316 Van Name and Hill — Solution of Metals 



influence of the chemical reaction velocity, are all lowered 

 by an increase in the viscosity. In other words, these results 

 illustrate the case, alluded to above, of a reaction velocity 

 determined by the simultaneous influence and mutual relation 

 of both factors, diffusion process jind chemical reaction. 



Nor is there any reason for believing that such cases are 

 unusual. On the contrary, it seems proper to regard the 

 observed velocity as being normally the resultant of the simul- 

 taneous action of the two factors just mentioned. Cases in 

 which one factor predominates to the virtual exclusion of the 

 other are, therefore, merely limiting cases, though no doubt 

 they occur very often in practice. 



This point of view appears to us to be the most reasonable 

 and helpful one in dealing with heterogeneous reactions of the 

 general type under consideration, and will be the one employed 

 throughout the present paper, in interpreting experimental 

 results. 



The results obtained with ferric chloride, summarized in the 

 lower section of Table VIII, tend to confirm, so far as their 

 evidence goes, the conclusions drawn above from the experi- 

 ments with ferric sulphate. These values are probably less 

 accurate than those for sulphate solutions, and less stress can 

 be laid upon small differences* They prove that cadmium and 

 iron dissolve in ferric chloride at the same rate in the presence 

 of Ol molar hydrochloric acid, and probably also when the 

 acid is - 5 molar. This indicates that in these cases the 

 observed reaction velocities are essentially rates of diffusion. 

 It is not clear whether the effect of this increase in acidity is 

 to raise or to lower the observed velocity, but the change is at 

 all events small, as would be expected in a case in which the 

 viscosity change is almost negligible (about 3 per cent).* 



The same increase in acidity produces a marked rise in the 

 rate of solution of copper. We would avoid laying any 

 emphasis on the absolute values of these copper constants, on 

 account of the complications already mentioned. Neverthe- 

 less, it may be pointed out that the relations of these two 

 velocities to one another, and to the corresponding values for 

 cadmium and iron, involves nothing unexpected or inconsistent 

 with the point of view stated above. The lower value for 

 copper in - l molar hydrochloric acid, as compared with cad- 

 mium and iron, seems to be clue to the influence of the rela- 

 tively slower rate, in the case of copper, of the chemical part 

 of the reaction, this rate being accelerated by the increase in 

 acidity to 0'5 molar, up to the point where the rate of diffusion 



*In alike manner, the fact that the ferric sulphate constants change so 

 little between 001 and - 25 naohtr sulphuric acid, may be ascribed largely 

 to the smallness of the accompanying viscosity change. 



