September 12, 1919] 



SCIENCE 



261 



takes place in an iron wire immersed in a 

 strongly oxidizing solution like concentrated 

 nitric acid, and is similarly associated witii a 

 return of tlie original sensitive or quasi-ir- 

 ritable properties of the film; this recovery is 

 also attended with a certain delay, analogous 

 to the refractory period of the protoplasmic 

 system. In this automatic return of passivity 

 both the general oxidizing action of the solu- 

 tion and the electrochemical oxidizing in- 

 fluence at the local anodic regions of the 

 metallic surface are factors. 



Iron wires which have previously been 

 rendered passive will frequently retain their 

 passivity for an indefinite time, if left undis- 

 turbed, in solutions whose oxidizing powers 

 are insuiEcient to impart passivity to already 

 active wires. This is the case, for example, 

 in solutions of nitric acid of less than 1.2 s.g. ; 

 in such solutions passivity remains unaltered 

 for an indefinite time, provided the continuity 

 of the surface-film is not interrupted (by me- 

 chanical or other means) over a sufficiently 

 great area. But activity, once it is estab- 

 lished, is permanent and the metal dissolves 

 in the acid. As a rule, extensive scraping or 

 vigorous jarring is required to activate me- 

 chanically a passive iron wire immersed in 1.2 

 HNOj, although different specimens of iron 

 vary in sensitivity. Apparently if the total 

 area of metallic surface which is thus freed 

 from film and exposed to the direct action of 

 the acid is less than a certain critical min- 

 imum, the local anodic action quickly reforms 

 the film, and the wire as a whole continues to 

 exhibit passivity. Hence a single scratch with 

 a glass rod may be ineffective, while if several 

 scratches are made simultaneously or close to- 

 gether the total or siunmated effect may be 

 sufficient for activation. This behavior throws 

 light upon the general nature of summation- 

 effects in film-covered systems of this class, 

 which include living protoplasm as well as 

 passive iron. Destruction of a sufficient area 

 of surface-film is followed by a rapidly pro- 

 pagated wave of activity which destroys the 

 whole film and renders the whole metal active. 

 Thus the physiological distinction between the 

 " local change " and the " propagated disturb- 

 ance" in irritable tissues, familiar to physiol- 



ogists from the work of Keith Lucas and 

 others, is exemplified in the behavior of such 

 wires. Any alteration of the film affecting less 

 than a certain critical area fails to propagate 

 itself and involve the whole surface. Ap- 



, , . active area , 



parently the ratio : must exceed a 



passive area 

 certain critical minimum if the activating 

 effect is to gain the predominance and involve 

 the whole surface of the metal; otherwise the 

 entire surface resumes the passive state. Local 

 conditions of either passivity or activity are 

 equally capable of spreading; and the final 

 state of the system as a whole depends upon 

 whether the one or the other condition gains 

 the upper hand. The tendency to revert to 

 passivity after local disturbance varies in 

 different metals and in different specimens of 

 the same metal; for example, in nickel (in 1.2 

 HIN'O,) it is much greater than in iron. 

 Hence the local state of the surface at any 

 time is determined by the relative intensity of 

 the two opposed processes, one of which tends 

 to form and the other to destroy the surface- 

 film. A similar statement holds true of the 

 protoplasmic systems; in the maintenance of 

 any living structure constructive or " ana- 

 bolic " processes are continually at work, which 

 compensate or offset the continually acting 

 destructive processes; this applies to the sur- 

 face-film as well as to other protoplasmic 

 structures. 



Another simple observation, continually re- 

 peated in these experiments, indicates still 

 further the active or self-regulating character 

 of the process by which the surface-film of 

 passive iron is preserved intact in an oxidizing 

 solution (e. g., 1.20 Hl^TO,) in spite of minor 

 disturbances or local alterations in the film. 

 All solutions of chlorides rapidly destroy 

 passivity, at a rate which is approximately pro- 

 portional to the concentration of the Cl-ions; 

 usually in the wires used in the following ex- 

 periments an exposure of 8 or 10 seconds to 

 m/1200 ISTaCl or KCl is required to render 

 a passive wire reactive to 1.20 HN"03. When, 

 however, such a wire is exposed to the salt 

 solution for less than this critical period, e. g., 

 for 6 seconds, and is then dipped momentarily 



