barely visible. If solid particles are injected into
the flame, much more light usually will be emitted. A gas-burner of the
Bunsen type, in which complete combustion is obtained by mixing air in
proper proportions with the gas, gives a hot flame which is of a pale
blue color. Upon the closing of the orifice through which air is
admitted, the flame becomes bright and smoky. The flame is now less hot,
as indicated by the presence of smoke or carbon particles, and
combustion is not complete. However, it is brighter because the solid
particles of carbon in passing upward through the flame become heated to
temperatures at which they glow and each becomes a miniature source of
light.

A close observer will notice that the flame from a match, a candle, or a
gas-jet, is not uniformly bright. The reader may verify this by lighting
a match and observing the flame. There is always a bluish or darker
portion near the bottom. In this less luminous part the air is combining
with the hydrogen of the hydrocarbon which is being vaporized and
disintegrated. Even the flame of a candle or of a burning splinter is a
miniature gas-plant, for the solid or liquid hydrocarbons are vaporized
before being burned. Owing to the incoming colder air at this point, the
flame is not hot enough for complete combustion. The unburned carbon
particles rise in its draft and become heated to incandescence, thus
accounting for the brighter portion. In cases of complete combustion
they are eventually oxidized into carbon dioxide before they are able to
escape. If a piece of metal be held in the flame, it immediately becomes
covered with soot or carbon, because it has reduced the temperature
below the point at which the chemical reaction--the uniting of carbon
with oxygen--will continue. An ordinary flat gas-flame of the
"bats-wing" type may vary in temperature in its central portion from
300 deg.F. at the bottom to about 3000 deg.F. at the top. The central portion
lies between two hotter layers in which the vertical variation is not so
great. The brightness of the upper portion is due to incandescent carbon
formed in the lower part.

When scientists learned by exploring flames that brightness was due to
the radiation of light by incandescent solid matter, the way was open
for many experiments. In the early days of gas-lighting investigations
were made to determine the relation of illuminating value to the
chemical constitution of the gas. The results combine