314 Van Name anil I /ill — Solution of Metal Is 



of diffusion of the active substance, and shall he practically 

 independent of the nature of the solid. JN'ow it cannot he 

 denied that disturbing influences may have slightly affected 

 some of the results in the tahle ; in particular, the nickel con- 

 stants are rather uncertain. On the other hand, there was no 

 evidence whatever of interfering effects in the cadmium exper- 

 iments. Moreover, the distinctly systematic nature of the 

 results in the tahle argues against the possibility that any con- 

 siderable number of them represent "anomalous" cases. 

 Finally, the decreasing constants obtained with silver were 

 clearly traced to the retardation of the chemical reaction by 

 the accumulation of silver sulphate, which would have only, a 

 trilling effect on the rate of diffusion. In short, these results 

 furnish grounds for seriously doubting the general validity of 

 Nernst's hypothesis. 



A study of the table seems to show that at one extreme we 

 have velocities determined largely or wholly by diffusion, at 

 the other, velocities determined chiefly by the rate of the 

 chemical reaction, and between them velocities in which we 

 can readily detect the simultaneous effect of both influences. 

 Although these results do not cover the full range from a 

 purely chemical reaction velocity to a pure diffusion velocity it 

 is evident that the transition from one to the other is gradual, 

 not abrupt. 



This point becomes clearer on further analysis of the results. 

 An increase in the concentration of free sulphuric acid above 

 0"25 molar produces in all cases a marked decrease in the 

 observed reaction velocity. In general, the activity of an oxi- 

 dizing agent is increased by a rise in the concentration of 

 hydrogen ion. It will hardly be doubted that this rule applies 

 in the cases with which we are dealing, and we should, there- 

 fore, expect that the free energy of the reaction with the metal 

 would increase with increasing acidity. This is supported by 

 the potentials in Table VI so far as inferences can safely be 

 drawn from the behavior of electrodes which are imperfectly 

 reversible. This table contains the single potentials of silver, 

 copper, and cadmium against solutions of the same composi- 

 tion as those used in the reaction velocity measurements. As 

 we have already noted, the values show that with increasing 

 concentration of sulphuric acid the metal in all cases becomes 

 less positive or more negative toward the solution, that is, the 

 change is in the direction of increasing free energy. 



For our purpose, however, the important question is the 

 effect of an increase in acidity upon the velocity of the chem- 

 ical part of the reaction with the metal. Although, in general, 

 there is no necessary correspondence, even in sign, between 

 variations in reaction velocity and variations in the magnitude 



