398 PRINCIPLES OF CHEMISTRY 



CHAPTER XXIV 



COPPER, SILVER, AND GOLD 



THAT degree of analogy and difference which exists between iron, 

 cobalt, and nickel repeats itself in the corresponding triad ruthenium, 

 rhodium, and palladium, and also in the heavy platinum metals, 

 osmium, iridium, and platinum. These nine metals form Group VIII. 

 pf the elements in the periodic system, being ther intermediate group 

 between the even elements of the large periods and the uneven, among 

 which we know zinc, cadmium, and mercury in Group II. Copper, 

 silver, and gold complete l this transition, because their properties 

 place them in proximity to nickel, palladium, and platinum on the one 

 hand, and to zinc, cadmium, and mercury on the other. Just as Zn, 

 Cd, and Hg ; Fe, Ru, and Os ; Co, Rh, and Ir ; Ni, Pd, and Pt, 

 resemble each other in many respects, so also do Cu, Ag, and Au. 

 Thus, for example, the "atomic weight of copper Cu = 63, and in all its 

 properties it stands between Ni = 59 and Zn = 65. But as the tran- 

 sition from Group VIII. to Group II., where zinc is situated, cannot be 

 otherwise than through Group I., so in copper there are certain pro- 

 perties of the elements of Group I. Thus it gives a suboxide, Cu 2 0, 

 and salts, CuX, like the elements' of Group I., although at the same 

 time it forms an oxide, CuO, and salts CuX 2 , like nickel and zinc. In 

 the state of the oxide, CuO, and the salts, CuX 2 , copper is analogous to 

 zinc, judging from the insolubility of the carbonates, phosphates, and 

 similar salts, and by the isomorphism, and other characters. 2 In the 

 cuprous salts there is undoubtedly a great resemblance to the silver 



1 The perfectly unique position held by copper, silver, and gold in the periodic system 

 of the elements, and the degree of affinity which is found between them, is all the more 

 remarkable, as nature and practice have long isolated these metals from all others by 

 having employed them for example, for coinage and determined their relative 

 importance and value in conformity with the order (silver between copper and gold) of 

 their atomic weights, &c. 



2 Cupric sulphate contains 5 molecules of water, CuS0 4 ,5H 2 O, and the isomorphous 

 mixtures with ZnSO4,7H 2 O contain either 5 or 7 equivalents, according to whether copper 

 or zinc predominates (Vol. II. p. G). If there be a large proportion of copper, and if the 

 mixture contain 5HoO, the form of the isomorphous mixture (triclinic) will be isomorphous 

 with cupric sulphate, CuS0 4 ,5H 2 O, but if a large amount of zinc (or magnesium, iron, 

 nickel, or cobalt) be present the form (rhombic or monoch'nic)"will be nearly the same 



