430 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 



single voltaic circuit, or of Wollaston's powerful single pair of metals. This diffi- 

 culty has arisen altogether from the antagonism of the chemical affinity engaged in 

 producing the current with the chemical affinity to be overcome, and depends entirely 

 upon their relalive 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 

 quantity, with the course of those which are exerting the most intense action. 



898. Water has generally been the substance, the decomposition 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. 316.) 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 intensity of their ordinary chemical affinities. This reason appeared in their 

 reaction back upon the affinities tending to cause the current ; and it appeared pro- 

 bable, that many substances might be found which could be decomposed by the cur- 

 rent 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 expe- 

 riments offer new and beautiful proofs of the direct relation and opposition of the 

 chemical affinities concerned in producing and in resisting 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, b, and c are platina 

 wires ; the decompositions were effected at x, and occasionally, indeed generally, a 

 galvanometer was introduced into the circuit at g : its place only is here given, the 

 circle at g having no reference to .the size of the instrument. Various arrangements 

 were made at x, according to the kind of decomposition to be effected. If a drop of 

 liquid was to be acted upon, the two ends were merely dipped into it ; if a solution 

 contained in the pores of paper was to be decomposed,, one of the extremities was 

 connected with a platina plate supporting the paper, whilst the other extremity rested 

 on the paper, e, fig. 12 : or sometimes, as with sulphate of soda, a plate of platina 

 sustained two portions of paper, one of the ends of a and c resting upon each piece, 

 c, fig. 14. The darts represent the direction of the electric current (667.). 



900. Solution of iodide of potassimn, being placed in moistened paper at the inter- 

 ruption of the circuit at x, was readily decomposed. Iodine was evolved at the a7iode, 

 and alkali at the cathode, of the decomposing body. 



901. Protochloride of tin, when fused and placed at x, was also readily decom- 

 posed, yielding perchloride of tin at the anode (779.), and tin at the cathode. 



902. Fused chloride of silver, placed at x, was also easily decomposed ; chlorine 

 was evolved at the anode, and brilliant metallic silver, either in films upon the sur- 

 face of the liquid, or in crystals beneath, evolved at the cathode. 



