Artificial Lighting 163 



In some plants a scrubber is used, the work of which is ex- 

 plained in the description of coal gas manufacture. 



Illuminating gas contains a large proportion of carbon 

 monoxid, which is odorless and poisonous. Water gas, a 

 mixture of hydrogen and carbon monoxid, is very dangerous 

 because of the impossibility of detecting it by its odor when it 

 escapes. For this reason water gas is enriched with odorous 

 gases from oils which also make it luminous. 



Coal gas. The manufacture of coal gas may well be studied 

 directly at a gas plant or by means of a model plant. The 

 diagram (Fig. 46) illustrates the chief parts of the plant, and 

 with the description of the processes may serve as a guide in 

 studying the actual plant. The general process is known as 

 destructive distillation. Coal, either bituminous or semi- 

 bituminous, is used. It cannot be distilled and condensed as 

 coal, but is broken down into simpler substances by the applica- 

 tion of heat. The distilled product is gas ; the by-products 

 coke, ammonia, and coal-tar turn out, however, to be more 

 valuable commercially than the gas. 



A ton of fairly good coal yields by destructive distillation 

 the following : 



Coke 1300 pounds 



Gas 10,000 cubic feet 



Coal-tar 115 gallons 



Ammonia liquor 18 gallons 



By-products. The coke is used in producing heat for 

 retorts, or sold for household heating. From the ammonia 

 liquor ammonia gas or ammonia water is obtained. The usual 

 method of making ammonia commercially is first to mix the 

 gas liquor with slaked lime, and then to boil the mixture so as 

 to drive off the ammonia gas. The gas is purified by being 

 combined in solution with dilute sulfuric acid. This solution 

 may be evaporated to form crystals of ammonium sulfate, or 

 it may be heated with slaked lime to drive off the purified 

 ammonia gas. For distribution the gas is either compressed 



