164 Mk. Napier on Copper Sheathing. 



used for sheathing, especially one analysed by Sir H. Davy that lasted 

 long, and contained 1'4 per cent, of tin, and another by R. Phillips, which 

 only lasted four years, and was the purest copper he had seen; it was 

 therefore considered that pure copper was not the best for sheathing, and 

 that the presence of tin and zinc were favourable to its durability." These 

 conclusions, be it observed, differ from that Sir H. Davy came to, and 

 that which R. Phillips analysed may be corroborative of Davy's opinion, 

 because such copper put upon a vessel in connection with impure or 

 alloyed sheets will cause its rapid destruction, and aU the conditions not 

 being given, the conclusion come to from an analysis may be erroneous. 

 Upon these conclusions Mr. Prideaux remarks, that the durability of the 

 sheathing does not depend so much upon the presence of these two metals, 

 tin and zinc, but that their presence guarantees the absence of suboxide 

 of copper, which he considers very injurious to sheathing, facilitating the 

 action of sea water both mechanically and chemically. Here we find the 

 presence of other metals even to 1'5 per cent, thought of no consequence, 

 except as a negative test for the presence of a compound siqyposed to act 

 deleteriously, which, I think, is sufficiently answered by the analyses 

 given by Mr. Prideaux of five difierent coppers, where it will be seen that the 

 presence of tin and zinc are greatest in the copper most rapidly destroyed : — 



New copper. In wear 30 jts. In wear 17 yrs. In wear 5 JTS. Rapid wear. 



Tin, — 0-08 0-07 . 



Zinc, 0-17 009 014 



Iron, 016 0-07 0-26 



Silver, 0-13 0-01 014 



Lead, trace trace — 



0-46 0-25 0-61 0-53 0-64 



It is to be regretted that Mr. Prideaux did not give the amount of copper 

 also in these analyses. 



Pieces of each of these coppers were put into sea water having a little 

 salammoniac in it, and exposed for twelve days, when the loss was as 

 under : — 



New. 30 years. 17 years. 5 yeai's. Rapid wear. 



5-1 5-7 5- 4-6 52 



Although there is not much to be deduced from these experiments, as 

 they would require repetition, and without salammoniac in the solution, 

 nevertheless the results correspond with Sir H. Davy's views. That 

 which had lasted thirty years is the most pure, and sea water has the 

 greatest action upon it. 



Mr. Prideaux also examined the effects of the nails used, and says, in 

 some cases they seemed to have acted as protectors to the copper, it being 

 thickest round them ; in other cases as negative, the copper being destroyed 

 round them. When tried by a galvanometer the nails were found mostly 

 negative to copper, but when the nails were covered with verdigris and 



