RALPH S. LILLIE 133 



is closely proportional to the excess of concentration of the acid above 

 a certain critical limit (of about 53 to 54 per cent) .■* Hence in strong 

 acid the film at its first deposition has a thickness (or a structure) 

 such that the relative quantity removed by the cathodic reduction 

 at the active-passive boundary when the metal is locally activated is 

 less than in weaker acid. The decrement of transmission is hence 

 from the first steeper in the strong acid, and a greater time is required 

 for the removal of the surplus fihn material (by the solvent action of 

 the acid) until it attains a degree of thinness permitting of rapid and 

 complete removal under the influence of the local circuit. 



The important fact from the point of view of the physiological 

 comparison is that after the decrement stage has passed the local 

 reaction is complete and does not admit of gradations. Transmis- 

 sivity is then complete and the wire as a whole exhibits an "all or 

 none" type of response. This stage is a permanent one, as long as 

 the metal remains undisturbed in the acid, and apparently it corre- 

 sponds to a condition of minimal thickness of film. Judging from the 

 analogy with adsorption processes in general, it seems probable that 

 at this stage the film is only 1 molecule in thickness.^ It is evident 

 that with a film of this thickness there would be no possibility of gra- 

 dation in the local action; either it would be complete or would not 

 occur. Uniform and complete action at the different areas of the sur- 

 face in a transmitting wire would thus be necessitated, with the 

 result that the activation wave would be transmitted for an unlimited 

 distance with a uniform velocity whose exact degree would be deter- 

 mined by the local rate of reduction of the film. 



These considerations lead to the conclusion that the condition 

 reached by the passivating film when complete transmissivity is 

 regained is one in which its material is spread out in a uniform layer 

 of 1 molecule in thickness. In this respect it corresponds in structure 

 to films of oil or fatty acid which have spread over the surface of water 

 to an equiUbrium stage, as in Langmuir's experiments.® The precise 

 composition and structure of the film do not concern us at present; 



^Cf. LiUie, R. S., /. Gen. Physiol., 1920-21, Hi, 119, Table II; and 125, Fig. 3. 

 ^ Cf. Freundlich, H., Kapillarchemie, Leipsic, 1909, 278. 

 ^Langmuir, I., /. Am. Chem. Soc, 1917, xxxix, 1848. 



