518 Prof. E. F. Herroun on the Electromotive 



give Pt = O05, An = — 0*05, which appear to be much 

 more probable values, and make gold, as its thermochemical 

 data require, negative as regards platinum. 



Under these circumstances it appeared worth while measur- 

 ing the actual E.M.F. set up between zinc and gold, and zinc 

 and platinum, in solutions of their own chlorides of equal 

 molecular strength. 



Zinc- Platinum Cell. 



The heat of formation of PtCl 4 is apparently unknown, but 

 as it appears to be impossible to prepare a neutral solution of 

 that salt, compounds such as PtCl 4 ,2HCl or PtCl 4 ,2NaCl 

 must be substituted, and their heats of formation are given 

 by Thomsen. I selected the latter salt on account of its 

 freedom from acid, and prepared a neutral solution having 

 the strength of •25(PtOl 4 2NaCl)100H 2 O, which therefore con- 

 tained about 2*75 grams of Pt in 100 cub. centim. of solution. 



Thomsen gives for [Pt,Cl 4 ,2NaCl, aq] the value 73720 

 + 8540 = 82,260 calories ; and for [Zn,Cl 2 , aq] the value 

 112,840 calories. These numbers would give as the heat 

 of replacement of one equivalent of platinum by zinc the 

 nett heat evolution of 35,855, which is equivalent to a theo- 

 retical E.M.F. of 1-548 volt. 



A cell was set up consisting of an amalgamated pure zinc 

 rod immersed in a solution of '25 ZnCl 2 100H 2 O, opposed to a 

 clean platinum plate, immersed in the solution of sodio-platinic 

 chloride above described. The two solutions were separated 

 both in this and other experiments by an ordinary porous 

 earthenware pot, and the E.M.F. was measured by balancing it 

 against a difference of potential by Poggendorff's method. 



The standards taken were a Latimer- Clark cell, which was 

 assumed to have an E.M.F. of 1*435 volt at 15° C., and a 

 chloride of silver battery (modified De-la-Rue cell), which 

 by comparison with the Clark cell was found to have an 

 E.M.F. of 1*045 volt. I find this cell more convenient in 

 using Poggendorff's, or any similar method, as its E.M.F is 

 not appreciably disturbed by its sending a small current, or 

 by shaking, and it has a smaller temperature-coefficient than 

 the Clark cell. The temperature of all the cells used in these 

 experiments only varied between the narrow limits of from 

 12° to 15° C, 



In one experiment the zinc-platinum cell, when first set up, 

 gave an E.M.F. of 1*647 volt. It was then allowed to send 

 a current through a low external resistance for five minutes, 

 and after further resting for five minutes its E.M.F. was again 

 measured, when it was found to have dropped to 1*473 volt. 



