It has been seen that flames have been prominent sources of artificial light; and although of low luminous efficiency, they still have much to commend them from the standpoints of portability, convenience, and subdivision.  The materials which have been burned for light, whether solid or liquid, are rich in carbon, and the solid particles of carbon by virtue of their incandescence are responsible for the brightness of a flame.  A jet of pure hydrogen gas will burn very hot but with so low a brightness as to be 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.