248 Sir Humphry Davy on the [April; 



Article IV. 



Further Researches on the Vreservation of Metals hy Ehctro- 

 chemical Means. By Sir Humphry Davy, Bart. Pres. RS.* 



In two papers read before the Royal Society, I have described 

 the effects of small quantities of electro-positive metals in pre- 

 venting the corrosion or chemical changes of copper exposed to 

 sea water, and I have stated that the results appear to be of the 

 same kind, whether the experiments are made upon a minute 

 scale, and in confined portions of water, or on large masses, and 

 in the ocean. 



The first and preliminary experiments proved, that the copper 

 sheeting of ships might be 'preserved by this method; but 

 another and a no less important circumstance was to be attended 

 to, — how far the cleanness of the bottom, or its freedom from the 

 adhesion of weeds or shell fish, would be influenced by this 

 preservation. 



The use of the copper sheathing on the bottom of ships is two- 

 fold ; first, to protect the wood from destruction by worms ; and 

 secondly, to prevent the adhesion of weeds, barnacles, and other 

 shell fish. No worms can penetrate the wood as long as the 

 surface of the copper remains perfect ; but when copper has 

 been applied to the bottom of a ship for a certain time, a green 

 coating or rust, consisting of oxide, submuriate and carbonate of 

 copper and carbonate of magnesia forms upon it, to which weeds 

 and shell fish adhere. 



As long as the whole surface of the copper changes or corrodes, 

 no such adhesions can occur ; but when this green rust has 

 partially formed, the copper below is protected by it, and there 

 is an unequal action produced, the electrical effect of the oxide, 

 submuriate, and carbonate of copper formed, being to produce a 

 more rapid corrosion of the parts still exposed to sea water ; so 

 that the sheets are often found perforated with holes in one part, 

 after being used five or six years, and comparatively sound in 

 other parts. 



There is nothing in the poisonous nature of the metal which 

 prevents these adhesions. It is the solution by which they are 

 prevented — the wear of surface. Weeds and shell fish readily 

 adhere to the poisonous salts of lead which form upon the lead 

 protecting the fore part of the keel; and to the copper, in any 

 chemical combination in which it is insoluble. 



In general in ships in the navy, the first effect of the adhesion 

 of weeds is perceived upon the heads of the mixed metal nails, 

 •which consist of copper alloyed by a small quantity of tin. The 



" From tl^e Flulosophical Transactions for 1825, Fart II. 



