40 Dr. Faraday's' Experimental Researches in Electricity, 



the obstacle to be overcome by the affinities exerted in the di- 

 lute sulphuric acid. 



896. This becomes the condition of a single pair of 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 c ; whereas, when metallic contact 

 is not allowed, two sets of opposing affinities must be con- 

 quered (894..). 



897. It has been considered a difficult, and by some an im- 

 possible, thing to decompose bodies by the current from a 

 single pair of plates, even when it was so powerful as to heat 

 bars of metal red hot, as in the case of Hare's calorimeter, 

 arranged as a single voltaic circuit, or of Wollaston's power- 

 ful single pair of metals. This difficulty has arisen altogether 

 from the antagonism of the chemical affinity engaged in pro- 

 ducing the current with the chemical affinity to be overcome, 

 and depends entirely upon their relative intensity; for when 

 the sum of forces in one has a certain degree of superiority 

 over the sum of forces in the other, the former gains the 

 ascendancy, determines the current, and overcomes the latter 

 forces so as to make the substance exerting them yield up its 

 elements in perfect accordance, both as to direction and quan- 

 tity, with the course of those which are exerting the most in- 

 tense action. 



898. Water has generally been the substance, the decom- 

 position of which has been sought for as a chemical test of the 

 passage of an electric current. But I now began to perceive 

 a reason for its failure, and for a fact which I had observed 

 long before (315. S16*.) with regard to the iodide of potassium, 

 namely, that bodies would differ in facility of decomposition 

 by a given electric current, according to the condition and in- 

 tensity of their ordinary chemical affinities. This reason ap- 

 peared in their reaction back upon the affinities tending to 

 cause the current; and it appeared probable, that many sub- 

 stances might be found which could be decomposed by the 

 current of a single pair of zinc and platina plates immersed 

 in dilute sulphuric acid, although water resisted its action. I 

 soon found this to be the case, and as the experiments offer 

 new and beautiful proofs of the direct relation and opposition 

 of the chemical affinities concerned in producing and in resist- 

 ing the stream of electricity, I shall briefly describe them. 



899. The arrangement of the apparatus was as in fig. 5. 

 The vessel v contained dilute sulphuric acid ; Z and P are the 

 zinc and platina plates ; a, Z>, and c are platina wires ; the de- 



[* These numbers refer to part of the author's Third Series of Re- 

 searches in Electricity, which will be found in Lonu. and Edinb. Phil. Mag. 

 vol. iii. p. 254. — Edit.] 



