54 SCIENCE 
passes along the fiber. Such difficulties are 
only apparent, however; in nerve conduction 
it is quite certain that an entirely new state of 
activity is aroused at each successive region 
of the fiber as the impulse passes; and all of 
the evidence indicates that the speed of trans- 
mission is determined mainly by the sensi- 
tivity and local rate of response of the nerve,* 
and not at all by the rate of transmission of 
the electric current in the bioelectric circuit. 
It is probable that in the local bioelectric 
cireuit set up by the initial stimulus the 
direct chemical influence of the current ex- 
tends for only a short distance, at most a few 
centimeters from the original site of stimula- 
tion; but one of its effects is to originate a 
new and similar circuit in the adjoining 
regions of the fiber; this process repeats itself 
as already indicated, and in this manner the 
impulse spreads. The observed speed of the 
activation-wave has thus nothing to do with 
the speed of the purely electrochemical dis- 
tance effect. What we seem to observe is a 
local electrical circuit which travels along the 
nerve together with the activation wave; but 
in reality there is a succession of new cir- 
cuits, each of which automatically arises at 
the boundary between resting and active re- 
gions as the front of the activation wave ad- 
vanees. The relatively slow rate of movement 
of the impulse and the absence of a decrement 
may thus be understood. 
The rapid passage of a wave of chemical 
decomposition (probably oxidative in nature 
and involving some structural change) over 
the surface of the reacting element, followed 
immediately by a reverse change which re- 
stores the original or resting condition, is 
what appears to take place in a nerve or other 
living structure during conduction. Asso- 
ciated with the chemical process is a local 
electrical circuit by whose electrolytic action 
the chemical change is apparently determined. 
Have we examples of similar processes in in- 
organic systems? It appears in fact that this 
general type of process is not unusual in metals 
in contact with solutions. Especially clear and 
4 Cf. Amer. Jour. Physiol., 1914, Vol. 34, p. 414; 
Vol. 37, p. 348. 
[N. S. Vou. XLVIII. No. 1229 
striking examples are seen in the transmission 
of the state of activity over the surface of 
metals, especially iron, which have been 
brought into the temporarily non-reactive or 
“passive” condition by immersion in strong 
nitric acid (or other suitable oxidizing agent) 
and are then placed in dilute acid and made 
to react. It has long been known that iron 
which has been thus “passivated” becomes 
resistant or refractory to reaction and (for ex- 
ample) no longer dissolves spontaneously when 
placed in dilute nitric acid (s. g. 1.20). But 
if while immersed in the dilute acid it is 
touched momentarily with a baser metal, or 
with a piece of ordinary non-passive iron, it 
is at once “activated” and reacts vigorously 
with the acid until dissolved.©5 The experi- 
ment is a striking one and easily performed. 
In my own demonstrations a piece of pure iron 
wire (No. 20 piano wire, bent at one end into a 
hook for handling) is passivated by immersion 
in strong nitric acid (s. g. 142) for a few 
seconds, and is then placed (by means of a 
glass hook) in a flat dish containing dilute 
acid (s. g. 1.20). The wire if left undisturbed 
remains bright and unaltered for an indef- 
inite time. If then it is touched at one end 
with a piece of ordinary iron, or with zine or 
another baser metal, the bright metallic sur- 
face is at once darkened (through formation 
of oxide) and active effervescence begins; this 
change is transmitted rapidly, though not in- 
stantaneously, over the entire length of the 
wire; the velocity of transmission varies with 
the conditions, and is of the order of 100 or 
more centimeters per second in this experi- 
ment. The wave of activation may also be 
initiated mechanically, e. g., by. bending the 
wire or tapping it sharply with a glass rod; or 
chemically, e. g., by contact with a reducing 
susbtance such as sugar; or electrically, e. g., 
by making the wire (while immersed in the 
acid) the cathode in any battery circuit (of 
two or more volts potential), preferably with 
another piece of passive iron wire as anode; 
5 For a recent extended study of the passive 
state in metals with full references to the literature, 
cf. Bennett and Burnham, Jour. Physical Chem., 
1917, Vol. 21, p. 107. 
