GAS FOR COOKING 127 



tain substances in it volatilize and pass through an exit pipe 

 into a trough which contains water, and is called the hydraulic 

 main. Some of the tarry vapors which distill over from the coal 

 condense in the hydraulic main as a thick tarry liquid. Other 

 gases dissolve in the water. Some of the vapors do not con- 

 dense in the water, but bubble through it into other receptacles. 

 The gases that bubble through the hydraulic main, uncon- 

 densed and undissolved, pass onward to a series of coils or tanks, 

 where they are cooled and where as a result of cooling they con- 

 dense and form valuable commercial products. Some of the 

 gas does not condense at all. This passes from the condensers 

 into " scrubbers " and "purifiers," where it is freed of objec- 

 tionable material. It is then ready to be used for cooking 

 and lighting. It is stored in huge gas holders from which it is 

 distributed through underground pipes to the buildings where 

 it is burned for light or heat. 



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 surrounded by 

 air, the central portion of it does not receive sufficient oxygen 

 to burn up the numerous carbon particles constantly thrown 

 off by the burning gas, and many carbon particles remain in 

 the flame as glowing, incandescent masses. If enough air were 

 supplied to the flame to burn up the carbon as fast as it is set 

 free, there would be no deposit of soot. 



Unburned carbon would be objectionable in cooking stoves, 

 where utensils are constantly in contact with the flame. For 

 this reason cooking stoves are provided with an arrangement 

 by means of which enough air is supplied to the burning gas to 

 insure complete combustion of the carbon particles. An open- 

 ing is made in the tube through which gas passes to the 

 burner, and as the gas moves past this opening, it carries with 

 it a draft of air. These openings are visible on all gas stoves, 



