CHEMISTRY. 253 



the cold rolling over the annealed alloy, while it will be seen that the silver 

 alloy, by taking the negative state in the mass of metal, hastened its destruc- 

 tion, while its own form and condition were such that it was separated as 

 the copper corroded. The average duration of the wear of copper on Ame- 

 rican ships is three years." 



Mr. Prideaux, after many trials and observations, as stated in his corre- 

 spondence already referred to, seems almost inclined to abandon the quality 

 of the metal, and seek the cause wholly in the conditions, which he states 

 thus : 



" 1. Friction from heavy shore work, faster sailing, and more active 

 service. 



" 2. Corrosive waters, as the drainage of mines, manufactures, sewers, and 

 putrescent matters in the sea. 



" 3. Climate. Corrosive action being increased by heat, and sheathing is 

 known to waste quicker in tropical climates. 



" 4. Weather. Electrical and thundery, storms, &c. 



" 5. Electro- Chemical. Xails and metal giving a positive tendency to 

 waste. 



" 6. Matters laid under sheathing as tar, paper, felt, which may have acid 

 or alkaline properties. 



" 7. Timber of the vessel. Some wood having acid properties, c." 

 Nevertheless, he seems driven back to the original question, and sums up his 

 inquiry with the following: " To whatever extent the recently increased 

 waste of sheathing may be due, such as constant employ, much greater velo- 

 city, &c.. there is reason to fear the fault is still to be sought too often in the 

 copper itself." 



As to the effect of speed and friction, Sir H. Davy found that, on a vessel 

 going at a speed of eight miles an hour, the copper most exposed to the 

 friction of the sea lost more than double that which was least exposed ; 

 and Mr. Prideaux found that pieces of the same quality of copper put into sea 

 water, from different localities, were differently acted upon. In thirteen 

 days' exposure the waste of copper in water from 



Heart of G-ulf Stream was 1-81 



,, Caribbean Sea . . . . . . O'4-O 



Plymouth Harbor 0'31 



Such circumstances as these are easily detinecl; but when two vessels 

 sheathed at one time, and kept nearly under the same conditions, and the 

 copper of the one lasting two or three times that of the other, or even one 

 vessel, her sheathing at one time lasting seventeen or twenty years, and at 

 another not more than three or four, and employed on the same service, are 

 circumstances not so easily accounted for. and require a more strict investi- 

 gation. 



Another element in the consideration of this question is the sea water. 

 Different seas act differently on sheathing, and often weak or diluted sea 

 water more than strong, so that at the mouths of rivers the sheathing is often 

 found to wear more rapidly than out at sea. As a sort of guide to this 

 inquiry, I made the following experiments, taking the different ingredients of 



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