252 THE REALITIES OF MODERN SCIENCE 



an anion and in other substances as a cation, thus in 

 ferrocyanide of potassium, K4Fe(CN) 6 , there are formed 



four positive jipns of potassium, K, and one complex 

 anion, Fe(CN) 6 , composed of an iron atom and six 

 molecular groups called cyanogen, each composed of 

 an atom of carbon and one of nitrogen. The cyanogen 

 ion consists of thirteen atoms and carries four extra 

 electrons. The iron atom in this case is not a cation 

 but a component part of the anion, and thus does 

 not enter into the characteristic chemical reactions 

 which distinguish it in those cases where it is a cation. 



When an ion is neutralized at an electrode, there are 

 three possible effects ; thus it may be liberated as a 

 gas, be deposited on the electrode, or enter into a re- 

 action with the solvent. The electrolysis of HC1, of 

 silver nitrate, AgN0 3 , and the decomposition of water 

 are examples respectively. 



Consider a solution of silver nitrate between elec- 

 trodes of silver. When the silver ions are neutralized 

 at the cathode they are deposited, granules of pure 

 silver being formed on the surface of the plate. At 

 the anode the neutralization of the radical NO 3 takes 



place by its combination with a silver ion, Ag, which is 

 released by the anode itself. Thus silver is trans- 

 ferred from the anode to the cathode. We may con- 

 sider N0 3 to react with the silver of the anode to form 

 AgN0 3 , or better we may consider that the silver dis- 

 solves as silver ions, that is, atoms deficient by one 

 electron each. A surplus of electrons is thus left on 

 the anode just as if the nitrate radicals had given up 

 to it their excess electrons. 



