Chap. 34.] Plating of Copper. 225 



with iron, but the properties of it have not been well 

 examined into. With lead it forms an alloy, which is 

 much more fufible than pure filver, and feems in all 

 refpecls to be of an intermediate nature between thefe 

 metals. Copper increafes the hardnefs of filver, and 

 renders it more fonorous, without impairing its duc- 

 tility or colour, when the copper does not exceed the 

 twelfth part of the weight of filver employed. The 

 itandard of filver coin is eleven ounces two penny- 

 weights, troy, of filver, and eighteen pennyweights of 

 copper. 



The purity of filver cannot be accurately afcertained 

 without fubmitting it to cupellation with lead, and its 

 purity is calculated according to the weight it lofes 

 in that procefs. Silver is divided into twelve ima- 

 ginary parts, called pennyweights. If it only lofes 

 one twelfth part of its weight by cupellation, it con- 

 tains eleven twelfths of pure filver., and is faid to be 

 eleven pennyweights fine ; if it lofes two twelfths, it is 

 ten pennyweights fine, and fo of other proportions. 

 For greater accuracy, each pennyweight is fuppofed to 

 be divided into twenty-four grains. 



The covering of the furface of copper with filver, 

 or plating, is performed in the following manner: 

 ' Upon fmall ingots of copper, plates of filver are 

 bound with iron wire, generally allowing one ounce of 

 filver to twelve ounces of copper. The furface of the 

 plate of filver is not quite fo large as that of the cop- 

 per ingot. Upon the edges of the copper, which are 

 not covered by the filver, a little borax is put, and by 

 expofing the whole to a ftrong heat, the borax melts, 

 and in melting contributes to melt that part of the 

 filver to which it is contiguous, and to attach it in 

 that melted (late to the copper. The ingot, with its 

 illvcr plate, is then rolled under fteel rollers, moved 



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