\ 
Juxy 19, 1918] 
reacting region extends upward the bright 
surface of the iron is darkened locally for a 
distance of two or three centimeters; behind 
this advancing active region the wire again 
becomes bright and inactive. The visible 
effect is that of a slight temporary darkening 
or clouding which travels upward along the 
wire. After the wave has passed over the 
whole length of the wire the latter, when 
tested by dipping in 1.20 acid, is found to 
be again passive; the temporary and reversible 
character of the activation is thus shown. A 
similar slow spreading of the active state takes 
place in a passive wire dipped in weaker (1.20) 
acid and then activated as above, but in this 
ease there is no spontaneous return to pas- 
sivity; the whole wire remains dark, and when 
_ again placed in 1.20 acid at once reacts vigor- 
ously in the usual manner of active iron. 
Spontaneous reversal thus takes place only in 
the strong acid. 
In the experiment just cited the rate of 
transmission is lowered by increasing the elec- 
trical resistance of the local circuit, but in 
other respects there is no essential difference 
from the conditions observed in immersed 
wires. A noteworthy feature of these phe- 
nomena is that after a wire has been activated 
while immersed in strong acid (e. g., 80 per 
cent. 1.42) some time elapses before a complete 
reaction can be again excited; 7. e., a period 
of insensitivity and imperfect transmission 
always follows the spontaneous return of pas- 
sivity. Contact with zine within the first four 
or five minutes after activation causes typ- 
ically only a local reaction which may be 
transmitted slowly for a few centimeters but 
then dies out; some minutes later transmission 
takes place more rapidly and through a longer 
distance; but it is usually only after ten or 
fifteen minutes (the exact time varying with 
the conditions) that perfect transmission 
through an indefinite distance becomes again 
possible. The recovery of the original con- 
dition thus requires some time, the exact in- 
terval varying with the concentration of acid, 
and in general decreasing with decreasing con- 
centration. This phenomenon also has its 
biological analogies, and may be compared to 
SCIENCE 57 
fatigue, or possibly to the refractory period 
which typically follows stimulation in all ir- 
ritable tissues. Evidently the reformed sur- 
face film regains its former sensitive proper- 
ties by a progressive and somewhat gradual 
process. 
This tendency to an automatic restoration 
of the protective surface layer of oxide after 
local removal is probably the essential condi- 
tion underlying another characteristic feature 
of the electrical activation of passive iron, 
namely, that a slowly increasing current passed 
through the wire (as cathode) is much less 
effective in causing activation than a current 
of similar strength which attains its full in- 
tensity rapidly or instantaneously. In this 
respect also the passive metal resembles the 
living irritable tissue. If the current leading 
to the two passive wires immersed in 1.20 
HNO, is derived by means of a stationary 
and a movable zine electrode from a bath of 
zine sulphate solution forming part of a cir- 
cuit of several storage cells—an arrangement 
enabling the potential of the “shunt current” 
to the iron wires to be varied at will—it is 
found that a gradual increase of the current, 
from near zero to an intensity which in itself 
is amply sufficient to activate with sudden 
closure, is typically without effect. Evidently 
a sudden change of surface polarization is 
needed; if the change is gradual it seems that 
the oxidative action of the acid in contact 
with the metal has time to reform the pas- 
sivating protective layer as fast as it is re- 
duced by the cathodal action. 
The chief of the foregoing resemblances be- 
tween the passive iron wire and the irritable 
living element may now be briefly summarized 
as follows: (1) Mechanical, electrical and 
chemical agencies have the same activating 
effect; (2) electrical activation is a polar phe- 
nomenon (analogy to the law of polar stimu- 
lation); (3) the local state of activity is prop- 
agated along the wire at a velocity which is 
similar in its order to that of the excitation 
wave in living tissues; (4) whenever activation 
is excited by any means in a passive wire 
immersed in a definite solution of acid (e. g., 
70 per cent. 142 HNO,) the whole wire is 
