[Fr., from I.] The illumination given by burning gas. Heat is produced by the chemical action which takes place during combustion, the temperature of the burning material being raised sufficiently high to give out light or to produce flame. The structure of flame can be most easily observed in a candle, lamp, or ordinary gas flame. When a gas flame is lit, it is seen that the part nearest to the burner is only feebly luminous. This consists of the gas which has just escaped from the pipe, and, though to some extent heated, has not begun to unite with the oxygen of the air. As the gas rises higher, it comes into contact with oxygen and unites with it. At first the hydrogen of the gas unites more rapidly with oxygen than the carbon, and in consequence a number of particles of carbon are set free, which, on becoming intensely heated, give rise to a bright flame. The illuminating power of the flame is due to the particles of glowing carbon, which, as they rise higher, are consumed on the border of the flame in contact with the atmosphere. In order to give out much light, a flame should contain solid matter ; but the most luminous flames are not necessarily the hottest.