GAS PIPES AND FIXTURES 225 



when steam is passed through a bed of hot hard coal or 

 coke. The second method gives water gas. At present 

 nearly all illuminating gas is water gas "enriched" with 

 gases obtained by the charring of petroleum ; these make it 

 light-producing. 



As illuminating gas comes to the consumer, it burns 

 with a brilliant, yellow flame. If burned from a circular 

 opening, it is smoky; but if we force it through 

 a narrow slit (a "tip"), it is almost smokeless, 

 because the gas has a large surface in contact 

 with the air. 



254. Incandescent Mantles. To get a brighter light 

 than that of the ordinary gas flame, we may burn the 

 gas in a Bunsen burner, and put a "mantle" in its hot, 

 colorless flame. This is the principle of the Welsbach Fig - 21 - 



, -., ,, ,1 ,) i mi T i i e , i Mantle Light. 



and other mantle lamps. Ine light comes irom the A gas stream 

 incandescent (white hot) mantle (Fig. 210). 



The best mantles contain the oxides of the two rare 

 metals cerium and thorium. The mantle is first knitted Jeat es to white 

 out of "stockingette." This is then soaked in a solution 

 containing cerium and thorium salts. When the stockingette is dried, 

 the solid salts fill its pores. The mantle is now set on fire. The 

 organic part (the thread) burns up, and the salts in its pores are 

 decomposed. The oxides of the metals remain as a thin, fragile shell. 

 Other gases, such as natural gas and gasoline vapor, can be used with 

 incandescent mantles. 



255. Gas Pipes and Fixtures. Gas is stored by the 

 gas company in large tanks inverted in water. They are 

 loaded with weights, which, with the weight of the tanks, 

 cause the pressure of the gas. As gas is used up, the 

 tanks sink into the water; but they rise again when they 

 are refilled. 



