COPPER, SILVER, AND GOLD 405 



gage hydrogen from it either at the ordinary or at high temperatures. 

 Nor does copper liberate hydrogen from the oxygen acids , these act on 

 it in two ways : they either give up a portion of their oxygen, form- 

 ing lower grades of oxidation, or else only react in the presence of 

 air. Thus, when nitric acid acts on copper it evolves nitric oxide, the 

 copper being oxidised at the expense of the nitric acid. In the same 

 way copper converts sulphuric acid into the lower grade of oxidation 

 into sulphurous anhydride, S0 2 . In these cases the copper is oxidised 

 to copper oxide, which combines with the excess of acid taken, and 

 therefore forms a cupric salt, CuX 2 . Dilute nitric acid does not act 

 on copper at the ordinary temperature, but when heated it reacts 

 with great ease ; dilute sulphuric acid does not act on copper except 

 in presence of air. 



Both the oxides of copper, Cu 2 and CuO, are unacted on by- 

 air, and, as already mentioned, they both occur in nature. 6 bis How- 

 ever, in the majority of cases copper is obtained in the form oi ! 

 "cupric oxide and its salts and the copper compounds used indus- 

 trially generally belong to this type. This is due to the fact that the 

 cuprous compounds absorb oxygen from the air and pass into cupric 

 compounds. The cupric compounds may serve as the source for the 

 preparation of cuprous oxide, because many reducing agents are 

 capable of deoxidising the oxide into the suboxide. Organic sub- 

 stances are most generally employed for this purpose, and especially 

 saccharine substances, which are able, in the presence of alkalis, to 

 undergo oxidation at the expense of the oxygen of the cupric oxide, 

 and to give acids which combine with the alkali : 2CuO O = Cu 2 O, 

 In this case the deoxidation of the copper may be carried further and 

 metallic copper obtained, if only the reaction be aided by heat. Thus, 

 for example, a tine powder of metallic copper may be obtained by heat- 

 ing an ammoniacal solution of cupric oxide with caustic potash 

 and grape sugar. But if the reducing action of the saccharine 

 substance proceed in the presence of a sufficient quantity of alkali in 



6 bu Copper, besides the cuprous oxide, Cu 2 O, and cupric oxide, CuO, gives two khown 

 higher forms of oxidation, but they have scarcely been investigated, and even their 

 composition is not well known. Copper dioxide (CuO 2 , or CuO 2 ,H. 2 O, perhaps CuOH 2 O 2 ) 

 is obtained by the action of hydrogen peroxide on cupric hydroxide, when the green 

 colour of the latter is changed to yellow. It is very unstable, and is decomposed even 

 by boiling water, with the evolution of oxygen, whilst the action of acids gives cuprio 

 Baits, oxygen being also disengaged. A still higher copper peroxide is formed by heating 

 a mixture of caustic potash, nitre, and metallic copper to a red heat, and by dissolving 

 cupric hydroxide in solutions of the hypochlorites of the alkali metals. A slight heating 

 of the soluble salt formed is enough for it to be decomposed into oxygen and copper 

 dioxide, which is precipitated. Judging from Fremy's researches, the composition of the 

 copper-potassic compound should be KaCuO 4 . Perhaps this ia a compound of the 

 peroxides of potassium, K,O 2 , and of copper, Cu0 3 . 



