152 ARTIFICIAL LIGHTING 



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

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

 and should be kept clean and free of clogging, in order to 

 insure complete combustion. So long as the supply of air 

 is sufficient, the flame burns with a dull blue color, but when 

 the supply falls below that needed for complete burning of 

 the carbon, the blue color disappears, and a yellow flame takes 

 its place, and with the yellow flame the deposition of soot is 

 inevitable. 



146. By-products of Coal Gas. Many important products 

 besides illuminating gas are obtained from the distillation of 

 soft coal. Ammonia is made from the liquids which collect in 

 the condensers ; anilin, the source of exquisite dyes, is made 

 from the thick, tarry distillate, and coke is the residue left in 

 the clay retorts. The coal tar yields not only anilin, but also 

 carbolic acid and naphthalene, both of which are commercially 

 valuable, the former as a widely used disinfectant, and the 

 latter as a popular moth preventive. 



From a ton of good gas-producing coal can be obtained 

 about 10,000 cubic feet of illuminating gas, and as by-prod- 

 ucts 6 pounds of ammonia, 12 gallons of coal tar, and 1300 

 pounds, of coke. 



147. Natural Gas. Animal and vegetable matter buried 

 in the depth of the earth sometimes undergoes natural distil- 

 lation, and as a result, gas is formed. The gas produced in 

 this way is called natural gas. It is a cheap source of illu- 

 mination, but is found in relatively few localities and only in 

 limited quantity. 



148. Acetylene. In 1892 it was discovered that lime and 

 coal fused together in the intense heat of the electric furnace 

 formed a crystalline, metallic-looking substance called calcium 

 carbide. As a result of that discovery, this substance was 

 soon made on a large scale and sold at a moderate price. 



