PRECIPITATION STRUCTURES SIMULATING ORGANIC GROWTH. 253 



men containing 2 per cent. K 3 FeCy 6 and 0.5 per cent. NaCl), 

 numerous hollow, vesicular, shell-like structures (Fig. 17) are 

 formed in addition to the slender tortuous filaments. These 

 precipitation-vesicles closely resemble those formed on cadmium. 

 They are finely and evenly striated, the striae being only about 5 

 microns apart. They grow so slowly that no rhythmic motion 

 can be detected, but it is probable that their formation is periodic, 

 as in the case of the cadmium precipitation-vesicles. 



A highly characteristic feature of the cobalt filaments is their 

 tortuous, tendril-like form. They curl and wind in all directions 

 without any apparent cause. Nickel filaments show a similar 

 behavior. The nature of the precipitate formed, no doubt, has 

 much to do with this phenomenon. The long filaments show no 

 striation and there is no tendency to form branches. 



4. Cadmium. The most characteristic structures formed 

 from this metal are hollow shell-like growths (Fig. 18). These 

 form best in a 2 per cent, egg albumen solution containing 

 2 per cent. K 3 FeCy 6 plus 0.5 per cent. NaCl. The structures 

 formed under these conditions are striated and exhibit a very 

 regular rhythmic motion (see below, page 259).- They are of 

 various shapes and sizes; occasionally they resemble a mussel- 

 shell both in shape and character of striation. The striations 

 vary in width in different cases- from 20 to 30 microns. In Fig. 

 19 some of the same structures are shown after 24 hours in the 

 solution. The largest of these are about I mm. in length. As 

 is shown, their surface becomes entirely covered with fine short 

 filaments giving them somewhat of the appearance of a burr. 

 Very little growth takes place in the main structure after these 

 fine filaments begin to form. As the reaction continues the 

 enclosing membrane becomes thick and firm and more resistant 

 to the forces which cause the rhythmic motion and periodic 

 out-pushing of the structure. The small, short filaments then 

 shoot out quickly at very regular intervals at different points 

 over the surface. Some filaments are also formed directly from 

 the surface of the metal. These are coarse and granular in 

 appearance and show much non-coherent precipitate scattered 

 along their course. If a strip of cadmium is put in contact with 

 a nobler metal, or if a more concentrated solution of ferricyanide 



