1. Passive steel wires were activated in a bath (Bath A) containing 70 v. per cent HNO3 (in which they undergo prompt repassivation), and immediately transferred to a second bath (Bath B) containing HNO3 of a concentration varying in different experiments. After varying intervals in this bath they were transferred while still passive to a third bath (Bath C) containing strong HNO3 (70 or 100 v. per cent) and there immediately activated.
2. During the immersion in Bath B the wires progressively recover their ability to transmit activation waves in strong HNO3. The measure of this recovery is the distance travelled by the activation waves in Bath C after the varying times of exposure in Bath B. Transmissivity as thus measured is at first incomplete (decremental) and later becomes complete. The minimal exposures in Bath B required to render wires completely transmissive in the strong acid of Bath C were determined for concentrations of HNO3 between 10 and 100 v. per cent. With 100 v. per cent HNO3 in Bath C, these exposures range from 40 minutes or more in 15 v. per cent to 10 minutes in 100 v. per cent HNO3 (temperature 19–20° in all baths).
3. The time required for complete recovery varies inversely with the concentration of the acid in the recovery bath (Bath B), but increases rapidly with the concentration of the acid in the testing bath (Bath C). Hence at a time when a wire has recovered just sufficiently to transmit non-decrementally in a given strong acid (e.g., 70 v. per cent) it still transmits decrementally in a stronger acid. Complete recovery for transmission in 100 v. per cent HNO3 requires about twice as long as for 70 v. per cent HNO3. In HNO3 of 50 v. per cent and less decremental transmission does not occur.
4. The indications are that recovery is an effect of the progressive solvent action of the external acid on the passivating oxide film, which at its first deposition appears to be relatively thick and hence resistant to electrochemical reduction. The final stage of recovery, when electrical sensitivity and speed of transmission are maximal, would on this hypothesis correspond to minimal thickness, possibly monomolecular.
5. The rate of recovery in Bath B is not far from proportional to the concentration of HNO3 in the more dilute solutions, but in the higher, especially the strongly passivating, concentrations (70 to 100 v. per cent) the rate becomes appreciably slower than proportional, apparently because of the intense oxidizing action of these solutions, which reinforces the oxide sheet and retards the thinning process.
6. The bearing of these observations on the problem of the conditions of recovery in irritable living tissues (such as nerve) during the absolute and relative refractory periods is briefly discussed.