142 TRANSMISSIVITY IN PASSIVE IRON WIRES 



high velocity of the recovery process in highly irritable tissues like 

 nerve, where the relative refractory period lasts for only a few thou- 

 sandths of a second, must be referred to special peculiarities of struc- 

 ture and metabolism whose detailed nature is entirely unknown at 

 present. 



SUMMARY. 



1. Passive iron (steel) wires, when activated after prolonged immer- 

 sion in nitric acid of 55 to 90 per cent concentration (volumes per cent 

 of HNO3, specific gravity 1.42) revert spontaneously to the passive 

 state, after a temporary reaction which is transmitted rapidly over 

 the whole length of wire. The duration of this reaction at any region 

 decreases rapidly with increase in the concentration above a certain 

 critical limit of 52 to 54 per cent. In weaker acid (50 per cent and 

 lower) the reaction continues uninterruptedly until all the metal is 

 dissolved. 



2. Immediately after this automatic repassivation the wire fails to 

 transmit activation through more than a short distance (1 to 2 cm.); 

 if left undisturbed in the acid it recovers by degrees its power of 

 transmission (as measured by the distance traveled by an activation 

 wave), at first slowly, then more rapidly; eventually, after an interval 

 varying with the concentration of acid and the temperature, the acti- 

 vation wave is transmitted through an indefinite distance as before. 



3. The return of complete transmissivity in 55 per cent acid occu- 

 pies less than a minute (at 20°) ; in stronger acid it is more gradual, 

 requiring in 90 per cent acid 20 minutes or more. This ''complete 

 recovery time" is nearly proportional to the excess of concentration 

 of acid above the limiting value of 53 to 54 per cent. 



4. In a given solution of acid the rate of recovery exhibits a tem- 

 perature coefficient closely similar to that of most chemical reactions 

 at this temperature (3-20°), and also to that of the rate of recovery 

 (refractory period) of irritable living tissues after stimulation (Qio = 

 about 3). 



5. Two definite phases are distinguishable in the recovery process: 

 (1) the redeposition of the continuous passivating surface layer (of 

 oxide or oxygen compound); and (2) the progressive change of the 

 newly passivated wire from the state of incomplete to that of complete 



