COPPER, SILVER, AttD GOLJD 403 



'Copper is characterised by its^red colour, which distinguishes it 

 from all other metals. Pure copper is soft, and may be beaten out by 

 a hammer at the ordin'ary temperature, and when hot may be rolled 

 into very thin sheets. Extremely thin leaves of copper transmit a 

 green light. The tenacity of copper is also considerable, and next' to 

 iron it is one of the most durable metals in this respect. Copper wire 

 of 1 sq. millimetre in section only breaks under a weight of 45 kilograms. 

 The specific gravity of copper is 8-8, unless it contains cavities due to the 

 fact that molten copper absorbs oxygen from the air, which is disen- 

 gaged on cooling,, and therefore gives a porous mass whose density is 

 much less. Rolled copper, and also that which is deposited by the electric 

 current, has a comparatively high density. Copper melts at a bright 

 red heat, about 1050, although below the temperature at which many 

 kinds of cast iron, melt. At a high temperature it is converted into 

 vapour, which communicates a green colour to the flame. Both native 

 copper and that cooled from a molten state crystallise in regular 

 octahedra. Copper is not oxidised in dry air at the ordinary tempera- 

 ture, but when calcined it becomes coated with a layer of oxide, and it 

 does not burn even at the highest temperature. Copper, when calcined 

 in air, forms either the red cuprous oxide or the black cupric oxide, 



while the Ag, Au and oxides of iron remain behind in the residue (from which the noble 

 metals may be extracted) ; (3) a portion of the copper in solution is converted into CuCLj 

 (and.CaSO 4 precipitated) by means of the CaCl 2 obtained in the fifth process ; (4) the 

 mixture of solutions of CuS0 4 and CuCl 2 is converted into the insoluble CuCl (salt of the 

 suboxide) by the action of the SOj obtained by roasting the ore (in the first operation), 

 sulphuric acid is then formed in the solution, according to the equation : CuSO^H- CuClj 

 ? + SO.j + 2H 2 O = 2H 2 SO 4 + 2CuCl; (5) the precipitated CuCl is treated with lime and 

 water, and gives CuCl 2 in solution and CuO in the residue ; and lastly (6) the Cu 2 O is 

 reduced to metallic Cu by carbon in a furnace. According to Crboke's, method the impure 

 copper regulus obtained by roasting and smelting the ore. is broken up and immersed 

 .repeatedly in molten lead, which extracts the Ag and Au occurring in the regulus. The 

 regulus is then heated in a reverberatory furnace to run off the- lead, and is then smelted 

 for Cu. 



The copper brought into the market often contains small quantities of varioijs impuri- 

 ties. Among these there are generally present iron, lead, silver, arsenic, and sometimes 

 small quantities of oxides of copper. As copper, when mixed with ,a small amount of 

 foreign substances, loses its tenacity to a certain degree, the manufacture of very thin 

 sheet copper requires the use of Chili copper, which is distinguished for its great softness, 

 and therefore when it is desired to have pure copper, it is best to take thin sheet copper, 

 like that which is used in the manufacture of cartridges. But the purest copper is electro- 

 lytic copper that is, that which is deposited from a solution by the action of an 

 electric current. 



If the copper contains silver, as is often the case, it is used in gold refineries for the 

 precipitation of silver from its solutions in sulphuric acid. Iron and zinc reduce copper 

 salts, but copper reduces mercury and silver salts. " The precipitate contains not only the 

 silver which was previously in solution, but also all that which was in the 9 copper.. The 

 silver solutions in sulphuric acid are obtained in the separation of silver from gofcl by 

 treating their alloys with sulphuric acid, which only dissolves the silver. 



