232 CERTAIN SAND MOUNDS OF 



Native copper is entirely free from combined oxygen, which is always found in 

 copper which has been in a state of fusion. Unfortunately, the thin sheets of 

 hammered native copper usually met with in the mounds are so greatly oxidized 

 exteriorly that total elimination of extraneous oxygen previous to analysis is a 

 matter of great difficulty, hence percentages of oxygen are frequently reported. 

 Therefore, except in the case of pieces of copper cut from solid implements, the 

 presence or absence of oxygen is not a final test. 



All native copper, so far as we have been able to learn, contains a percentage 

 of silver. The metal is finely distributed, and except in the case of occasional 

 masses of copper from Lake Superior, to which reference will be made later, its 

 presence is not visible in copper from North America so far reported. 



Crystallized native copper from Lake Superior, 1 which James R. Cooper, Esq., 

 Superintendent of the Lake Superior Smelting Company, kindly has had analyzed, 

 yielded 2 - 74 ounces of silver per ton (0'0093 per cent.), while the average amount 

 of silver in ordinary "Lake" copper is reported by the same high authority to be 

 about 6 ounces to the ton (00206 per cent.). 



Unfortunately for exact determination the presence of silver in copper does 

 not of necessity indicate native copper, unless the silver is visibly present in 

 streaks, seams, or flakes, since silver is very frequently found in the ores of copper, 

 and its elimination, as we have stated elsewhere, is difficult. 2 



Lead, we believe, has never been discovered in native copper. 3 It is true that 

 its presence has occasionally been reported in ingot copper from Lake Superior, but 

 this result is in every case due to the gangue or to impurities in the furnaces. The 

 intentional introduction of lead into drawn copper from the "Lake" was discon- 

 tinued thirty years ago. As to the absence of lead from "Lake" copper previous 

 to treatment, all experts are absolutely unanimous . 



Native copper, owing to absence of oxygen, is of a lighter color than copper 

 when melted and cast or smelted from the ore. 



We are indebted to James R. Cooper, Esq., for another method to distinguish 

 native copper from copper which has undergone treatment. 



"You can readily determine," he writes, "the fact whether the 'mound' cop- 

 per is a native metal, or whether it has been smelted. 



" Take a piece of the mound copper and hammer it thoroughly to harden it, 

 then bend it double and hammer it down flat. If it is native copper it will stand 

 the test without a show of cracking, but if it is smelted copper it will break short 

 in bending double. * * * * The fracture is entirely different. The fracture 

 of native copper is more like that of lead when it is bent back and forth and finally 

 broken." 



1 Practically all " Lake" copper is native. 



2 Silver is not volatile, as are some other elements, hence the tenacity of its union with copper. 



3 A certain writer has reported the discovery of lead in South American native copper. This report 

 lacks confirmation. Moreover, this assertion was made at a time when the chemistry of copper was in 

 its infancy. There is, against the discovery of lead in native copper, the fact that native lead, if found at 

 at all, is of extreme rarity. Lead ore may exist in the gangue. 



