ELECTROMOTIVE FORCES IN TKE VOLTAIC CELL. 487 



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

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

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

 uniformly with temperature. But it is permanent, remaining after the 

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

 oxidation proceeds with time alone. Time curves are logarithmic like 

 coolino" curves, and the most oxidisable metal varies most quickly both 

 for time variation and temperature variation. There seems to be a surface 

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

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

 ■changes with temperature. 



Mr. S. Lavington Hart, in 1881, ' describes a mercury dropper where 

 the mercury is contained in a funnel, and is connected with an electro- 

 meter by au iron rod dipping into it. The drops form inside an iron 

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

 any A'olta force that he considers the arrangement as an inversion of 

 Lippmann's electrometer, the advancing drops being oxidised. It can 

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

 and that is what I suppose it to be. He makes two interesting modifica- 

 tions : the first is to replace the air round the dropping mercury by coal- 

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

 exuding drops of mercury, unlike most pieces of metal, expose to the coal 

 gas a virgin surface which has probably contracted no condensed air sheet : 

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

 to. It' the experiment is regarded as sufficiently direct and simple, this 

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

 surrounding 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. 

 Hart 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 

 out by Pellat in a paper published in 1882. - Pellat says that if he places 

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

 half a millimetre more or less : variations from 12 to T), each metal under- 

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

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

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

 influencing metal is removed the other returns 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 

 attempt to account for this interesting phenomenon further than by 

 suggesting some possible connection with the smell of metals. 



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

 though they are by no means the same. He considers the case of two 

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

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

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



1 Hart: Brit. Assoc. York, p. 555, and Phil. Mag., Nov. 1881, 5 ser., xii., 324. 

 « Pellat : Comptes Ilciuhts, xciv., 18S2, p. 1247. Influence of metals on one 

 another at a distance. 



