54 GALVANISM. 



Powerful Batteries. 



Sir H. Davy's great voltaic battery consisted of 

 2000 double plates of copper and zinc, of four inches 

 square. 



Each plate of Mr. Children's large Galvanic battery 

 consists of 32 square inches, and produces, intense 

 heat. Iron was instantaneously converted by it to 

 blister steel, and diamond powder disappeared. 



Copper and Sea Water. 



The chemical action of bodies on each other may 

 be modified or destroyed by changing their electric 

 states. By bringing a body, naturally positive, arti- 

 ficially into a negative electric state, its usual powers 

 of combination will be destroyed. It was upon this 

 principle that Sir H. Davy applied a negative power 

 to prevent the corrosion of the copper sheathing of 

 ships. 



Copper, being a metal only weakly positive, could 

 be easily rendered slightly negative, when all its 

 chemical action on the sea water would cease, and it 

 was subsequently found that a piece of zinc no larger 

 than a pea or the point of a small iron nail, by effect- 

 ing this change, preserved from 40 to 50 inches of 

 copper, exposed for many weeks to the full flow of 

 the tide in Portsmouth harbour, while the zinc or 

 iron was slowly corroded. 



Copper sheathing, until it is worn into holes, cor- 

 rodes so fast that no permanent surface remains to 

 which weeds can adhere, but when there are inequa- 

 lities on the surface, they adhere readily enough even 

 to the poisonous oxide of copper. It is probable 

 that protectors, if made a little positive, or nearly in 

 equilibrio, will preserve the whole surface smooth, 

 and prevent the adhesion of either shell fish or weeds. 

 At present the requisite proportion is estimated to 



