520 Electromotive Forces of Gold and of Platinum Cells. 



contrasts in a striking manner with the variability of the pla- 

 tinum cells. 



Maximum E.M.F. = 1-855 volt. 

 Minimum ,, = 1'834 „ 



Mean „ = 1/844 „ 



Assuming the thermal values for gold to be accurate, the 

 actual E.M.F. thus measured is seen to be *2 volt below the 

 calculated value ; or, adopting the convention suggested by 

 Messrs. Wright and Thompson, — -0*2 volt is the thermo- 

 voltaic constant for gold in dilute neutral solutions of its 

 chloride. 



When a platinum plate was substituted for the gold plate 

 and immersed in the gold-chloride solution, the E.M.F. of 

 the cell thus formed was found to be 1*782 volt, i. e. less than 

 the gold-gold chloride, but greater than the platinum-platinic 

 chloride cell given above. 



From the thermochemical values one might conclude that 

 platinum would be capable of replacing gold from gold 

 chloride ; but, so far as my experiments have gone, I have 

 not found this to be the case, nor on the other hand is gold 

 able to replace platinum from platinic chloride, which, of 

 course, is not to be expected. 



When a plate of gold and another of platinum are im- 

 mersed together in pure water or in dilute hydrochloric acid, 

 the gold acquires slightly the higher potential. If strong 

 hydrochloric acid be substituted for the dilute, the direction 

 of the difference of potential becomes doubtful, and on the 

 addition of nitric acid to the strong hydrochloric, so as to 

 form aqua regia, the platinum acquires distinctly the higher 

 potential, and if the outside circuit be closed on a gal- 

 vanometer, a very decided current flows from the gold to the 

 platinum through the cell (cf. Ganot's Physics, article on 

 "Electromotive Series"). 



Why the nascent chlorine combines readily with the gold 

 where it evolves but little heat and slowly and reluctantly 

 with the platinum , in which reaction much more heat is dis- 

 engaged, is a problem which at present appears to admit of 

 no satisfactory solution. 



