STRUCTURES RESEMBLING ORGANIC GROWTHS. 145 



ferrous ferricyanide which is deposited as an incomplete vesicle 

 or ring of semipermeable membrane forming the beginning of the 

 tube; ferrous ions continue to be given off from the iron surface, 

 forming ferrous chloride with the chlorine ions present; the 

 interior of the tube thus contains a solution of ferrous salt which 

 is continually being propelled forward through the open ex- 

 tremity, and there meeting ferricyanide from the external solu- 

 tion forms the precipitate. 



The question of what maintains the flow is interesting. One 

 possibility is that each ferrous ion is hydrated, i. e., carries 

 water along with it as it enters solution ; water thus flows in the 

 same direction as the ferrous ions, the energy of the flow being 

 derived from the solution-pressure of the iron. Other factors, 

 however, probably enter; it is especially to be noted that the 

 point of origin of each tube represents an anodic area, from which 

 the positive electrical stream enters the solution, reentering the 

 metal at some cathodic region or regions outside of the tube. 

 An electric current thus passes outward along the tube; and 

 this must cause effects of the kind always found when such a 

 current traverses a system of capillary tubes containing a solu- 

 tion, e. g., a porous partition; these effects are of the kind known 

 as "electrical endosmose"; 1 when the substance of the capillary 

 wall is negatively charged the adjacent solution is positive and 

 travels with the positive stream. It is known that iron ferri- 

 cyanide particles are charged negatively in contact with water; 2 

 hence the layer of solution at the walls of a tube consisting of 

 this material must be positively charged and undergo transport 

 in the direction of the positive stream. The flow of solution 

 through the tube is thus probably in large part due to electrical 

 transport by the current of the local circuit. One necessary 

 factor in the process of formation of the tube is the semi-permea- 

 bility of its wall, which permits water but not salts to penetrate; 

 the solution of ferrous salt thus transported along the tube can 



1 For a description of this phenomenon see the larger textbooks of physics; a 

 good account of the essential conditions is given in Hober's " Physikalische Chemie 

 der Zelle und der Gewebe," 1914, pp. 234 seq. 



2 The ferrocyanides and the ferricyanides of the heavy metals form negatively 

 charged particles. The potential-difference of Prussian blue against water is 

 .056 volt. (Cf. Burton, "The Physical Properties of Colloidal Solutions," London, 

 1916, p. 135.) 



