1826.] Preservationof Metals hxf Electro-chemical Means. 253 



These results, which showed that a very small quantity only 

 of the imperfect or fluid conductor was sufficient to transmit the 

 electrical power, or to complete the chain, induced me to try if 

 copper nailed upon wood, and protected merely by zinc or iron 

 on the under surface, or that next the wood, would not be 

 defended from corrosion. For this purpose I covered a piece of 

 wood with small sheets of copper, a nail of zinc of about the 

 •^■^•o P^i* of *-'^^ surface of the copper being previously driven 

 into the wood : the apparatus was plunged in a large jar of sea 

 water: it remained perfectly bright for many weeks, and when 

 examined, it was found that the zinc had only suffered partial 

 corrosion ; that the wood was moist, and that on the interior of 

 the copper there was a considerable portion of revived zinc, so 

 that the negative electricity, by its operation, provided materials 

 for its future and constant excitement. In several trials of the 

 same kind, iron was used with the same results ; and in all these 

 experiments there appeared to be this peculiarity in the appear- 

 ance of the copper, that unless the protecting metal below was 

 in very large mass, there were no depositions of calcareous or 

 magnesian earths upon the metal ; it was clean and bright, but 

 never coated. The copper in these experiments was nailed 

 sometimes upon paper, sometimes upon the mere wood, and 

 sometimes upon linen ; and the communication was partially 

 interrupted between the external surface and the internal surface 

 by cement; but even one side or junction of a sheet seemed to 

 allow sufficient communication between the moisture on the 

 under surface and the sea water without, to produce the electri- 

 cal efi'ect of preservation. 



These results upon perfect and imperfect conductors led to 

 another inquiry, important as it relates to the practical applica- 

 tion of the principle ; namel}"-, as to the extent and nature of the 

 contact or relation between the copper and the preserving metal. 

 I could not produce any protecting action of zinc or iron upon 

 copper through the thinnest stratum of air, or the finest leaf of 

 mica, or of dry paper; but the action of the metals did not seem 

 to be much impaired by the ordinary coating of oxide or rust ; 

 nor was it destroyed when the finest bibulous or silver paper, as 

 it is commonly called, was between them, being moistened with 

 sea water. I made an experiment with different folds of this 

 paper. Pieces of copper were covered with one, two, three, 

 four, five, and six folds ; and over tliem were placed pieces of 

 zinc, which were fastened closely to them by thread; each piece 

 of copper so protected was exposed in a vessel of sea water, so 

 that the folds of paper were all moist. 



It was found in the case in which a single leaf of paper was 

 between the zinc and the copper, there was no corrosion of the 

 copper ; in the case in which there were two leaves, there was % 

 very slight effect ; witlj three, the corrosion was distinct; and it 



