STRUCTURES RESEMBLING ORGANIC GROWTHS. 14! 



once leads to a rapid formation of precipitate. The growth of 

 filaments in gelatine or egg-white solutions containing K 3 FeCy 6 

 is in fact dependent upon the inorganic salts associated with the 

 protein. Apparently salts must be present whose anions form 

 soluble compounds with the filament-forming metal; the reason 

 for this requirement will become clear when the process of fila- 

 ment-formation is described in detail. The following experiment 

 shows that the rate of filament-formation varies directly with 

 the proportion of chloride (or similar salt) present in the solution 

 of ferricyanide. A small iron nail (one centimeter long) was 

 placed in each member of a series of solutions of 4 per cent. 

 K 3 FeCy 6 containing progressively decreasing proportions of NaCl, 

 viz., 4, 2, i per cent., etc., down to 1/128 per cent. An abundant 

 precipitate of iron ferricyanide was formed within ten minutes 

 or less in all solutions containing 1/16 per cent. NaCl or more, 

 and the more rapidly the higher the proportion of NaCl. With 

 1/128 per cent. NaCl the precipitate formed gradually; while in 

 the entire absence of NaCl none was perceptible after several 

 hours. Similar experiments with pieces of zinc in contact with 

 copper wire gave the same result. 



The presence of egg-albumin or gelatine, i. e., an emulsoid 

 colloid capable of having a "protective" action upon finely 

 divided insoluble material, greatly favors the formation of co- 

 herent filaments from iron and zinc; with copper this condition 

 is less important. Thus in a solution of i per cent. K 3 FeCy 6 in 

 distilled water containing a trace of HC1 (nj 200-^/400) an iron 

 nail rapidly forms precipitate, but this is largely separated in 

 the form of a loose non-coherent deposit and only partly as 

 filaments. In a similar solution containing 2 per cent, egg- 

 albumin relatively little amorphous precipitate is formed, and 

 the filaments are more numerous, regular, and coherent. Similar 

 conditions hold to an even greater degree for the formation of 

 zinc filaments; in simple aqueous solution the zinc ferricyanide 

 is deposited chiefly in amorphous form or in the form of coarse, 

 brittle, and irregular tubular structures; while with protein 

 present an abundance of slender hypha-like filaments results. 

 This type of phenomenon is undoubtedly of much biological 

 interest. In the absence of the protective colloid the crystals of 



