COPPER, SILVER, AND GOLD 899 



salts thus, for example, silver chloride, AgCl, is characterised by its 

 insolubility and capacity of combining with ammonia, and in this respect 

 cuprous chloride closely resembles it, for it is also insoluble in water, 

 and combines with ammonia and dissolves in it, &c. Its composition is 

 also BC1, the same as AgCl, NaCl, KC1, <fec., and silver in many com- 

 pounds resembles, and is even isomorphous with, sodium, so that this 

 again justifies their being brought together. Silver chloride, cuprous 

 chloride, and sodium chloride crystallise in the regular system. 

 Besides which, the specific heats of copper and silver require that they 

 should have the atomic weights ascribed to them. To the oxides Cu 2 O 

 and Ag 2 there are corresponding sulphides Ag 2 S and Cu 2 S. They 

 both occur in nature in crystals of the rhombic system, and, what is 

 most important, copper glance contains an isomorphous mixture of 

 them both, and retains the form of copper glance with various pro- 

 portions of copper and silver, and therefore has the composition B 2 S 

 where R = Cu, Ag. 



Notwithstanding the resemblance in the atomic composition of the 

 cuprous compounds, CuX, and silver compounds, AgX, with the com- 

 .pounds of the alkali metals KX, NaX, there is a considerable degree 

 of difference between these two series of elements. This difference is 

 clearly seen in the fact that the alkali metals belong to those elements 

 which combine with extreme facility with oxygen, decompose water, 

 and form the most alkaline bases ; whilst silver and copper are 

 oxidised with difficulty, form less energetic oxides, and do not decom- 

 pose water, even at a rather high temperature. Moreover, they only 

 displace hydrogen from very few acids. The difference between them 

 is also seen in the dissimilarity of the* properties of many of the 

 corresponding compounds. Thus cuprous oxide, Cu 2 O, and silver oxide, 

 Ag 2 0, are insoluble in water : the cuprous and silver carbonates, 

 chlorides, and sulphates are also sparingly soluble in water. The 

 oxides of silver and copper are also easily reduced to metal. This 

 difference in properties is in intimate relation with that difference in 

 the density of the metals which exists in' this case. The alkali metals 

 belong to the lightest, and copper and silver to the heaviest, and there- 

 fore the distance between the molecules in these metals is very dis- 

 similar it is greater for the former than the latter (tables in Chapter 

 XV.). From the point of view of the periodic law, this difference 

 between copper and silver and such elements of Group I. as potassium 

 and rubidium, is clearly seen from the fact that copper and silver 



as that of zinc sulphate, ZnSO 4 ,7H 2 0. Supersaturated solutions of each of these salts 

 crystallise in that form and with that amount of water which is contained in a crystal 

 of one or other of the Baits brought in contact with the solution (Chapter XIV., Note 27). 



