CARBON AND THE OXIDES OF CARBON 



333 



(SiC, silicon carbide), manufactured in large quantities at Niag- 

 ara Falls (Acheson's process). The coke and sand (silicon dioxide 

 SiO 2 ) are piled between the ter- 

 minals, and the resistance of 

 the former causes the produc- 

 tion of the heat (Fig. 87) : 



3C + Si0 2 - SiC + 2CO t . 



FIG. 87 



Here the carbon reduces the 

 oxide, and combines with the 

 element (Si) as well. The 

 product (SiC) is exceedingly hard, and, after pulverization and 

 mixing with other materials, is moulded into grinding wheels. 



In the other type of furnace the air between the terminals 

 furnishes the resistance, and the arc (a discharge carried by the 

 badly conducting air and carbon vapor) furnishes the heat. 



The arc is used in making calcium carbide (CaC 2 ), by heating a 

 mixture of lime (CaO) and coke : 



Cold water acts vigorously with calcium carbide, giving acetylene 

 gas C2H 2 (see p. 351) and calcium hydroxide (slaked lime): 

 CaC 2 + 2H 2 -4 Ca(OH) 2 + C 2 H 2 1 . 



CARBON DIOXIDE 



Occurrence. Carbon dioxide is found in nature issuing from 

 the ground, especially in volcanic neighborhoods, and dissolved 

 in effervescing natural waters, such as Saratoga and Vichy. It is 

 found in the air (3.5 liters in every 10,000 liters of air) and in the 

 breath (37 liters per 1000 liters). 



Preparation. 1. Carbon dioxide is most easily prepared in 

 the laboratory by the action of an add such as hydrochloric acid 

 upon a natural carbonate like calcium carbonate (marble or lime- 

 stone). The action occurs in two stages. The first is a double 



