302 Van Name and Hill — Solution of Metals 



peculiarities which make it easy to determine the cause of the 

 abnormal results. These anomalous cases can only be inter- 

 preted individually, and must, for the present, be excluded 

 from our general discussion. 



The hypothesis of a diffusion layer, together with its 

 immediate consequences, is usually enough to account for the 

 observed results. Nernst, however, whose opinion carries 

 special weight since he is to a large extent the originator of 

 the diffusion theory,* has introduced a second hypothesis. 

 Nernst argues that at the boundary surface between two phases 

 considerable differences in chemical potential must exist 

 between points infinitely near together, which should make 

 the velocit} 7 of the chemical reaction infinite, or at least in 

 practice extremely high.f Consequently the diffusion process 

 will always be slow in comparison, and will therefore deter- 

 mine the observed reaction velocity in all cases except, of 

 course, the anomalous ones already mentioned. 



If this hypothesis of Nernst's is to be understood literally it 

 means that the observed velocity of a reaction between a solid 

 and a dissolved substance will never represent, even approxi- 

 mately, the true rate of the chemical reaction. No place is 

 left for cases other than those in which the velocity is governed 

 solely by diffusion, and those rendered abnormal by disturbing 

 influences. From this it follows that when the same dissolved 

 substance reacts with different metals the velocity of the reac- 

 tion, in all cases free from secondary disturbances, should be 

 the same irrespective of the specific nature of the metal. This 

 test of Nernst's hypothesis is applied in the experiments to be 

 described. 



EXPERIMENTAL PART. 



The method and apparatus used have been described in a 

 previous article.^ Such minor modifications in the procedure 

 as were found desirable in dealing with the different reactions 

 will be mentioned in their proper connection. The different 

 metals, in the form of circular disks 0"5 mm in thickness and 

 gg.gmm j n (Ji ame ter, were exposed to the action of the solution, 

 under carefully regulated conditions as to temperature, rate of 

 stirring, and position of the disk. Samples of the solution, 



*Noyes and Whitney (Zeitschr. phys. Chem., xxiii, 689, 1897) were the 

 first to suggest the conception of a diffusion layer, but they applied it only 

 in a special case. Nernst, in 1904, gave the idea a more exact formulation, 

 showed its wide applicability, and made it the basis for a "general theory 

 of heterogeneous reactions." (See following footnote.) 



t Zeitschr. phys. Chem., xlvii, 52, 1904, also " Theoretical Chemistry," 

 3d Eng. ed., p. 586. 



\ This Journal (4), xxxii, 207, and in part, ibid., xxix, 237. 



