384 



SMITH'S INTERMEDIATE CHEMISTRY 



Limestone is used for building and in road-making. Much of it 

 is employed in making quicklime, cement (p. 471), and glass 

 (p. 361), and as a flux in metallurgical operations (see p. 486). 

 Marble, often variegated by the presence of impurities, is used 

 in building and sculpture. 



FIG. 93 FIG. 94 



As we have seen (p. 333), calcium carbonate reacts with adds to 

 give carbonic acid: 



CaC0 3 + 2HC1 -> CaCl 2 + H 2 C0 3 -* H 2 O + C0 2 1 . 

 When heated, all forms of calcium carbonate give off carbon 

 dioxide, and leave calcium oxide : 



CaC0 3 <=CaO 



CABBOH 



""*" DIOXIOf 



Calcium Oxide CaO, Manufacture. The manufacture of 



calcium oxide or quicklime (i.e. 

 live lime) is one of the most ancient 

 chemical industries. The lime- 

 stone is thrown into a kiln lined 

 with brickwork (Fig. 95). The 

 -flames and heated gases from the 

 fire pass through the limestone and 

 the carbon dioxide is liberated and 

 carried off by the draft. When 

 this gas is to be used, as in the 

 Solvay process or in the refining 



of sugar, coke (smokeless) is chosen 



FIG. 95 j.i f i TTTI ' , 



as the fuel. When no use is to be 



made of the escaping gas, coal may be employed. 

 The use of as low a temperature as possible is important. A 



