58 SCIENCE 
involved and the reaction lasts for a definite 
time; 7. e., the character, intensity and dura- 
tion of the reaction are independent of the 
nature of the activating agent; the metal 
either reacts completely or not at all (analogy 
to the “all-or-nothing” behavior of irritable 
living elements); (5) a wire which is polar- 
ized anodically while immersed in acid is 
activated with difficulty and the activation 
wave tends to travel for only a short distance 
(analogy to anelectrotonus in nerve) ; (6) the 
spontaneous return of passivity in strong acid 
is immediately succeeded by a period during 
which the metal is less responsive than before 
(analogy to fatigue effect or refractory 
period); (7) a current which reaches its full 
intensity gradually is less effective than one 
which reaches the same intensity suddenly; 
and finally (8) the local chemical surface re- 
action of activation is constantly associated 
with a variation of electrical potential, the 
active region becoming negative relatively to 
the inactive regions (analogy to the bioelectric 
variation or “action current” of an active 
living tissue). 
The chief characteristics of this electrical 
variation are readily demonstrated as follows. 
When two iron wires connected with the bind- 
ing-posts of a voltmeter of suitable scale are 
passivated and placed side by side in a vessel 
containing 1.20 HNO,, no potential difference 
is shown. If then one wire is activated the 
instrument at once indicates a P.D. of 0.7 to 
0.8 volt, with the active wire negative; this 
P.D. remains constant while the reaction con- 
+inues in the one wire; if then the other wire 
is also activated the P.D. again falls to zero. 
The active wire is thus anodal, the passive 
wire acting like a noble metal. If the same 
experiment is performed with stronger acid 
(55 per cent. 1.42 or higher) a similar but 
temporary excursion of the needle is seen, 
lasting for the period of the reaction. In 
acid of 55 or 60 per cent. the potential ex- 
hibits irregular rhythmical fluctuations for the 
few seconds during which the reaction con- 
tinues, and the needle swings by degrees back 
to zero and somewhat beyond as the reaction 
subsides and ceases. Immediately after the 
[N. S. Vou. XLVIII. No. 1229 
return of the passive condition the activated 
wire is always found slightly more positive 
than before, usually by ca. 0.02 volt; after an 
interval of some minutes—corresponding ap- 
parently in its duration to the insensitive or 
refractory period above described—the original 
potential returns. The wire may then be 
again reactivated and the same process is 
repeated. This tendency to overpass the orig- 
inal potential after the return of passivity re- 
calls the similar phenomenon in nerve known 
as the “positive after-variation,” and sug- 
gests a similarity in the general conditions 
under which the surface film is reconstituted 
in the two cases. 
The variation of potential associated with 
the transmission of the activation wave may 
be demonstrated in a single wire which is con- 
nected near its opposite ends with a sensitive 
string galvanometer (by means of thin passive 
iron wires) and immersed in 70 or 80 per cent. 
acid. If the wire is activated at one end the 
string shows a quick excursion, first in the 
one direction, then in the other, the deflec- 
tion showing that at each leading-off region 
the wire becomes first negative and then posi- 
tive. The curve of movement is thus com- 
parable to the typical “ diphasic ” action cur- 
rent curve of a nerve conducting an impulse. 
The amplitude of these variations of poten- 
tial in metals is of course much greater than 
that found in living tissues, but in their gen- 
eral characteristics both classes of phenomena 
give unmistakable evidence of being condi- 
tioned in the same manner. In the case of 
the metal it is certain that the effect depends 
upon a sudden alteration of the electromotor 
properties of the surface layer. In living 
tissues and cells there is also much evidence 
that a change in the protoplasmic surface 
layer (or so-called plasma-membrane) involy- 
ing increased permeability and altered meta- 
bolism is constantly associated with stimula- 
tion, and that the variation of electrical poten- 
tial is due primarily to this change. Thus in 
both living system and metal the electrical 
variations are the expression or indication of 
changing chemical and structural conditions 
in the surface layer. The local interruption 
