172 JIr. Napier on Copper Sheathing. 



only conclusion yet apparent is, that chloride of magnesium is more destruc- 

 tive to the cojjper than any of the other salts found in sea water; and> 

 according to the principle of diffusion recently defined by Professor 

 Graham, the chlorides being most easily diflPiised, there is an excess of 

 chlorides over sulphates near to the mouth of large rivers, which may 

 account for the rapid destruction of sheathing sometimes observed on 

 vessels lying near the mouths of rivers. However, I do not give this as 

 any explanation of the general question. 



I have again taken copper alloyed with from one to two per cent, of 

 other metals, as zinc, iron, lead, arsenic, tin, bismuth, cobalt, nickel, and 

 antimony, and submitted them to the action of sea water. The results, so 

 far as they have gone, only tend to verify the general question, that pure 

 copper is acted upon more rapidly in salt water than alloyed copper. One 

 general principle seems indicated, namely, that copper alloyed with a 

 metal electro-positive to it, is more rapidly acted upon by sea water than 

 when alloyed with a metal electro-negative to it, so that I would infer 

 that a ship sheathed with copper of various qualities of alloy, however 

 minute, will be more rapidly destroyed than if sheathed with copper of 

 one quality. The sample alloyed with antimony was least acted upon, 

 that with no alloy most, the proportion being as three to seven ; there- 

 fore, if a vessel were sheathed with a mixture of these qualities the waste 

 would necessarily be rapid, and an analysis of any single sheet from the 

 vessel would not give the true cause. This, I believe, has led to much 

 error. One sheet having worn well is analysed and found to contain an 

 alloy, another wearing thin in two or three years is found nearly pure ; 

 hence alloyed copper is recommended, and this is no doubt the cause of 

 the haphazard manner in which different applications have been made, to 

 the disappointment and loss of many. 



I had intended calling attention to the various patents taken out for 

 improvements in sheathing, as an illustration of the great want of prac- 

 tical knowledge, and of the application of principles to such questions 

 which often tends to give our merchants and manufacturers a very indif- 

 ferent opinion of the value of science, but time will not permit. Only 

 one of the numerous patents taken out has stood the test of experience, 

 as I have already noticed, viz. Muntz's yellow metal; and any want of 

 uniformity in the sheets of this alloy, also causes more rapid destruction 

 of the sheathing. 



Mr. James Thomson explained his patent apparatus for obviating 

 priming in steam engines. 



\^th Fehrvwy, 1851. — Mr. "William Murray in (lie Chair. 



Messrs. Archibald M. Fyfe and James Milne were admitted members. 

 A letter was read from the Swansea Literary and Scientific Society, 



