I4O RALPH S. LILLIE. 



form highly characteristic structures of this kind, especially if 

 in contact with carbon or a nobler metal (Ag, Hg, Pt, Cu, Fe). 

 Pieces of pure zinc, without other metal, form filaments in the 

 above solution much more slowly than iron, presumably because 

 of the more homogeneous structure of zinc and the relative 

 absence of regions differing sufficiently in solution-tension (and 

 hence in potential) to form efficient local couples; but when 

 in contact with a noble metal the rate of formation is rapid. 

 Copper in contact with platinum or carbon also forms filaments 

 in ferricyanide solutions containing sodium chloride in sufficient 

 quantity; for reasons that will appear below, the presence of 

 this or a similar salt is necessary to the process. On the other 

 hand, certain metals which readily form insoluble ferricyanides 

 do not deposit the precipitate in filamentous form; this is true 

 of aluminium, lead, tin, and (in part) manganese. Pieces of 

 aluminium and lead in contact with copper or platinum wire 

 form precipitates rapidly in the above solution; but the deposit 

 merely collects in the form of more or less coherent or vesicular 

 masses at various regions of the surface without forming fila- 

 ments. Apparently only those metals whose ferricyanides form 

 semi-permeable precipitation-membranes can form filaments. 

 Under the above conditions tin forms a slight and non-coherent 

 precipitate; with manganese some formation of filaments is 

 usually observed, but most of the precipitate is deposited in 

 amorphous form. The following observations have been made 

 chiefly with filaments of iron, zinc, and copper ferricyanide. The 

 more minute structural details of these formations will not be 

 described at length in the present paper, both in order to save 

 space and also because the experiments themselves are so simple 

 and the materials so accessible that anyone can readily make 

 these observations for himself. The following description will 

 therefore be confined to the more general features of structure 

 and the essential conditions of formation. 



Conditions of Formation of Filaments. In pure solutions of 

 KsFeCye (2 to 4 per cent, in distilled water) the formation of 

 precipitate is slight and gradual. Pieces of iron or zinc remain 

 for hours in such a solution without perceptible change. The 

 addition of a little soluble alkali salt, however, e. g., NaCl, at 



