STRUCTURES RESEMBLING ORGANIC GROWTHS. 149 



exhibit the characteristic red color of the indicator in alkaline 

 solution. Each filament thus originates from an anodic area in 

 a local electric circuit; the precise mechanism of the foimative 

 process has already been considered. 



Specimens of ordinary iron always exhibit a large number of 

 local couples of this kind, because of the characteristic lack of 

 electro-chemical homogeneity of this metal. As has frequently 

 been pointed out, this condition explains the susceptibility of 

 iron especially impure iron to rust; an iron surface is com- 

 posed of numerous areas which differ in physical condition and 

 hence in electrolytic solution-tension; areas of higher solution- 

 tension form local anodes, and rusting is a consequence of elec- 

 trolysis at these local circuits. Iron prepared in as high a state 

 of purity as possible is surprisingly resistant to corrosion. 1 What 

 is true of rusting is true also of the formation of precipitates 

 with electrolytes like ferricyanide ; filaments form more slowly 

 from chemically pure iron wire than from pieces of ordinary 

 commercial metal (nails, etc.). Zinc and aluminium, although 

 metals of higher solution-tension than iron, are more readily 

 obtained in a relatively homogeneous condition; hence they 

 resist corrosion better and form precipitate more slowly in 

 ferricyanide solutions. 



Such considerations explain the accelerating influence which 

 the contact of a nobler metal, like platinum or copper, has upon 

 the formation of precipitation-filaments from zinc, iron, or copper. 

 The nobler metal (that of lower solution-tension) has a higher 

 potential in contact with the solution than the less noble; hence 

 a circuit is formed in which the latter acts as anode, i. e., enters 

 solution as cations which form precipitate with the ferricyanide. 

 Conversely, a less noble metal, e. g., magnesium or manganese 

 in relation to iron or zinc, inhibits the formation of filaments 

 from the membrane-forming metal. Both the reinforcing and 

 the inhibiting influences have very pronounced and evident 

 effects, which are perceptible not only at the immediate region 

 of contact of the two metals, but for a considerable distance, 

 e. g., of several centimeters, beyond this region. The degree of 

 this influence at any point on the surface of the filament-forming 



1 For references to the literature in this field see my paper above cited, Amer. 

 Journ. Physiol., 1916, Vol. 41, p. 126. 



