RALPH S. LILLIE 131 



effect reduction; during transmission, therefore, we are to assume 

 that the removal of the film by cathodic reduction is continually in 

 progress through a certain distance in advance of the boundary. 

 The rate of this reducing action, and the distance from the boundary 

 through which it is effective (i.e. the length of XY) are the two chief 

 factors determining the speed of propagation. 



It seems probable that in the decrement type of transmission the 

 removal of the passivating film by cathodic reduction in the region 

 XY adjoining the boundary is everywhere incomplete, and that the 

 progressive decline in the traveling power of the activation wave is 

 thus to be explained. In the case just considered, however, in which 

 the local reduction is assumed to be complete, with consequent expo- 

 sure of a free surface of metallic or "active" iron over the entire area 

 XY, there will inevitably be complete transmission; i.e., the activa- 

 tion wave, once set up, will travel through an indefinite distance, 

 since then the area secondarily activated by the local current will 

 have the same properties as the original active area; accordingly the 

 current conditions at the boundary of this new active area will be 

 the same as at the original area, and these conditions will be repeated 

 at every new boundary as the active region extends. With exact 

 repetition of the same reducing effect at the cathodal (i.e. passive) 

 area adjoining each new boundary as it is formed, there is nothing to 

 prevent transmission through an indefinite distance. In transmission 

 with a decrement, however, it is evident that the chemical effect at 

 each new boundary is somewhat less than before, as shown by the 

 progressive decline in the traveling power of the activation wave. 

 If, as the above hj^othesis holds, the removal of the passivating film 

 near the boundary is in this case only partial, this partially activated 

 area will not reach the full anodal potential characteristic of a freely 

 exposed iron surface; and the reducing effect at the passive area ad- 

 joining will be less than at the original active area; i.e., there will be 

 a progressive decline in the reducing power of the local circuit as the 

 active area advances. Eventually, when the boundary of the active 

 region has advanced for a certain distance from the starting point, 

 the p.D. of the local circuit will fall below the critical value (i.e. the 

 decomposition voltage) required for appreciable reduction ; the advance 

 of the active region will then cease, and the line of cessation will be 



