256 RALPH S. LILLIE AND EARL N. JOHNSTON. 



3. Condition of Surface of the Metal. Oxide-covered areas 

 on the surface of a metal form local cathodes in the ferricyanide 

 solution; hence filaments are formed only where the pure metal 

 is exposed. Fig. 28 shows a piece of rusty iron wire with the 

 rust removed along one side; filaments were formed almost 

 exclusively on the bright surface, which is anodic. Fig. 29 shows 

 both ends of a fine iron wire (bent into a U-shape) 1.8 mm. 

 apart, one end being rusty the other polished bright. Filaments 

 were formed only on the bright end. It is interesting to note 

 that nearly all the filaments formed on the bright end were 

 directed toward the rusty end. Apparently the main direction 

 of growth is determined by the direction of the current-lines 

 between anodic and cathodic areas. 



4. External Conditions. All filaments are extremely senstive 

 to outside influences. Slightly jarring or causing currents in 

 the solution sometimes changes both the direction of growth 

 and shape of filaments. Encountering small obstacles or other 

 filaments has the same effect. Figs. 10 and 30 show the effect 

 of filaments encountering small particles of sand. 



5. Modifications in Filaments at the Surface of the Solution. 

 Some of these effects have already been mentioned. A widening 

 and flattening effect is common with all filaments on reaching 

 the surface of the solution. In some cases there is a display of 

 different combinations of colors, and very beautiful figures of 

 various shapes are formed. Filaments formed from nickel show 

 no interesting features while entirely submerged in the solution, 

 but when they run over the surface they become brightly colored, 

 showing alternate light blue and brown cross-striations with a 

 blue border along each side. Some interesting figures having 

 the general shape of a bunch of leaves tinted with brown form 

 at intervals along the filaments. 



Another interesting surface-phenomenon is the formation of 

 groups of small sac-like structures. This effect is very common 

 with zinc filaments. As soon as a zinc filament in its tortuous 

 course comes to the surface of the solution these small structures 

 begin to form (Fig. 12). They push out around a common 

 center, one forming every one or two seconds, at a rate varying 

 somewhat in different cases. At the central point there is a 



