COPPER, SILVER, AND GOLD 409 



instance, the nitrite and carbonate) leave oxide of copper 8bis in the 

 form of friable black powder, after being ignited. If the ignition be 

 carried further, Cu 2 O may be formed from the CuO. 8 trl Anhydrous 

 cupric oxide is very easily dissolved in acids, forming cupric salts, CuX 2 . 

 They are analogous to the salts MgX 2 , ZnX ? , ]STiX 2 , FeX 2 , in many 

 respects. On adding potassium or ammonium hydroxide to a solution 

 of a cupric salt, it forms a gelatinous blue precipitate of the hydrated 

 oxide of "copper, CuH 2 2 , insoluble in water. The resultant precipitate 

 is redissolved by an excess of ammonia, and gives a very beautiful 

 azure blue solution, of so intense a colour that the presence of small 

 traces of cupric salts may be discovered by this means. 9 An excess of 



8 bit The oxide of copper obtained by igniting the nitrate is frequently used for. 

 organic analyses. It is hygroscopic and retains nitrogen (1*5 c.c. per gram) when the 

 Citrate is heated in vacuo (Richards and Rogers, 1893); 



8 trl Oxide o.f copper is also capable of dissociating when heated. Debray and 

 Joannis showed that it then disengages oxygen, whose maximum tension is constant 

 lor a given temperature, providing that fusion does not take place (the CuO then 

 dissolves in the molten Cu 2 O) ; that this loss of oxygen is followed by the formation of 

 fiuboxide, and that on cooling, the oxygen is again absorbed, forming CuO. 



9 Cupric oxide and many of its salts are able to give definite, although unstable, 

 compounds with ammonia. This faculty already shows itself in the fact that cupric 

 oxide, as well as the, salts of copper, dissolves in aqueous ammonia, and also in the fact 

 that salts of copper absorb ammonia gas. If ammonia be added to a solution of any 

 cupric salt, it first forms a precipitate of cupric hydroxide, which then dissolves in an 

 excess of ammonia. The solution thus formed, when evaporated or on the addition of 

 alcohol, frequently deposits crystals of salts containing both the elements of the salt of 

 copper taken and of ammonia. Several such compounds are generally formed. Thus 

 cupric chloride, CuCl 2 , according to Deherain, forms four compounds with ammonia 

 namely, with one, two, four, and six molecules of ammonia. Thus, for example, 

 if ammonia gas be passed into a boiling saturated solution of cupric chloride, on 

 cooling, small octahedral crystals of a blue colour separate out, containing 

 CuCl 2 ,2NH 3 ,H 2 O. At 150 this substance loses half the ammonia and- all the water 

 contained in it, leaving the compound CuCl 2 ,NH 3 . Nitrate of copper forms the com- 

 pound Cu(NO 3 ) 2 ,2NHj. This compound remains unchanged on evaporation. Dry 

 cupric sulphate absorbs ammonia gas, and gives-a compound containing five molecules of 

 ammonia to one of sulphate (Vol. I., p. 257, and Chapter XXII., Note 85). If this com- 

 pound is dissolved in aqueous ammonia, on evaporation it deposits a crystalline substance 

 containing CuS04,4NH 3 ,H 2 0. At 150 this substance loses the molecule of water and 

 one-fourth of its ammonia. On ignition all these compounds part with the remaining 

 ammonia in the form of an ammoniacal salt, so that the residue consists of cupric oxide. 

 Both the hydrated and anhydrous cupric oxide are soluble in aqueous ammonia. 



The solution obtained by the action of aqueous ammonia and. air on copper turnings 

 (Note 6) is remarkable for its faculty of dissolving cellulose^ which is insoluble in water, 

 dilute acids, and alkalis. Paper soaked in such a solution acquires the property of not 

 rotting, of being difficultly combustible, and ^aterproof, &c. It has therefore been 

 applied, especially in England, to many practical purposes for example, to the con- 

 struction of temporary buildings, for covering roofs, &c. The composition of the 

 substance held in solution is Cu(HO) 2 ,4NH 3 . 



If dry ammonia gas be passed over cupric oxide heated to 265, a portion of the oxide 

 of copper remains unaltered-, whilst the other portion gives copper nitride, the oxygen of 

 the eopper oxide combining with the hydrogen and forming water. The oxide of copper 

 which" remains unchanged is easily removed by washing the resultant product with 



