(I) The century that has just ended saw many new materials adapted for battery use. (II) These include mercury, silver oxide and nickel-cadmium. (Ill) At first, little thought was given to what would happen when millions of tons of these substances were discarded into the environment. (IV) But recently mercury has been eliminated from batteries because of its toxicity, and cadmium may follow. (V) For devices used only occasionally, the zinc-carbon battery can be more costeffective than other types.