Jury 19, 1918] 
or removal of the surface film of oxide in the 
metal is comparable with the increase of per- 
meability in the living element. 
A significant general analogy to physio- 
logical conditions is also to be seen in the 
readiness with which the active state is trans- 
mitted from an active to a passive metal by 
contact. Transmission of excitation from one 
eell or cell element to another by contact is 
frequent in organisms; and many character- 
istic structural arrangements, especially in the 
nervous system, give evidence that such trans- 
mission is a normal and constant physiolog- 
ical process; the interlacing of dendrites from 
different neurones, contact of nerve cells with 
one another by “ end-feet” and similar struc- 
tures, the histological characters of the myo- 
neural junctions and other nerve endings— 
which typically form contact with the surface 
of the cell—may serve as examples. Instances 
of transmission by contact in metals have been 
given above. A good demonstration is the 
following: if a number of passive iron wires 
are placed in contact with one another in a 
dish of nitric acid, and any single wire is 
touched with zinc, all immediately become 
active. A long fine passive wire in contact 
at one end with a large piece of passive iron, 
e. g., a nail, will on activation at its other end 
rapidly conduct and transmit the state of 
activation to the terminal object. Another 
remarkable feature of this phenomenon is that 
the transmission between different metals may 
be irreciprocal; this may be shown by using 
wires of the two metals, iron and nickel, which 
differ in the readiness and rate with which 
activation takes place. A momentary contact 
with active nickel will instantly activate a 
piece of iron wire in 1.20 HNO,, but under 
the same conditions a piece of passive nickel 
is activated slowly and only after prolonged 
contact. Consequently, while briefly touching 
passive iron with active nickel immediately 
and completely activates the iron, touching 
passive nickel with active iron is typically in- 
effective or has only slight local effect. In 
other words, transmission of activation takes 
place rapidly and readily from nickel to iron, 
but not in the reverse direction. The differ- 
SCIENCE 59 
ence depends upon the relative slowness of the 
activation process in nickel; in this metal the 
local reaction tends to start slowly and to reach 
its maximum slowly, and the rate of transmis- 
sion is correspondingly gradual. Such facts 
suggest the possibility that the characteristic 
irreciprocality of transmission in reflex arcs 
may depend upon similar differences in the 
time factors of excitation of the interacting 
neurones at the synapses. The recent work 
of Lapicque and Keith Lucas has shown clearly 
the fundamental importance of the time fac- 
tor in the excitation process.7 
It seems clear that variations in the electro- 
motor properties of the surfaces concerned— 
respectively the metallic surface and the cell 
surface—form the essential feature of activity 
which is common to both types of system and 
upon which the above various similarities of 
behavior depend. These variations are due 
to changes in the physical and chemical char- 
acter of the surface layer, which in both cases 
is water-insoluble, chemically unstable, and in 
contact with an eléctrolyte solution. Experi- 
ment shows that in the passive metal this sur- 
face film is in a characteristic state of equilib- 
rium which is readily disturbed, and the same 
appears to be true of the protoplasmic surface 
film in an irritable cell or cell element. 
This general similarity probably explains 
another peculiarity of behavior common to 
both systems, namely a tendency to automatic 
rhythmical fluctuations of potential and chem- 
ical action; this phenomenon is seen in the 
solution of many metals in nitrie acid, and 
also in the well-known rhythmical catalytic 
decomposition of hydrogen peroxide in con- 
tact with mereury. Alternation of activity 
and passivity, due to rhythmical formation and 
dissolution of a surface film of oxide or other 
protective material, appears to underlie these 
phenomena in metals. In living organisms 
rhythmical action is also highly characteristic, 
and is presumably due to analogous conditions. 
The general view that the semi-permeable 
7 Similarly with transmission from element to 
element. Lapicque’s work indicates that the fail- 
ure of transmission from nerve to muscle in curare 
poisoning is due to ‘‘heterochronism.’’ 
