PRECIPITATION STRUCTURES SIMULATING ORGANIC GROWTH. 239 



own special physical and structural peculiarities which vary 

 with the nature of the heavy metal. The observed structural 

 difference between these two kinds of filaments appears to be 

 largely determined by the difference in the physical nature and 

 consistency of the precipitate; this is more finely divided and 

 coherent with iron than with zinc, and greater smoothness of 

 contour and uniformity of structure in the growing filaments 

 are the result. Again, the two closely related metals, nickel 

 and cobalt, whose ferricyanides have almost identical physical 

 properties, illustrate the same principle in a highly characteristic 

 manner; the precipitation-structures formed from these two 

 metals are strikingly similar in appearance, exhibiting what 

 might be described as a close family resemblance, e. g., in the 

 coarse texture and opacity of the filaments and in the char- 

 acteristically winding and tortuous course which they both adopt 

 (see Figs. 16, 17, 21); this latter peculiarity is found in none of 

 the filaments formed from other metals. Similarly zinc and 

 cadmium form filaments and vesicular formations (Figs. 12, 13, 

 18, 19) which are closely alike in structure and appearance. 



It is interesting to note that this resemblance extends not 

 only to the form-characters of the completed structure but also 

 to the peculiar type of physical activity which it exhibits during 

 its growth. The precipitate forming the walls of the striated 

 vesicular or shell-like structures characteristic of zinc and cad- 

 mium (see Figs. 13, 18, 19) is deposited rhythmically or inter- 

 mittently; the striation is a direct consequence of this rhythm, 

 which itself is due to an intermittency in the outflow of the 

 solution from the interior of the growing vesicle; this intermit- 

 tency depends upon the alternating formation and rupture of 

 the precipitation-membrane forming the wall of the vesicle. 

 The rate of this rhythm, and hence the rate at w y hich the pre- 

 cipitate is deposited and the resultant character of the striation, 

 are similar in the two metals. Evidently these peculiarities of 

 behavior are to be referred ultimately to the special physico- 

 chemical properties of the precipitate and of the metal; zinc and 

 cadmium, being chemically closely related metals, form ferri- 

 cyanides having similar properties; accordingly they -give rise 

 to structurally similar deposits, which under similar external 



