102 TH^'PRODTTCTrO'N AND USE OF HEAT 



heated. The hydrocarbons at the same time are broken down 

 into simpler compounds. The gas also contains hydrogen 

 and carbon monoxide. Because of the hydrocarbons in coal 

 gas it burns with a luminous flame. The manufacture of 

 coal gas has gradually declined of late owing partly to the 

 fact that but few kinds of coal are suitable for use in making 

 it and these are becoming more expensive, and partly to the 

 decrease in cost of electricity. 



116. Water-Gas. WATER-GAS has come to replace coal 

 gas in many cities. When steam is passed over red-hot 

 carbon, the former is decomposed into hydrogen and oxygen. 



6A8 



GENERATOR CARBUfiCTCR. SUPERHEATER 



FIG. 81. Water-gas apparatus. 



The carbon then unites with the oxygen from the steam to 

 form carbon monoxide or carbon dioxide depending upon 

 conditions, while the hydrogen from the water is set free. 

 By proper control of conditions, carbon monoxide rather than 

 carbon dioxide may be formed, which, with the hydrogen set 

 free, makes a mixture of highly combustible gases. The 

 mixture is called WATER-GAS. During the chemical reaction 

 of the steam and the hot carbon, heat is absorbed, and conse- 



