Forces of Gold and of Platinum Cells. 517 



reasonably supposed that gold evolves more heat in the forma- 

 tion of its chloride than does platinum. On referring to the 

 values for the heats of formation of the chlorides of these two 

 metals, as given by Julius Thomsen *, one finds, however, that 

 the heat attending the formation of auric chloride is, per 

 equivalent, only about half as great as that in the case of 

 platinic chloride. 



Assuming that the voltaic constants of metals are deducible 

 from the thermochemical values of their compounds, the 

 above facts would compel us to regard gold as more negative 

 than platinum, at least when immersed in chloride solutions. 

 (The same observations would also apply if oxygen were the 

 attacking medium, as Thomsen gives the heat of formation of 

 platinic hydrate as a considerable positive number, while that 

 of auric hydrate is a large negative quantity.) 



It was, therefore, an interesting point to determine how far 

 the actual electromotive forces obtained with gold and with 

 platinum agreed with these conclusions, and I endeavoured to 

 find records of the electromotive forces of cells in which these 

 metals are immersed in solutions of their chlorides opposed 

 to some other metal in a solution of its corresponding salt. 

 While there are many references to the E.M.FVs set up in 

 single fluid cells in which platinum is one of the metals used, 

 the references to gold are scanty, and even with platinum I 

 have only succeeded in finding one recorded measurement in 

 which the platinum was immersed in a solution of its own 

 salt. This was a measurement made by Wheatstonef, in 

 which liquid zinc amalgam was opposed to platinum in a 

 solution of platinic chloride. He found in measuring the 

 E.M.F. of this cell that it required 40 turns of his rheostat, 

 as compared wiih 30 turns required for a form of Daniell cell. 

 Now, assuming his Daniell cell to have had an E.M.F. of 

 1*09 volt, the value in volts for the zinc amalgam, platinum- 

 platinic chloride cell would be 1*453. 



In a list of the potential differences between different metals 

 and graphite simply immersed in water, Gotz and KurzJ 

 give the values 0*48 volt for gold and 0*37 volt for pla- 

 tinum, the value for zinc and graphite being 1'37 volt. 

 This would make platinum more electronegative than gold by 

 0*11 volt; but these values cannot be accepted as in any 

 degree expressing the actual electromotive forces concerned. 



Exner and Tuma§, on the other hand, taking carbon =0, 



* Thermochemische Untersuchungen, iii. pp. 412 & 430. 

 t Wheatstone's Scientific Papers, p. 115. 

 X Electrotechnie Zeit. ii. p. 30. 

 § Wien. Ber. xcvii. p. 017. 



