Forces of Gold and of Platinum Cells. 519 



After a further rest of about ten minutes it recovered to 1*507 

 volt, at which value it remained tolerably constant. During 

 the passage of the current from the cell the platinum plate 

 became covered with a black deposit of finely divided platinum, 

 and I thought it not improbable that this alteration of the 

 surface might be the cause of the marked falling off in E.M.F.; 

 but on removing the solution surrounding the platinum plate 

 and replacing it with fresh, the E.M.F. regained its initial 

 high value although the surface of the plate had not been 

 disturbed. It appeared from this that the high initial value 

 was probably due to oxygen dissolved in the liquid, which the 

 platinum would be very apt to occlude superficially, and 

 which would account for the uncertain values of the E.M.F. 



Whatever may be the cause, this variability renders the 

 measurement of the E.M.F. of this form of cell very difficult. 

 In other experiments values as high as 1*7 volt were 

 obtained on first setting up the cell, which, after sending a 

 current and resting, fell to the tolerably stable value of about 

 1*525. 



Maximum E.M.F. = 1*70 volt. 



Minimum „ = 1*473 „ 



Average „ 1*525 „ 



The average value (1*525) is seen to be slightly lower than 

 the E.M.F. calculated from the thermochemical equation 

 (1*548); but the difference is small (*023 volt), and is well 

 within the limits of experimental error with such a variable 

 cell. There is therefore no reason to assume that its actual 

 E.M.F. departs from the theoretical value. 



Zinc- Gold Cell. 



The heat of formation of [Au,Cl 3 , aq] is given by Thomsen 

 as 27,270 calories, and that of [Zn,Cl 2 ,aq] being 112,840, 

 the difference per equivalent gives 2*044 volts as the theo- 

 retical E.M.F. of zinc, displacing gold from weak solutions 

 of its chloride. 



A cell consisting of an amalgamated zinc rod immersed 

 in a solution of zinc chloride having the strength of 

 *25ZnCl 2 100H 2 O opposed to a plate of gold in a solution of 

 auric chloride of equal molecular strength, was set up and its 

 E.M.F. immediately measured. It was found to give an 

 E.M.F. of 1*855 volt, and after actual short-circuiting for 

 five minutes it had only fallen to 1*834. These values were in 

 fact the extreme limits of the variations that 1 have observed 

 on repeating the experiment, and the constancy of this cell 



