56 
with a piece of zinc a momentary flash-like 
wave of activation is seem to pass rapidly 
along the whole wire; the local reaction lasts 
for only a small fraction of a second and is 
instantly reversed; a slight temporary darken- 
ing of the metallic surface and a trace of 
brown coloration (due to reduction of the acid 
to lower nitrogen oxides) are the only visible 
effects; in 90 per cent. acid a similar though 
somewhat more prolonged reaction takes place; 
in 80 per cent. acid there is slight visible 
effervescence for a fraction of a second; in 
70 per cent. the effervescence lasts for about 
one second and the darkening of the metallic 
surface is more pronounced; while in 60 per 
cent. the reaction occupies two or three sec- 
onds and in 55 per cent. five seconds or more, 
and a considerable accumulation of brown 
oxide is formed at the surface of the metal. 
It would thus appear that in the stronger solu- 
tions the oxidation which forms the protective 
surface film takes place so rapidly that only 
a momentary reaction of the metal with the 
acid is possible; as the concentration of acid 
decreases the surface film forms more slowly 
and the reaction lasts longer, until at a certain 
eritical concentration (about 50 per cent.; 
ca. 1.20 s. g.) the surface oxidation becomes 
so gradual that its passivating influence is 
insufficient to interfere with the continued 
solution of the metal in the acid. 
Two chemical reactions of opposite character 
thus take place successively as the activation 
wave passes any region of the metallic sur- 
face; first, the local cathodic reduction which 
remoyes the protecting layer of oxide and 
enables the metal to react with the acid; and 
second, the immediately succeeding oxidative 
process which reforms the protective surface 
film and arrests the reaction. A factor of im- 
portance in this process of repassivation is 
apparently the electrochemical oxidative action 
at the local anode. As the activation wave 
advanees, the surface film is disintegrated at 
the cathodal region immediately in advance 
of the wave front; this region then instantly 
becomes active, 7. ¢., anodal; in other words, 
it undergoes a change of condition which in . 
itself tends to check or arrest the reaction. 
SCIENCE 
[N. S. Vou. XLVIII. No. 1229 
This is because of the characteristic passiva- 
ting influence at the anode; the reaction of a 
piece of active iron wire in 1.20 HNO, may in 
fact be brought to rest and the wire rendered 
passive by passing a strong current through it 
as anode for a few seconds.® As the activation 
wave passes, each region of the metallic sur- 
face thus becomes alternately cathodal and 
anodal. Making the passive metal cathode has 
an activating effect, while making the active 
metal anode tends to passivate. This latter 
electrochemical action is added to the direct 
passivating action of the acid. Hence in acid 
of a sufficient strength the local reaction is 
automatically self-limiting as well as self- 
propagating. This peculiarity depends di- 
rectly upon the properties of the surface film, 
which when the metal is cathode undergoes 
dissolution, and when the metal is anode is 
reformed. Apparently in strong acid the 
metal is in a condition where a slight local 
increase of reducing influence initiates the 
activating reaction and a slight increase of 
oxidizing influence inhibits it. We have here 
another parallel to the condition in an ir- 
ritable element like a nerve fiber, where cath- 
odal polarization promotes and anodal polar- 
ization inhibits the local reaction (electro- 
tonus). In both eases the alternate disin- 
tegration and reformation of a surface film 
under electrochemical influence appear to be 
the essential features of the local process. 
The passage of the wave of activation can be 
observed with especial clearness in a passive 
iron wire which has been dipped in a test-tube 
containing 1.42 HNO, and is then suspended 
vertically in air and touched at its lower end 
with zine. The adhering layer of acid is so 
thin as to increase greatly the resistance of 
the local circuit between active and inactive 
regions, and the local reaction spreads with 
corresponding slowness, at a rate of only a 
few (5 to 10) centimeters a second. As the 
6 Contact of a piece of platinum foil with a re- 
acting iron wire has the same effect; near the plati- 
num the iron soon ceases reaction and becomes pas- 
sive, and the effect then spreads over the whole 
wire. This phenomena is biologically interesting, 
as a case of transmission of inhibitory influence. 
