404 PRINCIPLES OF CHEMISTRY 



according to the temperature and quantity of air supplied. Tn an 

 at the ordinary temperature, copper as everyone knows becomes 

 coated with a brown layer of oxides or a green coating of basic salts, 

 due to the action of the damp air containing carbonic acid. If this 

 action continue for a prolonged time, the copper is covered with a thick 

 coating of basic carbonate, or the so-called verdigris (the aerugo nobilis 

 of ancient statues). This is due to the fact that copper, although 

 scarcely capable of oxidising by itself, 5 in the presence of water and 

 acids even very feeble acids, like carbonic acid absorbs oxygenfrom 

 the air and forms salts, which is a very characteristic property of it (and 

 of lead). 6 Copper does not decompose water t and therefore does not disen- 



* Schiitzenberger showed that when the basic carbdnate of copper is decomposed by 

 an electric current it gives, besides the ordinary copper, an allotropic form which grows 

 on the negative platinum electrode, if its surface be smaller than that of the positive 

 copper electrode, in the form of brittle crystalline growths of sp. gr. 8'1. It differs from 

 ordinary copper by giving not nitric, oxide but nitrous oxide when treated with nitric 

 acid, and in being very easily oxidised in air, and coated with red shades of colour. It 

 is possible that this is copper hydride, or copper which has occluded hydrogen. Spring 

 (1892) observed that copper reduced from the oxide by hydrogen at the lowest possible 

 temperature was pulverulent, while that reduced from CuClj at a somewhat high tem- 

 perature appeared in bright crystals. The same difference occurs with many other 

 metals, and is probably partly due to the volatility of the metallic chlorides. 



6 This is taken advantage of in practice ; for instance, by pouring dilute acids ovei 

 copper turnings on revolving tables in the preparation of copper salts, such as verdigris, 

 or the basic acetate 2C 4 H 6 CuO 4 ,C uH 2 O 2 ,5H2O, which is so much used as an oil paint (i.e. 

 with boiled oil). The capacity of copper for absorbing oxygen in the presence of acids 

 is so great that it is possible by this means (by taking, for example, thin copper shavings 

 moistened with sulphuric acid) to take up all the oxygen from a given volume of air, and 

 this is even employed for the analysis of air. 



The combination of copper with oxygen is not only aided by acids but also by alkalis, 

 although cupric oxide does not appear to have an acid character. Alkalis do hot act jon 

 copper except in the presence of air, wheja they produce cupric oxide, which does not 

 appear to combine with such alkalis as caustic potash or soda. But the action of 

 ammonia is particularly distinct (Chapter V., Note 2). In the action of a solution of 

 ammonia not only is oxygen absorbed by the copper, but it also acts on the ammonia, 

 and a definite quantity of ammonia is always acted on simultaneously with the passage 

 of the copper into solution. The ammonia is then converted into nitrous acid, according 

 to the reaction : NH 3 + O 3 = NH0 2 + H-jO, and the nitrous acid thus formed passes into the 

 state of ammonium nitrite, NH 4 NO^. In this manner three equivalents of oxygen are 

 expended on the oxidation of the ammonia, and six equivalents of oxygen pass over to 

 the copper, forming six atoms of cupric oxide. The latter does not remain in the state 

 of oxide, but combines with the ammonia. 



A strong solution of common salt does not act on copper, but a dilute solution oi tUe 

 salt corrodes copper, converting it into oxychloride that is, in the presence of air. 

 This action of salt water is evident in those cases where the bottoms of ships are coated 

 .with sheet copper. From what has been said above it will be evident that copper vessels 

 should 'not be employed in the preparation of food, because this contains salts and acids 

 which act on copper in the presence of air, and give copper salts, which are poisonous, 

 and therefore the food prepared in untinned copper vessels may be poisonous. Hence 

 tinned vessels are employed for this purpose that is, copper vessels coated with a thin 

 layer of tin, on which acid and saline solutions do not act. 



