Chemical Affinity in terms of Electromotive Force. 355 



on altering the current-strength to some value previously 

 experimented with, the value of e always immediately altered 

 to something very close to the value it previously was found 

 to possess for the altered current, which could not have been 

 the case had the increase in value of e found for a stronger 

 current been simply due to the increase in the value of h! 

 owing to the accumulation of copper sulphate solution round 

 the + electrode. 



Very little difference in the value of E was noticed when 

 gold was substituted for copper as the negative electrode 

 (the dimensions and the positive electrode remaining un- 

 changed), the curve deduced by plotting the values of E — CR 

 as ordinates and the rates of flow per square centimetre as 

 abscissae being nearly, though not quite, the same whether 

 the — plate were copper or gold; what little difference did 

 exist was of such a nature that the gold — electrode curve lay 

 slightly above the copper one. On substituting platinum for 

 copper, the curve was very distinctly depressed ; i. e. the values 

 of E — CR, for a given rate of flow and a constant copper posi- 

 tive electrode were smaller with a platinum electrode than with a 

 copper one, and smaller with a copper one than with a gold one. 

 Whether the values thus registered by the aid of the quadrant- 

 electrometer are strictly comparable, or whether they are 

 complicated by a difference of contact action taking place 

 between the electrode and the brass quadrant of the electro- 

 meter (by means of the connecting-wire), is, however, open 

 to discussion *. Admitting that the contact action does not 

 materially affect the values obtained, the numbers found with 

 the gold, copper, and platinum — electrodes would indicate 

 that the value of n x is greater with platinum than with copper 

 and greater with copper than with gold; i. e. the "condensing " 

 actions of the three metals for "nascent" hydrogen lie in 

 the order, platinum greatest, then copper, and lastly gold. As 

 regards platinum and gold, this is precisely the conclusion to 

 be drawn from the experiments described in § 90. 



Experiments bearing further on the phenomena observed 



* It is manifest that, whatever may "be the nature of the contact actions 

 between two dissimilar metals &c, and however this may affect the determi- 

 nation of the true difference of potential between two dissimilar plates con- 

 nected respectively with the two quadrants of an electrometer, such actions 

 have no influence at all on the result when the two metals are the same ; so 

 that whilst the mercury in a mercury-cup and two thick copper wires 

 dipping into it may differ in potential, yet the two wires will be at the 

 same potential, i. e. just away from the mercurial surface; and the dif- 

 ference of potential between the two quadrants of an electrometer is the 

 same as that between the two plates of a voltameter &c. respectively 

 connected with them when these plates are of the same material, e. g. 

 platinum, although the intermediate connecting- wires and plates them- 

 selves may differ in their absolute potentials from the quadrant-potentials. 



