Unburned carbon would be objectionable in cooking stoves where utensils are constantly in contact with the flame, and for this reason cooking stoves are provided with an arrangement by means of which additional air is supplied to the burning gas in quantities adequate to insure complete combustion of the rapidly formed carbon particles.  An opening is made in the tube through which gas passes to the burner, 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-products 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 distillation, and as a result gas is formed.  The gas produced in this way is called natural gas.  It is a cheap source of illumination, 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.  The cheapness of calcium carbide has made it possible for the isolated farmhouse to discard oil lamps and to have a private gas system.  When the hard, gray crystals of calcium carbide are put in water, they give off acetylene, a colorless gas which burns with a brilliant white flame.  If bits of calcium carbide are dropped into a test tube containing water, bubbles of gas will be seen to form and escape into the air, and the escaping gas may be ignited by a burning match held near the mouth of the test tube.  When chemical action between the water and carbide has ceased, and gas bubbles have stopped forming, slaked lime is all that is left of the dark gray crystals which were put into the water.