444 APPLIED SCIENCE 



rolled, hammered, or worked into sheets, it is used to a 

 great extent for roofing and sheathing vessels, and for mak- 

 ing cylindrical pipes and copper wire. When hammered or 

 worked in the cold, copper becomes brittle, but its toughness 

 is restored by heating it to 500 F. When heated to redness 

 it can be drawn apart and forged, but if overheated it be- 

 comes coated with black oxide of copper. In the ingot or 

 cast condition it contains much oxide and therefore is not so 

 strong as when it is rolled out into sheets. 



514. Chemical Properties. When a piece of copper is 

 heated in the air it combines slowly with the oxygen and two 

 oxides are formed: cuprous oxide (Cu 2 O) and cupric oxide 

 (CuO). The cuprous oxide is red and the cupric oxide is 

 black. In a moist atmosphere, the carbon dioxide of the air 

 combines with these oxides forming a green layer which 

 contains the hydroxide, Cu(OH) 2 , and the carbonate, 

 CuCO 3 verdigris. 



Copper is not affected by hydrochloric acid, but a weak, 

 cold or hot concentrated acid will quickly dissolve it. 



515. Properties of Aluminum. Aluminum resembles sil- 

 ver in its whiteness but is much lighter. The ore of this 

 metal is obtained from pure clay, a substance with which 

 man was familiar for ages without suspecting the treasure it 

 contained. The metal is not found in a pure state although 

 its compounds are common. It is now made on a large 

 scale by an electrolytic process which has greatly reduced 

 its cost. 



When an electric current is passed through cryolite (a 

 fluoride of aluminum and sodium found abundantly in Green- 

 land), the aluminum oxide which is added is decomposed and 



