114 TRANSMISSIVITY IN PASSIVE IRON WIRES 



immediate darkening of the bright surface (from formation of oxide) , 

 followed by active effervescence; both of these changes then spread 

 rapidly along the whole length of the wire. A completely transmis- 

 sive wire thus reacts as a whole, in a manner which in acid of a defi- 

 nite concentration and temperature is constant and independent of 

 the mode of activation. The reaction continues for an indefinite 

 time in acid below a certain concentration ; this critical concentration 

 is about 50 volumes per cent of 1.42 HNO3 (equal to acid of 1.2 specific 

 gravity or about 7.5 n by volume); in all stronger solutions the reac- 

 tion soon comes automatically to rest and passivity is regained. In 

 such cases the duration of the local reaction varies in a characteristic 

 manner with the concentration of HNO3; e.g., in acid of 85 volumes per 

 cent (about 12 n by volume) effervescence lasts (at 20°) for only a 

 small fraction of a second, while in 55 per cent HNO3 it continues for 

 several seconds, at first uniformly and later declining and finally ceas- 

 ing somewhat abruptly, usually after 6 to 8 seconds. During the 

 reaction a dark brown oxide is deposited on the surface of the metal; 

 the thickness of this coating depends on the duration of the reaction; 

 hence immediately after spontaneous repassivation in 55 per cent 

 acid a wire is almost black in color, while in 80 per cent acid it is only 

 slightly bronzed. This deposit gradually dissolves away if the metal 

 is left in the acid; it is loosely adherent and is entirely distinct from 

 the thin, resistant, and invisible surface film which imparts passivity. 

 The recently formed oxide-covered region furnishes a convenient indi- 

 cator of the distance traveled by the activation wave in cases of par- 

 tial transmission, since its boundary against less recently activated 

 regions is always sharply defined. 



On activation the potential of the metal changes suddenly from that 

 characteristic of passive to that of active iron; in nitric acid of the 

 above concentrations the active metal is more negative (zinc-like or 

 anodal) by 0.7 to 0.8 volt. Hence the duration and certain other 

 features of the reaction can be observed conveniently by the use of 

 a voltmeter or string galvanometer. When two iron wires, bent into 

 an appropriate shape and connected through keys to a voltmeter (pref- 

 erably one with a scale of 1 to 2 volts and a central zero), are passi- 

 vated by dipping into 1.42 HNO3, and are then placed side by side a 

 short distance apart in a flat dish containing 55 per cent acid, on 



