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XXXVII. Note on a new Method of Measuring certain 

 Chemical Affinities. By Alfeed Tribe*. 



WHEN a metal is immersed in an electrolytic field (i. e. 

 in an electrolyte in the act of electrolysis), and the 

 electromotive force set up on any part of its surface suffices 

 to overcome the affinities of the radicals of the medium, the 

 positive ion separates on that part of the surface which may 

 be supposed to have received — electrification, and the nega- 

 tive ion on that part which receives + electrification. 



When the metal is in the form of a rectangular plate and 

 placed so that the lines of force are perpendicular to its sur- 

 face, the maximum electromotive force set up is on the central 

 parts of such a plate, becoming less towards the edges, where, 

 and for some little distance from which, it is insufficient to 

 initiate electrochemical action. 



When the rectangular plate is placed in the electrolytic 

 field so that the lines of force are parallel with one of its edges, 

 the maximum electromotive force is on the end of the plate, 

 becoming less and less towards the central parts until it no 

 longer suffices to bring about electrochemical action. This is 

 denoted by the boundaries of the deposits, which in many cases 

 are very sharply defined. A plate in fact, in the position just 

 named, may be regarded, in so far as electrical power is con- 

 cerned, as a series of pairs of electrodes, the limits of the 

 electro-deposits representing a pair, the electromotive force of 

 which is just incapable of resolving the electrolyte into its 

 constituent ions (Proc. Roy. Soc. nos. 209, 214, 1881; Phil. 

 Mag. vol. xi. p. 446, 1881). 



The very intimate relation between electromotive force and 

 chemical affinity was a long time ago pointed out by Sir W. 

 Thomson, and more recently in an elaborate research by Dr. 

 Wright. It was therefore anticipated that if, in a series of 

 trials, the chemical affinities were altered while other circum- 

 stances remained the same, the magnitude of the intermedial 

 space between the boundaries of the electro-deposits would 

 increase along with the force required to overcome the affini- 

 ties of the ions of the electrolyte. 



A series of experiments with molecular solutions of the 

 chloride, bromide, and iodide of zinc with plates (analyzers) 

 of silver, copper, iron, and zinc showed that this is the case ; 

 ft. e. the intermedial space with the chloride was greatest, the 

 bromide less, and the iodide least. Another series with silver 



* Communicated "by the Author, having been read at the Meeting of 

 the British Association, York, September 1881. 



