphide solution. 



The next experiment appears to confirm in a marked way 

 the view that the difference of potential between two metals 

 in contact is due principally, if not altogether, to the difference 

 of their affinities for one of the elements of some compound 

 gas in the atmosphere surrounding them. The experiment is 

 a modification of one devised by Sir William Thomson, and 



