LIMESTONE 



121 



soda and a dilute acid. When the stream of carbon dioxide 

 is turned upon the burning object, it expels the air, and 

 as there is then no free oxygen, the fire is put out. Fire 

 extinguishers (Fig. 104) of various sorts con- 

 tain about the same materials as chemical 

 engines. 



132. Limestone. Limestone is found in 

 large deposits, usually as a gray solid. It is 

 composed of carbon, oxygen, and the metal 

 calcium. Its chemical name is calcium car- 

 bonate. Limestone is insoluble in water; but 



./.., .. , -I -I / /. ( 



it the water contains carbon dioxide (cf. 



12o), the limestone dissolves to some extent. 



When thus dissolved, it causes the temporary hardness of 



water (cf. 82) . Water containing limestone is ' ' broken " 



acid 6 .' 8 when 



these substances' 



react, carbon ai- 



oxide is formed. 



FIG. 105. 



Marengo Cave, Ind. The masses hanging from the ceiling and those rising from 

 the floor are both deposited by water that drips from the ceiling. 



