eli:(t::( "MOTIVE joucivs ix the voltaic tEi.r.. 



4'Jl 



thongli 

 jat may 



sopliical 

 3gard to 

 athcr of 

 jets old- 

 ndcncy,' 

 course is 

 ubt that 

 ■o due to 

 ndency ' ; 



iiavo also 

 with fair 

 803-000, 



n orvcnt'tniH 

 correct". 

 trikiiiK sen- 



M-Jil iiiaiiiKr 

 lace, 1 must 

 r case as tin' 

 on the '■on- 

 any currout 

 iiic'al action. 

 :lu>r than you 

 wo dilVcrent 



1 a cliciaii-'i'l 



;o substances 



circulation.' 



licrc called a 



ling but that 

 orco clectre- 

 |ove stated, a 

 lan be drawn, 

 lifect (it uiorc 

 ;h a seusiblo 

 the common 

 ly contact. 



|(> cause of i'- 

 w now very 

 uric acid, tb' 

 tt autlicientlv 

 hi the decoiii- 

 Int to prodiKT 

 luical atlir.ii,^ 

 it ; and lliat 

 ^ices, for til'' 



^anilthetluil 



Ictween it a'"' 

 lie conductiii'-' 

 Tribod [that ot 



fposing it ;;'' 



1 voltaic celP, 



For much discns.sion of contact eloctricity, and for some interesting^ 

 statements of the views of Marianini, Davy, and others, refer to 'Experi- 

 iiicntal Researches,' vol. ii. p. 20, &c. From what is there said it appears 

 that Karsten and Marianini held a modified contact theory, placing the 

 K.I\I.F. at the metal-fluid junction ; and that IJecquerel admitted as a pos- 

 sibility the ellicieiicy of chemical attraction, asdistinct from combination, 

 soniothinof in the same Avay as Schonbein. 



Pinf. Tait, in his ' Thermodynamics,' lends his powerful support to 

 the contact view of the activity of the pile as tantrht by Sir W. Thomson. 



S(ime work has been done in the direction of observing reversible heat 

 ( fleets at metal-liqnid junctions, notably by Joule, Thomson, and liosscha. 



Joule in 18il sent currents through several dilute acid voltameters 

 with different electrodes, and measured the e.Kcess or reversible heat 

 If-KC- generated in the whole cell; with the re.sult that the excess 

 of heat ob.served is tliat due to the observed hack E.!M.F. of the cell, 

 inimi^ that concerned in the decomposition of water. A table of his 

 results is given by Chrystal, ' I'^ncy. IJrit.,' p. '.»1. For ^laxwell on the 

 same subject ave ' Elementary Electricity,' p. 14G. 



Tiiomson (Math, and Pliys, Papers, pp. 400, oO^) says thai of two 

 (leeornposition cells, one with zinc cathode, the other with platinum 

 cathode, the former showed the most heat when the same current was 

 sent through both. iSeparating the electrodes by a porous cell, zinc 

 cathode showed more heat than zinc anode ; but platinum anode more 

 heat than platinum cathode.' He speaks of the local heat developed at 

 a tin surface, and shows that it is greater where hydrogen is liberated 

 than where tin is dissolved; and suggests thermal observations on four 

 dilute acid voltameters in one circuit with zinc and platinum electrodes, 

 arranged according to the permutations, zinc zinc, zinc ])latinum, 

 ]ilotinum zinc, and platinum platinum. Thomson attributes the extra 

 lieat at an electrode to opposing chemical affinities which have to be 

 overcome — a doctrine of ' chemical resistance.' 



'■lecir.isc, wlicn t/wi/ are used, tlie c]u>mical atfinitics liotween tlirm and tlie zinc 

 I'rodiicc a contrary and opposing action to that wliidi is inlluenti;d in the dilute 

 >ul]ihuric aei<l ; or if that action lie hut .small, still the alHnity of their coinjionent 

 j'lirls fdr each other has to lie overconu", for they cannot conduct wirliout sujferin<;' 

 <lecouip(i>itiiin; and tins decomposition is found c.rpi'rinii'iititllii to react bai^k upun 

 die forcu'N whicii in the acid tend to ]U'oduce the cun-i'nt, .'ind in numerous cases 

 iiitirely to neutralise them. Where dirtr;.'t contact (jf the zinc and platina occurs, 

 I hese obstructing forces are not brought into action, and therefore the production 

 and (be circulation of the cdectric current, and the concomitant action of dccomiiosi- 

 liiu are then highly fa\oure(l. 



' (>S!i."(.) It is evident, however, tliat one of these op])osing actions may be dismissed, 

 ■i.d yet an electrolyte be used for tiie purpose of completing tlu^ circuit between the 

 iiiie and platina immersed separately into the dilute acid; for if iri the above 

 experiment the platina wire be retained in metallic contact withthozinc platt?, and a 

 'livi>i(in of the platina be made elsewhere, then the solution of iodide placed there, 

 being in contact with platina at both surfaces, exerts no chemical aflinities for that 

 metal ; or if it does, they are eriual on botli sides. Its power, therefore, of forming 

 •■> current in opi)osition to that dependent upon the action of the acid in the vessel 

 1- removed, aiul only its resistance to dei?ompos£tion n>mains as the obstacle to be 

 ovcre.onic by the alhnities exerted in the dilute sulphuric acid. 



'(s'.)ti.) This becomes the condition of a single pair of active ])lates where 

 'iictdllir contact is allowed. In such cases, only one set of opposing a!linities are to 

 '■oevcrconio by those which an- dominant in the vessel: whereas, when metallii'. 

 lontactis not allowed, two sets of opposing affinities must bo conquered (894).' 



' Showing, I suppose, that while zinc attracts oxygen much, and hydrogen not at 

 ■dl, platinum attracts livdro<ren more than oxvgen. 



i 



M! 





