ARTIFICIAL LIGHT 293 



Artificial or coal gas is made by heating coal in fur- 

 naces where air or oxygen cannot get to it. The gas comes 

 from the coal containing many impurities which are re- 

 moved by washing the gas, that is, by passing it 

 through water and other substances; the pure gas is 

 then collected in tanks for use. The by-products which 

 come from the production of coal gas are ammonia, coal 

 tar, carbolic acid, naphthalene, and anilin dye. Coke is 

 the residue left after the gas is driven out of the coal. 



Acetylene gas is made by allowing calcium carbide 

 crystals to fall continuously into a tank containing water. 

 The tank is air-tight and so arranged that the inside part 

 can move up and down according to the gas pressure. 

 The gas is distributed through pipes to the burners. In 

 acetylene gas lamps the water is permitted to drip into 

 the can containing the calcium carbide. The carbide is 

 made by fusing coal and lime together in an electric 

 furnace. Slaked lime is the residue left after acetylene 

 gas is made by the action of water on the carbide. 



QUESTIONS AND EXERCISES 



1. Give the history of the development of artificial lighting. 



2. How are oil and gas obtained for lighting? 



3. Have oil and gas always been used? 



4. Explain how a gas flame makes light. 



5. Explain how an electric lamp makes light. 



6. What are the substances now used for lighting? 



