220 PROTOPLASMIC ACTION AND NERVOUS ACTION 



ing the physico-chemical conditions under which chemical 

 reactions actually are accelerated in systems of the kind 

 under consideration, we must note as especially signifi- 

 cant the fact that reactions occurring under electrical 

 influence at surfaces (especially metallic surfaces) may 

 be influenced in their velocity by the contact of materials 

 which change locally the electrical state of the surface. 

 The rusting of iron in water or salt solution is a good 

 instance of this type of effect; the reaction may be 

 greatly accelerated by placing another metal, e.g., 

 copper or platinum (which itself does not undergo 

 change) in contact with the iron. The apparently 

 catalytic effect in this case is due to the formation of 

 an electrical circuit between the two metals, the iron 

 becoming anodal and hence freeing Fe ions with increased 

 rapidity; these can then react to form carbonate or 

 hydrate with the anions present in the solution. Another 

 simple and striking demonstration of a '^catalytic" 

 action of this kind is made by placing in a solution of 

 KjFeCye (containing a little NaCl to allow a soluble 

 zinc salt to be formed) two similar strips of metallic zinc, 

 one of which is marked with a lead pencil or bound with 

 a small piece of copper or platinum, while the other is 

 free from such contact. In a few hours a luxuriant 

 ^'growth" of filaments and tubules of zinc ferricyanide 

 is formed from the first strip, while the second remains 

 almost unaltered. The carbon, or the noble metal, 

 acts ''catalytically" in this reaction because it furnishes 

 a surface of lower solution-tension, which formes the 

 cathode of the local electric couple; and since these two 

 areas are in metallic connection and immersed in the 

 electrolyte solution, a current flows which enables 



