ELKCTUOMOTIVE FOllCKS IN TUE VOLTAIC CELL. 



487 



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the same metal at different temperatares, using the condenser or Kohl- 

 rausch method. He found that iron, copper, zinc, and probably tin, were 

 neirativo when hot to the same metals cold ; and the effect increases 

 uniformly with tomperatui-e. But it is permanent, remaining after the 

 hot plates have cooled down ; hence it must be duo to oxidation. A slow 

 oxidation proceeds with time alone. Time curves are logarithmic like 

 cooling curves, and the most oxidisable metal varies most quickly both 

 tor time variatiou and temperature variation. There seems to be a surface 

 couditiou of a metal proper to each temperature which no polishing can 

 change, for it establishes itself in ft few seconds after cleaning, and only 

 eliauii'es with temperature. 



^li'. S. Laviiigtou llart. in 1881,' describes a mercury dropper where 

 ll:e ir.c'.'cury is eoiitiiincd in a funnel, and is connected with an electro- 

 nieler by ai) iron rod dipping; into it. ^riic drops form inside an iron 

 inductor, atul they fall negatively charged. Mr. Hart so far ignores 

 any \"olta force that ho considers the arrangement as an inversion of 

 iii|ipniann's electrometer, the advancing drops being oxidised. It can 

 plainly be regarded, however, as a mere Fe/Hg contact arrangement, 

 and that is wliat I suppose it to be. He makes two interesting modilica- 

 tious : the first is to replace the air round the dropping mei'cury by coal- 

 gas ; the electrical elfect is then zero. This is interesting because the 

 exuding drops of mercury, unlike most pieces of metal, cxpo-so to the coal 

 <_'as a virgin suri'ace which has })robably contracted no condensed air sheet : 

 only coal gas is a rather sophisticated substance for it to be tirst exposed 

 to. If the experiment is regarded as sulliciently direct and simple, this 

 iaet lends support to the view that Volta forces depend on the medium 

 -lUTouuding the metals. 



The second modification is to bring an earth-connected iron bar 

 close to the drops, and to show that it reduces the deflection. Mr. 

 llart thinks it reduces the oxidation by proximity; and certainly, 

 provided the obvious action of a mere electrostatic screen has been 

 considered and provided against, this action by proximity is very 

 remarkable. A similar effect has been observed and more fully worked 

 oat by Pellat in a paper published in 1882. -^ Pellat says that it he places 

 two metallic surfaces parallel to one another and very close together (say 

 half a niillimeli'c more or less : variations from 12 to "1), each metal under- 

 iroes a slight alteration of the properties of its superficial coat, and thereby 

 changes its position in the voltaic series. The alteration takes some 

 niinutes to produce, increases with time, but tends to a limit. When the 

 iufluencing metal is removed the other i-eturns gradually to its primitive 

 ^tate. Lead and iron produce the largest influence effects ; copper, gold, 

 and platinum give smaller but distinct effects ; zinc produces hardly any, 

 unless it be put within a hundredth of a millimetre or so. Pellat does not 

 iittempt to account for this interesting phenomenon further than hy 

 -iig^esting .some possible connection with tho smell of metals, 



^Ir. Hart's theoretical views are at first sight analogous to my own, 

 though they are by no means tho same. Ho considers the case of tv^ t 

 nietals immersed in liquid electrolytes, and dismisses air by calling it 

 a giiseous electrolyte. He believes zinc and copper in contact to be at 

 the same potential, and throws the variation of potential en the air between 



' Ihnt : Jlrif. A.i.^tic. l'iirl\ p. ."),";.', and J'/iil. Mai/., Nov. I SSI, ."> ser., xii., 321. 

 ■ I'ollat : C(ini/>ti\t /'iiii/iis, xciv., 1882. p. 1247. Inlinence of na^tala on one 

 another at a dLstainv. 



ii 

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