GAS FOR COOKING 151 



ridding it of impurities ; and finally as illuminating gas it 

 makes its way into a huge gas holder from which it is dis- 

 tributed through underground service pipes to the buildings 

 where it is burned for light or heat. 



In practice the gas holder is constructed with a sliding top, 

 which rises as gas enters by the supply pipes, and falls when 

 gas leaves through the service pipes. By this arrangement 

 the pressure of the gas within the tank is kept constant, and 

 the flow through the service pipes remains uniform at all times. 

 The quantity of illuminating gas manufactured in the United 

 States is enormous, amounting to more than 68,000,000,000 

 cubic feet per year. 



145. Gas for Cooking. If a cold object is held in the bright 

 flame of an ordinary gas jet, it becomes covered with soot, or 

 particles of unburned carbon. Although the flame is sur- 

 rounded by air, the central portion of it does not receive suf- 

 ficient oxygen to burn up the numerous carbon particles con- 

 stantly thrown off by the burning gas, and hence many carbon 

 particles remain in the flame as glowing, incandescent masses. 

 That some unburned carbon is present in a flame is shown by 

 the fact that whenever a cold object is held in the flame, 

 it becomes " smoked " or covered with soot. If enough air 

 were supplied to the flame to burn up the carbon as fast as 

 it was set free, there would be no deposition of soot on ob- 

 jects held over the flame or in it, because the carbon would 

 be transformed into gaseous matter. 



Unburned carbon would be objectionable in cooking stoves 

 where utensils are constantly in contact with the flame, and 

 for this reason cooking stoves are provided with an arrange- 

 ment by means of which additional air is supplied to the 

 burning gas in quantities adequate to insure complete com- 

 bustion of the rapidly formed carbon particles. An opening 

 is made in the tube through which gas passes to the burner, 



