146 RALPH S. LILLIE. 



form precipitate only where it flows out at the open extremity 

 and meets the ferricyanide outside; part of the precipitate is 

 scattered outside the tube, as already described, or remains 

 finely suspended within its interior; but part adheres at the 

 outlet and contributes to the growth of the tube. 



According to this conception of the tube-forming process, the 

 rate of flow of fluid along the tube must vary directly with the 

 intensity of the electric current through the tube i. e. (assuming 

 constant E.M.F.), with the electrical conductivity of the solu- 

 tion within the tube. This explains why the rate of growth of the 

 filaments varies directly with the concentration of NaCl (or 

 similar salt) in the ferricyanide solution. The only salt that 

 can remain in solution inside the tube and contribute to the 

 conductivity of its contents must obviously be one that is not 

 precipitated by the ions of the tube-forming metal. 



Structure of Filaments. The structure of a tubule is partly 

 determined by the nature of the material composing it, partly 

 by the conditions under which the material is laid down, such 

 as the size of the anodic area, concentration of salts, presence of 

 protective colloid, external mechanical influences, etc. In general 

 the consistency of the precipitate is coarsest in the case of zinc, 

 and finest in the case of copper; hence tubes of zinc ferricyanide 

 have a coarsely granular aspect and are more irregular in form, 

 while those of iron and copper have smoother contours and a 

 more uniform appearance. There is always considerable varia- 

 tion in the diameters.of the tubes formed in any single experiment ; 

 in general an increase in the concentration of NaCl in the solu- 

 tion (i. e., more rapid rate of formation) favors the production 

 of wider tubes. Apart from differences in caliber there is little 

 variation in the structure of those tubes which lie entirely below 

 the surface of the solution; typically these are cylindrical or 

 slightly tapering in shape, and follow a more or less tortuous 

 course, varying according to the number and nature of the 

 mechanical obstacles encountered during their formation ; zinc fila- 

 ments show more irregularities of this kind than iron or copper 

 filaments. Those filaments which reach the surface of the solu- 

 tion during their formation undergo various characteristic modifi- 

 cations; thus vesicular or bladder-like thin-walled structures may 



