Me. Napier on Copper Sheathing. 163 



upon copper is negative to metallic copper, that the nails used were nega- 

 tive to the sheathing, and also that copper alloyed with small quantities 

 of tin, zinc, iron, or arsenic, promotes the formation of insoluble com- 

 pounds upon the surface of copper in sea water, and that being negative, 

 hastens the destruction of the sheathing. Here was ample source for 

 explaining the diversity of wear in sheathing; and, it may be observed, it 

 brino's us back to the original idea, that impure copper causes more rapid 

 destruction when used as sheathing than pure; and although the direct 

 action of sea water be a little greater upon the latter, it being regular 

 and not subject to local galvanic influence, it will be more lasting, and 

 also more effective for prevention of fouling, &c. 



To overcome the evils both of the local and total destruction of sheath- 

 ing by electrical conditions. Sir H. Davy suggested the rendering of the 

 whole surface of the sheathing negative, and destroying all local electrical 

 influences, by bringing a positive metal into contact with it. The trials 

 and experiments made upon these suggestions were eminently successful. 

 " When a piece of zinc having a surface equal to jljjth of the copper was 

 attached, there was no corrosion or decay; with smaller quantities, such 

 as from a^cth to juoth, the copper underwent a loss of weight which was 

 greater in proportion as the copper was smaller." Trials were made upon 

 an extensive scale with vessels under various circumstances, and all proved 

 the correctness of the principle. But it was found that copper thus pro- 

 tected soon became covered with an earthy coating, composed of carbonate 

 of lime and hydrate and carbonate of magnesia, and to this coating weeds 

 and sheU-fish easily adhered and produced fouling, so that this beautiful 

 application of a philosophic principle had to be, in less than two years, 

 completely abandoned. 



The late Professor Daniell, thinking that the abandonment of the prin- 

 ciple was too absolute and premature, proposed a mode of partial protec- 

 tion, by arranging the protectors in such a manner that they could be with- 

 drawn in part or in whole, and that the sheathing might be fully protected 

 where there was no liability to foul, and where this liability existed, the 

 protectors would be partly or wholly removed. We are not aware if 

 trials were made upon this suggestion. The subject of fouling, and its 

 prevention by copper, I may mention, have been viewed differently. Some 

 suppose it to be owing to the poisonous quality of the copper compound 

 formed by the action of the sea water, which kills the barnacles and other 

 organisms that attempt to adhere and thus drop off, while others, and 

 among whom ranked Sir II, Davy, consider that the weeds and shell-fish 

 are prevented from adhering by losing their hold from the corrosion of 

 the surface. 



The next who drew public attention to this subject, was Mr. Prideaux, 

 chemist at the Plymouth dock-yards, in a paper to the British Associa- 

 tion at their meeting in 1841. In this paper wo find it stated that the 

 popular opinion then was, that alloyed copper was best, originating, says 

 that authority, " from observations made upon several samples of copper 



