ELECTROMOTIVE JOKCE.s IN THE VOLTAIC CELL. 491 



For much discnssion of contact electricity, and for some interesting 

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

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

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

 E.M.F. at the metal-fluid junction ; and that Becquerel admitted as a pos- 

 sibility the efficiency of chemical attraction, as distinct from combination, 

 something in the same way as Schonbein. 



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

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



Some work has been done in the direction of observing reversible heat 

 effects at metal-liquid junctions, notably by Joule, Thomson, and Bosscha. 



Joule in 1841 sent currents through several dilute acid voltameters 

 with different electrodes, and measured the excess or reversible heat 

 H— RC' 2 generated in the whole cell; with the result that the excess 

 of heat observed is that due to the observed back E.M.F. of the cell, 

 minus that concerned in the decomposition of water. A table of his 

 results is given by Chrystal, ' Ency. Brit.,' p. 91. For Maxwell on the 

 same subject see ' Elementary Electricity,' p. 146. 



Thomson (Math, and Phys. Papers, pp. 496, 503) says that of two 

 decomposition cells, one with zinc cathode, the other with platinum 

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

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

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

 heat than platinum cathode. 1 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 platinum, 

 platinum zinc, and platinum platinum. Thomson attributes the extra 

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

 overcome — a doctrine of ' chemical resistance.' 



because, when they are used, the chemical affinities between them and the zinc 

 produce a contrary and opposing action to that which is influential in the dilute 

 sulphuric acid ; or if that action be but small, still the affinity of their component 

 parts for each other has to be overcome, for they cannot conduct without suffering' 

 decomposition; and this decomposition is found experimentally to react back upon 

 the forces which in the acid tend to produce the current, and in numerous cases 

 entirely to neutralise them. Where direct contact of the zinc and platina occurs, 

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

 and the circulation of the electric current, and the concomitant action of decomposi- 

 tion are then highly favoured. 



' (895.) It is evident, however, that one of these opposing actions may be dismissed, 

 and yet an electrolyte be used for the purpose of completing the circuit between the 

 zii.c and platina immersed separately into the dilute acid ; for if in the above 

 experiment the platina wire be retained in metallic contact with the zinc plate, and a 

 division of the platina be made elsewhere, then the solution of iodide placed there, 

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

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

 a current in opposition to that dependent upon the action of the acid in the vessel 

 is removed, and only its resistance to decomposition remains as the obstacle to be 

 overcome by the affinities exerted in the dilute sulphuric acid. 



'(896.) This becomes the condition of a single pair of active plates where 

 metallic contact is allowed. In such cases, only one set of opposing affinities are to 

 be overcome by those which are dominant in the vessel ; whereas, when metallic 

 contact is not allowed, two sets of opposing affinities must be conquered (894).' 



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

 all, platinum attracts hydrogen more than oxygen. 



