162 Mk. Xapier on Copper Sheathing. 



The object required is a metal or alloy that will combine the qualities 

 of keeping the vessel clean, and protecting it from destniction by worms 

 at the same time within a reasonable cost. Notwithstanding many sug- 

 gestions and trials with different metals and aUoys, none has been found 

 more suitable than good copper. 



Sheathing with copper was first adopted in the navy in 1761. In the 

 course of forty years after, it was observed that there was some diversity 

 in the wear of the sheathing, especially in that which had been more 

 recently applied, which diversity has continued greatly to increase, and, 

 according to observations made, this has become much more so since about 

 1832. The extent of the diversity may be stated to range from thirty 

 months to thirty years, and this under all the varied circumstances in 

 which a vessel may be placed. The natural idea suggesting itself to all 

 observers as to the cause of this, was impurity in the copper used, and 

 that the wear was in proportion to its quality. This was put to the test 

 of experiment in 1824. At the commencement of his researches, Sir H. 

 Pavy found that sea water acted more rapidly upon pure copper than 

 when slightly alloyed. It is to be regretted that the nature of the alloy 

 experimented upon was not given ; however, later experiments made upon 

 sheathing which had been in wear, some remarkable for their durability, 

 others for their rapid decay, he found theu- action upon sea water to 

 differ so little as induced him to look for other causes of destruction than 

 the quality of the metal, and he conceived it probable to be owing to the 

 electrical condition of the metals in relation to the sea water, under certain 

 circumstances which had to be sought for by experiment. If it depended 

 whoUy upon the electrical condition of sea water and copper, or, in other 

 words, upon the solubility of copper in sea water, then as pure copper 

 had been found more soluble in sea water than impure, it would conse- 

 quently follow that pure metal was not the best for sheathing; therefore 

 the conditions which regulated the electrical relations of the water and 

 metal upon a ship's bottom became the object of Sir H. Davy's inquiry, 

 and this being of great importance to the present inquiry, I may here 

 briefly state what is meant by the electrical conditions of metals and sol- 

 vents. If any two metals be put into a solvent, say an acid, this acid 

 will act more rapidly upon one of these than upon the other; if the two 

 metals while in the solvent be made to touch each other, the one which 

 had been least acted upon will now be dissolved with less, and the other 

 with greater facility. Thus, suppose a piece of copper in sea water be 

 acted upon with a force equal to one, and a piece of iron in the same 

 water acted upon with a force equal to three; if the two metals are brought 

 into contact, the copper will cease to be acted upon, while the iron will 

 now be dissolved with a force equal to four. The copper in this experi- 

 ment is said to be rendered electro-negative, the iron electro-positive. 



Sir H. Davy found that oxide of copper is negative to metallic copper. 

 An alloy of tin and copper is negative to pure copper, and hammered or 

 hard copper is negative to soft copper. That the green rust which forms 



