ERRORS IN PREVIOUS INVESTIGATIONS. 



21 



this paper the section on " Corrosion of Strained Metal," which is the only 

 part to be discussed here, is no more conclusive than Andrews's work, 

 although Hambuechen states his conclusions in unqualified terms. The 

 work was very carefully performed in every particular except one, namely, 

 the fact that he chose ferric chloride solution as the electrolyte in which to 

 measure the electromotive force of the iron test-pieces. This unfortunate 

 choice completely invalidates all the results. The reason is very simple. 

 Ferric chloride is always hydrolyzed to a considerable extent that is, it 

 contains acid. When such a solution comes in contact with iron the three 

 following reactions take place: 



Fe + 2Fe +++ = 3Fe ++ (or Fe + 2FeCl 3 3FeCl 2 ). 



Fe + 2H + = Fe ++ + H 2 (or 2HCI + Fe = FeCl 2 + H 2 ). 



H (nascent) + Fe +++ = Fe ++ + H + (or H + FeCl 3 = FeCl 2 + HC1) . 



There exists, then, at the electrode a continually changing concentration of 

 ferric, ferrous, and hydrogen ions, and reliable work is out of the question. 

 The effect of all these influences combined would cause the electromotive 

 force to change steadily, quite independently of any stress and strain effects. 

 The rate of this change would depend on such indeterminate and chance rela- 

 tions as the area of the iron surface to the volume and the concentration of 

 the solution, and on the mechanical arrangement, allowing more or less 

 rapid diffusion, etc. The following experiments are enough to show this. 

 Wires of pure iron were cleaned with fine emery cloth, carefully wiped, and 

 simply immersed in a ferric chloride solution. 28 They were under no arti- 

 ficial strain of any kind when measured. 



Table 4. The electromotive force of iron in ferric chloride against the 



decinormal electrode. 



During the first day hydrogen was steadily evolved from both wires, but 

 the second day this side reaction was greatly diminished. As is seen in the 

 table, the electromotive forces rose in these cases respectively 0.192 and 0.123 



Similar results were found by Finkelstein in 1902. (Z. phys. Chem., 39, 91). 



