When gases are burned a light is given off. This light is called a flame. Flame is due to the combination of a gas with the oxygen of the air. A flame may be luminous, as in the case of an ordinary gas light, or it may be non-luminous, as in the case of the blue flame of a gas-burner. The luminosity of a flame is due to the glowing of small particles of carbon. A yellow flame is caused by incomplete combustion.