58 Van Name and Hill— Solution of Silver in Chromic Acid. 



sivo changes in the rate of solution of silver observed 

 in the present work, have nearly always been in the 

 opposite direction, — that of decreasing reaction velocity. 

 They are therefore of a kind which would be expected to 

 occur, if at all, only in the cases of the second class, that is, 

 in those in which the rate of the chemical reaction proper 

 has an influence upon the result. As was shown in our 

 former article, it is to this class that the case of silver 

 in chromic acid belongs. . 



The opposite proposition should also be true. Varia- 

 tions in the rate dependent upon the physical state of the 

 metal and not caused by roughness or impurities should 

 not occur in cases where the rate is governed solely by 

 diffusion. On this point the evidence is rather limited. 

 In our work on the rates of solution of metals we have 

 usually confined our experiments with a given metal to 

 specimens cut from one and the same sheet, so that the 

 conditions were not very favorable for detecting variations 

 of the kind in question, but so far as our experience goes, 

 it confirms the truth of the above proposition. One case, 

 however, which is clearly of the first class, has been much 

 more fully investigated. This is the reaction between 

 cadmium and dissolved iodine, which has been the subject 

 of several papers from this laboratory. 4 Although the 

 cadmium used in these investigations was rolled out into 

 sheets in small quantities at various times, and without 

 any attempt at uniformity of treatment, no characteristic 

 differences were observed in the rates of solution of the 

 numerous different samples so prepared, nor between the 

 behavior of the superficial and the interior layers of 

 metal in a single specimen. 



4 This Journal, 32, 207, 1911; 36, 543, 1913; 43, 449, 1917. 



