THE ATMOSPHERE 



229 



order to plainly see that combustion and rusting are essentially 

 similar actions, inasmuch as both are the formation of oxides, 

 the only difference being that in burning this is attended with 

 the production of light. How is this light brought about ? It is 

 owing to the heat consequent on the chemical union. When two 

 substances unite to form a new compound, heat is generally 

 evolved, and this may be sufficient to raise the substances and 

 the products of the combustion to such a temperature that they 

 become luminous. If gases are present among the products of 

 combustion they may become luminous and pass away as flames, 

 which hence consist of gases, produced by the combustion, raised 

 to such a high temperature that they become self-luminous. 



If a jet of combustible gas be allowed to burn in air, it is clear 

 that combustion can only occur where the gas and air are in 

 contact, i.e., on the outside of a jet, hence flames are generally 



FIG. 116. Hollow structure of a 

 Flame. 



FIG. 117. Bunsen 

 Burner. 



hollow, as may be seen by placing a piece of wire gauze over the 

 flame of a candle or Bunsen burner (Fig. 116). 



EXPT. 232. Place one end of a piece of glass tubing into the 

 middle of a Bunsen burner flame, or a candle flame. Slope the 

 tubing upwards away from the flame, and apply a light. 



It will be found that the interior of the flame consists of 

 unburnt gas capable of combustion, which, from what has been, 

 previously said, is what we should expect. 



EXPT. 233. Light a Bunsen burner and examine the flame. 

 Hold in it for a short time a piece of glass tubing. Now close 

 the air holes, d, at the base of the burner (Fig. 117) and 



