in Ferric Salts and in Chromic Acid. 



325 



pared with that of the diffusion process, a condition which 

 Nernst assumes to he always fulfilled except when secondary 

 effects interfere. 



Such a case, as Brunner* has shown, may conveniently be 

 represented hy a diagram, fig. 1, in which the ahscissas repre- 

 sent distances from the surface of the metal, and the ordinates 

 concentrations. The shaded portion is the metal, OY its sur- 



Fig. 1. 



Fig. 2. 



Fig. 3. 



\A 



3 



face, the dotted line is the outer limit of the diffusion layer, 

 and the broken line OAB the concentration of the oxidizer at 

 different points. 



In the case of our results this explanation is applicable only 

 to those experiments in the strongly acid solutions, in which, 

 within certain limits, the reaction velocity is independent of 

 the nature of the metal. ' Under conditions where different 

 metals show different specific velocities (exemplified by the 

 experiments at the lower acidities), the above explanation can 

 not apply, yet we find that in general the reaction velocity in 

 such experiments obeys the expression for a reaction of the 

 first order quite as well as when the velocity is independent of 

 the metal. 



The point of view which we have adopted (pp. 316 and 324) 

 assumes that the influence of the metal manifests itself in such 

 cases because the chemical reaction is not sufficiently rapid in 

 comparison with the diffusion process. In other words, the 

 concentration of the oxidizing agent at the surface of the metal 

 is not zero but has at every instant a finite value determined 

 by the relative velocities of the two consecutive reactions in- 

 volved. How this state of affairs can be reconciled with the 

 observed obedience to the laws of a monomolecular reaction, is 

 a point which calls for explanation. 



The case with which we are dealing, represented on a diagram 

 of the same type as fig. 1, gives fig. 2, in which the ordinate OC 

 represents the concentration at the surface of the metal. 

 Similarly fig. 3 shows the possible case of a reaction whose 

 chemical stage is extremely slow in comparison with the dif- 

 *Zeitsehr. phys. Chem., xlvii, 68, 1904. 



