RALPH S. LILLIE 113 



is associated with a change in the electrical potential of the metallic 

 surface, from that characteristic of active iron (in contact with acid) to 

 that of the passive or oxide-covered surface. This variation of po- 

 tential is a direct and readily observable index of the return to the 

 passive state ; it occurs simultaneously with the decline and cessation 

 of the reaction (effervescence, etc.) and lasts for only a short time, 

 e.g. a fraction of a second; the succeeding phase of gradually return- 

 ing transmissivity is much more prolonged. A somewhat similar 

 division of the recovery process into two stages can also be distin- 

 guished in living tissues, especially those with relatively long refrac- 

 tory periods, e.g. heart muscle, as already indicated. These resem- 

 blances justify the belief that a study of the conditions of recovery in the 

 metallic system may throw light on the general nature of the recovery 

 process in living protoplasm. 



//. Phenomena of Automatic Repassivation in Iron Wires. 



The following account is based on experiments with iron wires of 

 standard composition and properties, known as "music steel wires," 

 manufactured by the Spencer Wire Company of Worcester, Massa- 

 chusetts. Wires of 1 to 1.2 mm. diameter were used (Nos. 20 and 

 25). The metal is bright, elastic, and highly tempered. Its behavior 

 in nitric acid of different concentrations is definite and regular, within 

 certain well defined limits of variation to be described below. It is 

 important in experiments of this kind to use metal of uniform quality, 

 for ordinary specimens of iron {e.g. "black iron" sheeting, soft iron 

 wire, wire nails, etc.) are often highly variable and capricious in their 

 manner of reaction with strong nitric acid. 



I have already described briefly the appearances accompanying 

 the activation of these wires in different dilutions of pure nitric acid.^ 

 Before use, the freshly cut wires are allowed to react for 15 seconds 

 or so with dilute acid (specific gravity 1.2); they are then rinsed in 

 water, wiped clean with a coarse cloth, and passivated by immersion 

 in strong HNO3 (specific gravity 1.42). The passive wires are steel 

 bright and undergo no change if left undisturbed in dilute nitric acid 

 (from specific gravity 1.2 up). When they are activated in dilute 

 acid, e.g. by touching with zinc or scraping with glass, there is an 



