148 RALPH S. LILLIE. 



stream necessary for the formation of filaments. Once formed, 

 these precipitation-vesicles may be displaced or detached by the 

 pressure of others formed later, and large groups or heaps of 

 such structures may thus collect. In the case of copper one 

 highly characteristic type of formation, frequently found in 

 solutions of K 3 FeCy 6 containing considerable chloride (e. g., \ per 

 cent. NaCl plus 2 per cent. KsFeCye), is a group of slender 

 tapering filaments grouped radially about a central vesicular 

 mass of precipitate, the whole structure suggesting a miniature 

 chestnut-burr. 



It should be noted that these various types of structure, together 

 with many others, are produced as is also the case with the 

 majority of living structures by combinations of microscopic 

 hollow elements, vesicular or tubular in form, with walls con- 

 sisting of semi-permeable membranes. The above resemblances 

 to organic growths are undoubtedly dependent upon this funda- 

 mental similarity of physico-chemical constitution. 



Influence of Contact of other Metals upon Filament-formation. 

 Before proceeding to describe observations showing the inhibiting 

 and reinforcing influence which the contact of one metal may 

 exert upon the formation of filaments from another, the general 

 theory of this effect ought briefly to be considered. As already 

 explained, the formation and growth of these structures depend 

 upon a process of electrolytic local action or electrolysis; the 

 region from which the filament grows out represents an anodic 

 area, i. e., an area where ions of the membrane-forming metal' 

 enter solution; these ions then interact with the ferricyanide 

 ions to form the precipitate ; to each such anodic area corresponds 

 a cathodic area (or areas) at some other region of the metallic sur- 

 face, where the positive stream reenters the metal to complete the 

 circuit; at this area hydrogen ions are deionized and hydrogen 

 gas is freed with the formation of alkali. In the case of iron 

 these areas may readily be demonstrated by the addition of a 

 little phenol phthalein to the ferricyanide solution (as in the 

 ferroxyl reagent of Walker) j 1 the cathodic regions, which typ- 

 ically remain bright and free from filaments for some time, then 



1 Cf. Walker, Cederholm and Bent, Journ. Amer. Chem. Soc., 1907, Vol. 29, p. 

 1251. 



