226 



JONES AND SCHUMB. 



temperature coefficient of the difference is therefore —.0045/25 = 

 — .00018 volt per degree. 



Our experiments, therefore, corroborate the estimate made by Lewis 

 and Randall in 1914: — 



" The temperature coefficient of the e. m. f . between the decinormal 

 and normal calomel electrodes should be determined more accurately. 

 At present we can obtain only an approximate result as follows: The 

 e. m. f. at 25° we have shown to be —0.0530 v., and at 18°, Sauer has 

 found for the same combination —0.0514. Hence, for the tempera- 

 ture coefficient we find —0.00023. If, instead of using Sauer's result, 

 we take 0.0530 at 25°, and assume the e. m. f. of the combination to be 

 proportional to the absolute temperature, then we find —0.00018 for 

 the coefficient. For want of better information we shall average these 

 two values and assume the temperature coefficient to be independent 

 of the temperature." 



Following is a summary of values of the normal potential of thal- 

 lium, including only those results with which the values obtained in 

 the present research may fairly be compared. Starred values are 

 calculated results based on determinations under somewhat different 

 experimental conditions : 



TABLE X. 



Section 4. The Potential of the Iodine Electrode at 25°C. and 

 0°C. 



The normal potential of the iodine electrode is the only remaining 

 quantity required for the calculation of the free energy of formation 



