ELECTROMOTIVE FORCES IN THE VOLTAIC CELL. 465 



that these views are agreed with, they may yet serve as links with which 

 to connect the facts and the multifarious observations thereon. 



1. In the course of my reading on the subject, I have found only two 

 great and epoch-making papers, that of Volta in 1801, and that of Sir 

 William Thomson in 1851. Other contributions are some of them 

 keen, like those of Faraday and Clerk Maxwell ; some of them laborious, 

 like those of Hankel and Ayrton and Perry ; but none contain anything 

 essentially and powerfully new except those two : unless indeed we 

 include in the subject the immensely important phenomena of Seebeck 

 and of Peltier, and Faraday's fundamental law of electro-chemical decom- 

 position. 



Volta l showed that when two metals were put into contact and sepa- 

 rated, the insulated one was charged with electricity sufficient to make 

 gold leaves diverge. He also stated that the contact force between any 

 two metals was independent of intermediate metals, so that the metals 

 could be arranged in a definite numerical series ; and he gave the first 

 series of the kind — 



Zn^-^Pbw->Sn^^Fe— -Cu^^Ag. 

 5 112 3 



Moreover he started a hypothesis to account for the action, a sort of im- 

 pulsion or attraction of electricity by matter — an idea subsequently elabo- 

 rated by Helmholtz. Fabroni 2 objected to Volta's explanation of his ex- 

 periment. He denied contact force and considered that the electricity 

 was developed by chemical action. 



Then the fight began, and lasted on and off some half-century. On 

 the one side were Volta, Davy, Pfaff, Peclet, Marianini, Buff, Fechner, 

 Zamboni, Matteucci, and Kohlraasch. On the other were Fabroni, 

 Wollaston, Parrot, GErsted, Ritchie, Pouillet, Schonbein, Becquerel, De 

 la Pave, and Faraday. 



It was not all fighting ; part of it resulted in a more thorough investi- 

 gation of voltaic phenomena, and very often the original point of dispute 

 was lost sight of, and Volta's fact itself was doubted in the eagerness to 

 disprove Volta's explanation. The experiments of Pfaff and Peclet, 3 

 however, fairly well established the correctness of his observation, and 

 Kohlrausch showed how, by means of a Daniell's cell combined with a 

 condenser, to measure Volta forces absolutely, thus inventing a method 

 which has been employed with modifications by Hankel, by Gerland, by 



■Volta: Gehler's Worterbvch, iv. G16. See also a carefully edited version 

 Annates de Cliim. xl. 1 ser., p 225, 1801. 



2 Fabroni : Journal de Physique de I'abbe Rozier, xlix. 348. 



8 Peclet on the contact of good conductors.— Comptes llendus, 1838, p. 930.— Pogg. 

 Ann, xlvi. 1S39, p. 346. Ann. de Cltim. 1842 and 1841, 3 ser. ii. 233. 



Pfaff, letter to Gay Lussac.— Ann. de Udm. 2 ser. xli. 236, 1829. Pfaff.—' For 

 and against the production of E. by chemical processes, as a consequence of some 

 experiments on the E.M.F. of liquids and metals.'— Pogg. Ann. xli. 1840, pp. 110 and 

 197. Pfaff—' Experimentum crucis in favour of the contact theory.'— Pogg. liii. 1841, 

 p. 303. This crux is on p. 306, and consists in substituting ZnS0 4 for H 2 S0 4 in a Grove 

 cell, and showing that the current through a thin wire galvanometer is stronger than 

 before. This, lie says, leaves no further shift or evasion (Ausflucht) for the chemical 

 i e °£ y ix ** iS a fact we have g rown accustomed to, but it is rather surprising that 

 the E.M.F. yiven by ZnS0 4 should be even higher than that given by H„S0 4 . A con- 

 venient ' Ausflucht ' could nevertheless be provided for the chemical theory by pointing 

 out that the combustion heat Zi,2N0 3 is greater than Zn,S0 4 -H„,S0 4 + 2(H,N0 3 ), if 

 t%e faCt ,,e so " Another shift is to talk about basic sulphate and the sourness 

 of ZnS0 4 : anothor is to use the word ' dissociation.' 



1884. H H 



