480 



RErORT — 1884. 



Ml 



TLorason, that wlicroas copper i.^ Tipe:'^tivo with respect to iron in air, it 

 is positive with rcspcet to iron in H.jS. It would appear therefore tliat 

 xho reason why the results of metals l)y the ordinary "contact force" ex- 

 periments liarmoniso so well with the comparison by dip])infr both metals 

 in water or an oxidisin*^ electrolyte, is not because the E.M.F. betweon n 

 metal and a ^na or an electrolyte is small, but because the properties of 

 air af»reo to a certain extent with those of ordinary electrolytes. For 

 if the active comj»onent of the electrolyte is sul[)hur the results are (piirc 

 different, and the same kind of dilferencc occurs when hydrogen sulphide 

 is substituted for air.' 



In their third ])aper,' therefore, we find Ayrton and Perry's views 

 changed, and they clearly state that their experiments, like all those based 

 on inductive methods, leave the question of air contacts quite nndecidcd. 



They then go on to say: 'One way of determining the E.AI. F. of 

 contact in volts between a substance and air, and a way we hope shortly 

 to eni])loy, is to i-epeat all these exact contact experiments in different 

 gases [it is not quite true tho*-. this would give the results required, 

 because it would only give differential effects ; very important to observe, 

 no doubt, but not the same as observing the actual contact force of air or 

 of gas]. . . . Wo shall thus ascertain whether the contact difl'erence of 

 potentials of a substance and a gas differs much for different gnses. 

 Qualitative exf)eriments in this direction have already been made with 

 very interesting results by ^Mr. Brown, of IJclfast, but his experiments 

 differ from ours in not being quantitative.' [Only, as their experiments 

 do not yet exist, IMi'. Brown's have still some value.] 



In vol. xii. of ' Wiedemann's Annalen ' - I find some interesting experi- 

 nients by Schultze-Berge on contact force between metals and gases, lie 

 uses a condenser and difl'erent gases, but the plates of his condenser aro 

 both of the same metal, and he coats one of the plates with a film of tbo 

 gas, say chlorine, or hydrogen, or ozone, and leaves the other covered with 

 air. To measure the potential difference he employs the compensation 

 method of Pellat, and his arrangement seems fairly Hati.sfactory. But ho 

 does not explicitly outer on the question as to the seat of E.M.F. in the 

 Volta experiment (except in a controversy with Professor Exner). He 

 assumes that a contact force between metals and metals and between 

 metals and liquids has been established, and ho wishes to extend it to the 

 contact of a metal and a gas. Believincf firmly in the existence of films 

 of condensed gas at the surface of a solid, which films require time for 

 their formation or removal, he deems it sufficient to soak one of the plates 

 of his condenser in the gas to be examined, and then to take it out and 

 measure the difiercnce of potential between it and the other plate coated 

 only with a film of air. Tried thus, ozone rendered gold, platinum, and 

 brass negative as against the corresponding air-covered plate. Hydrogen 

 rendered its platinum strongly positive, but its influence on gold was 

 slight, and on brass tincertain. Chlorine and bromine made platinum 

 negative, and ammonia made brass positive. 



It may be readily objected that what the soaking with gas accomplished 

 ■was the formation of not only a film of gas, but a film of actual chloride, 

 oxide, or other combination. Against this is to bo urged the fact that 

 after removal from the gas the effect diminished with time, and the plates 



' P/iil. Tnuix., 1880. 



- Kcluiltzc-15orp;o : ' Uobcr die Elektricitiifscrrepung bcim Contact von Jlctallen 

 und Gason.' Ann. dir I'/iys. u. Chvm., xii. 2'j:<, 1881. 



