STRUCTURES RESEMBLING ORGANIC GROWTHS. 155 



inhibits the protective influence of the zinc upon the iron, 

 evidently because the zinc forms the cathode in the zinc-mag- 

 nesium couple, the current from which opposes and partly com- 

 pensates that from the zinc-iron couple. In another experiment 

 a similar strip of zinc was in contact with platinum wire at one 

 end and with magnesium at the other; in this case the zinc 

 remained free from filaments for a long time; after 18 hours a 

 dense growth of filaments had developed near the platinum; 

 toward the magnesium the growth decreased, and near the con- 

 tact with this metal the zinc was entirely free from filaments. 

 In a similar manner the contact of iron or zinc with a platinum- 

 bound copper wire inhibits the formation of filaments from the 

 copper. A piece of copper wire 4 cm. long, encircled at one end 

 with platinum wire, was placed in a solution containing 4 per 

 cent. K 3 FeCye and 4 per cent. NaCl; filaments and bur-like 

 tufts of copper ferricyanide appeared rapidly. But a similar 

 piece of platinum-bound copper which was placed in contact at 

 the other end with an iron wire showed only slight and gradual 

 formation of precipitate; under these conditions filaments form 

 rapidly from the iron, and their formation apparently has the 

 effect of inhibiting their development from the copper. What 

 really happens is that the current of the copper-iron circuit is 

 opposed in direction to that of the copper-platinum circuit and 

 compensates the latter. 



Attention should also be called to a type of experiment in 

 which the two precipitate-forming metals are not in direct contact 

 with each other but are connected by a conductor which itself 

 undergoes no chemical change, like platinum, carbon, or copper. 

 For example, a piece of iron and a piece of zinc connected by a 

 stretch of copper wire I cm. long were placed in 2 per cent. 

 K 3 FeCy 6 in dilute egg-white; the zinc immediately formed fila- 

 ments, but not the iron. Similar pieces of iron and zinc, each 

 in contact with a separate piece of copper, both form filaments 

 rapidly, as already described; but when the two are in contact 

 with the same conductor the zinc prevents the formation of 

 filaments from the iron in the same manner (though to a less 

 degree) as when the metals are in direct contact. It is further 

 to be noted that under these conditions the iron, though not 



