34- Sir H. Davy on Corrosion of Copper Sheeting by Sea-xvater. 



As the ocean may be considered, in its relation to the quan- 

 tity of copper in a ship, as an infinitely extended conductor, 

 I endeavoured to ascertain whether this circumstance would 

 influence the results; by placing two very fine copper wires, 

 one undefended, the other defended by a particle of zinc, in 

 a very large vessel of sea-water, which water might be consi- 

 dered to bear the same relation to so minute a portion of 

 metal as the sea to the metallic sheeting of a ship. The re- 

 sult of this experiment was the same as that of all the others ; 

 the defended copper underwent no change ; the undefended 

 tarnished, and deposited a green powder. 



Small pieces of zinc were soldered to different parts of a 

 large plate of copper, and the whole plunged in sea-water : it 

 was found that the copper was preserved in the same manner 

 as if a single piece had been used. 



A small piece of zinc was fastened to the top of a plate of 

 polished copper, and a piece of iron of a much larger size was 

 soldered to the bottom, and the combination placed in sea- 

 water. Not only was the copper preserved on both sides in 

 the same manner as in the other experiments, but even the 

 iron ; and after a fortnight, both the polish of the copper and 

 the iron remained unimpaired. 



7. I am continuing these researches, and I shall communi- 

 cate such of them as are connected with new facts, to the Royal 

 Society. 



The Lords Commissioners of the Admiralty, with their 

 usual zeal for promoting the interests of the Navy by the ap- 

 plication of science, have given me permission to ascertain the 

 practical value of these results by experiments upon ships of 

 war; and there seems every reason to expect (unless causes 

 should interfere of which our present knowledge gives no 

 indications) that small quantities of zinc, or, which is much 

 cheaper, of malleable or cast iron, placed in contact with the 

 copper sheeting of ships, which is all in electrical connection, 

 will entirely prevent its corrosion. And as negative electricity 

 cannot be supposed favourable to animal or vegetable life; 

 and as it occasions the deposition of magnesia, a substance 

 exceedingly noxious to land vegetables, upon the copper 

 surface; and as it must assist in preserving its polish, there 

 is considerable ground for hoping that the same application 

 will keep the bottoms of ships clean ; a circumstance of great 

 importance both in trade and naval war. 



It will be unnecessary for me to dwell upon the economical 

 results of this discovery, should it be successful in actual prac- 

 tice, or to point out its uses in this great maritime and com- 

 mercial country. 



I might 



