490 Mr, J. Napier on Copper Smelting. 



Srd, Further poling. Colour lighter, with more lustre, frac- 

 ture hard, not sandy, but vitreous. Spec. grav. 8*481. 



4th. Longer poling. Colour brighter, with more of the 

 metallic lustre, less vitreous than former, and having minute 

 cavities. Spec. grav. 8*397. 



5th. Near to tough pitch. Colour still brighter, fracture full 

 of minute cavities, as if the metal had set during escape of 

 gaseous matters, slightly vitreous. Spec. grav. 8*582. 



6th. Tough pitch. The appearance of this was singular; the 

 cavities in the last specimen seemed now collapsed, giving a 

 fracture resembling a honey-comb, strongly compressed, the 

 colour of silvery brightness and a soft metallic appearance. Spec, 

 grav. 8'654. 



7th. A little overpoled. The collapsed cells extended some- 

 thing like No. 5 ; some of the cavities oval, as if a gas had 

 escaped ; fracture, a reedy hard appearance. Spec. grav. 8*594. 



8th. Further overpoled. Fracture crystalline and vitreous, 

 full of cavities. Spec. grav. 8*518. 



9th. Much overpoled. Fracture like a crystalline network, 

 very hard and vitreous. Spec. grav. 8*298. 



These appearances seem to indicate the changes going on 

 within the metal during poling, and the specific gravity of 

 each corresponds with these appearances. In Nos. 4 and 8, 

 when the cavities are very numerous, the specific gravities are 

 lowest. 



From these experiments and observations, we think that the 

 first reaction in poling is the reduction of the oxide of copper, 

 and the complete reduction of the oxide constitutes the tough 

 pitch. Overpoling we think to be the imbibition of carbonic 

 oxide or carbonic acid, and not the formation of a carburet of 

 copper. That metals imbibe gas when in a melted state is well 

 known ; some of them, as silver, give it out when setting. 

 That overpoling is due to gaseous matter contained in the metal, 

 we think is evident by its effiervescence, and by its becoming 

 tough if kept melted until this effervescence ceases. 



It is worthy of remark, that green wood is much more effiect- 

 ive in poling than dry wood or charcoal ; probably the hydrogen 

 in the moisture and juices is more eff*ective in reducing the oxides. 



When copper is pure and of good quality, the ingot when 

 struck by a hammer or thrown upon another ingot gives a dead 

 solid sound, not sonorous; while copper with the slightest 

 quantity of alloy when so treated gives a sharp clink. By this 

 character the founder often judges of the quality of the metal. 

 We have seen copper received by a purchaser with great sus- 

 picion because of its sound, which gave by analysis — 



