248 RALPH S. LILLIE AND EARL N. JOHNSTON. 



This may be done by coating the metal with paraffin and then, 

 before placing into the solution, removing several very small 

 areas. Only one or two structures may be formed from such 

 exposed surfaces. 



Experiments have been performed with the following metals: 

 Fe, Zn, Cd, Co, Ni, Cu, Pb, Sn, Cr and Al. For each metal 

 which forms a precipitate with KsFeCye there is a definite and 

 characteristic type of precipitation structure. 



The presence or absence of a protective colloid has a marked 

 influence on the kind of structure formed. Definite tubular 

 structures or filaments are formed from Zn, Cd and Co only in 

 the presence of a protective colloid; in its absence most of the 

 precipitate is deposited in an "amorphous" state. Copper 

 differs from these metals in forming filaments readily in the 

 absence of the protective colloid. 1 It forms its characteristic 

 structures in a 4 per cent, solution of KsFeCye in distilled water 

 containing 4 per cent. NaCl. 



III. Character of Structures Produced with Different Metals. 



i. Iron. Iron gives rise to a greater variety of well defined 

 precipitation-structures than any of the other metals used. 

 The structure most frequently formed is a hollow filament cir- 

 cular in cross-section and somewhat tapering to the end. Such 

 a filament has a typically straight or slightly curved form if the 

 latter is not influenced by some modifying external condition. 

 The smaller filaments are very easily influenced and are therefore 

 less regular in the direction of growth than the larger filaments. 

 Figs. 3, 4 and 5 show some of the typical iron ferricyanide struc- 

 tures. All were photographed from the same preparation at 

 intervals of 10 minutes, I hour, and 24 hours, respectively, after 

 placing in the salt solution (2 per cent, egg albumin solution 

 containing 2 per cent. K 3 FeCy<j and 0.5 NaCl). 



In the earlier stages of development these structures are thin 

 walled, translucent, and comparatively smooth. Later the 

 walls become thicker and more opaque. The filaments shown 

 in Fig. 3 are all under the surface of the solution and the presence 

 of a distinct double contour proves beyond doubt that even the 



1 Lillie, loc. cit., p. 141. 



