Juny 12, 1918] 
the cathodal wire is instantly activated, while 
the anodal wire remains unchanged. Acti- 
vation with the electric current is thus 
typically a polar phenomenon, just as is the 
excitation of a living irritable element like a 
nerve. 
Activation by contact with active iron or a 
baser metal is in reality an instance of elec- 
trical activation, the activating metal form- 
ing the anode of the local circuit arising at 
the region of contact. At the local cathode, 
i. e., the adjoining passive iron, the metal is 
at once activated, and the effect spreads in the 
manner already indicated by means of the 
-eireuit which automatically arises at the 
boundary between active and passive areas. 
Any metal which thus activates by contact . 
must be of such a nature that the passive iron 
becomes the cathode of the local circuit 
formed. A metal which is nobler than passive 
iron, like platinum, not only does not cause 
activation, but it renders the iron locally more 
resistant to activation; thus the passage of 
the activation wave may be blocked by the 
contact of a platinum wire. This latter effect 
depends upon the formation of a local cireuit 
of the reverse orientation, the iron becoming 
anodal, a condition which furthers passivation 
and hinders activation. Active iron is a base 
metal in relation to passive iron, being more 
negative that the latter by ca. 0.75 volt in 
1.20 HNO,; hence when any region of a pas- 
sive wire is rendered active it immediately 
activates the adjoining areas. 
In passivation the surface layer of the iron 
is modified in a peculiar manner, apparently 
by the formation of a thin resistant layer of 
higher oxide. Any condition that interrupts 
locally this surface film of altered iron forms 
necessarily a local circuit by whose action the 
whole metal is activated in the manner just 
described. Apparently at any cathodal area 
the surface film of oxide is reduced to metallic 
iron; contact of a reducing substance has a 
similar effect; while a mechanical agent 
breaks the continuity of the film and exposes 
the unaltered iron beneath, thus forming the 
local cireuit. The reason why mechanical, 
chemical and electrical influences all produce 
-plus 10 ¢c.c. water, ete.). 
SCIENCE 55 
the same effect is thus evident. The parallel 
to the living irritable tissue is plain; local 
alteration of the protoplasmic surface film 
produces effects of a closely comparable na- 
ture, which spread in an analogous manner 
by means of the local electrical circuits 
formed. We are thus enabled to understand 
why any rapid local alteration of the cell 
surface may activate the whole cell—in other 
words why the cell is so characteristically 
“irritable.” The iron wire in its passive state 
may be compared to the irritable living ele- 
ment in a state of rest. The state of in- 
activity continues in both cases only so long 
as the surface layer is intact and homogeneous. 
The reason why the whole cell (or the whole 
iron wire) responds completely to a local stim- 
ulus is simply because transmissign over the 
entire surface follows automatically and in- 
evitably upon local activation. The “all-or- 
none” behavior thus becomes intelligible. 
Under normal conditions an irritable nerve 
or muscle returns spontaneously to an in- 
active or “resting” state after stimulation, 
and for renewal of activity a second stimulus 
is required. The resting condition thus repre- 
sents a condition of equilibrium, which is 
temporarily disturbed by the stimulating 
agent. The same is true of the passive con- 
dition of iron in strong solutions of nitric 
acid. In weaker solutions, of s. g. 1.20 and 
less, the reaction once initiated continues un- 
checked until all of the iron is dissolved; but 
in stronger solutions the reaction is temporary 
and the metal returns spontaneously to the 
passive condition. A wave of temporary activ- 
ity thus sweeps over the surface of a passive 
iron wire which is activated (e. g., by touch- 
ing with zinc) in nitric acid of s. g. 1.25 or 
higher; the state of local activity lasts in 
such a solution for a brief period only, which 
is the shorter the higher the concentration of 
the acid. An interesting gradation of effect 
may thus be shown by activating a series of 
passive wires in different dilutions of strong 
(s. g. 1.42) acid, e. g., 90, 80, 70, 60 and 55 
volumes per cent. (i. e., 90 c.c. 142 HNO, 
When a wire im- 
mersed in pure 1.42 acid is touched at one end 
