THE SINGLE POTENTIAL OF THE PURE IRON ELECTRODE. 31 



falling off of a higher potential artificially and temporarily created in the 

 metallic electrode with the help of hydrogen conveyed to the system from 

 without, as will be explained in the succeeding section of the paper. Hence 

 there can be little doubt that the potential of porous iron reduced at about 

 i,ooo and immersed in normal ferrous sulphate is really as high as 

 0.79 0.612 = 0.18 volt, if the decinormal calomel electrode is taken as 0.612, 

 the normal calomel electrode being taken as 0.56. Pure iron which has been 

 fused in a vacuum gives a value nearly 0.03 volt lower, or about 0.15 volt. 

 This latter value, corresponding to the flat surface of a coherent mass, is the 

 one to be chosen ordinarily as the normal value. Even this is nearly twice 

 as large as that usually ascribed to it. The errors of the work of others were 

 probably insufficient time of immersion and the presence of acid in the electro- 

 lyte, both of these tending to lower the observed value, as is shown above. 



