the
numerous carbon particles constantly thrown off by the burning gas,
and hence many carbon particles remain in the flame as glowing,
incandescent masses. That some unburned carbon is present in a flame
is shown by the fact that whenever a cold object is held in the flame,
it becomes "smoked" or covered with soot. If enough air were supplied
to the flame to burn up the carbon as fast as it was set free, there
would be no deposition of soot on objects held over the flame or in
it, because the carbon would be transformed into gaseous matter.

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
localit