flame, phenomenon associated with the chemical reaction of a gas that has been heated above its kindling temperature with some other gas, usually atmospheric oxygen (see combustion). The heat and light given off are characteristic of the specific chemical reaction (or reactions) going on; the luminosity of the flame is usually caused by solid particles of foreign matter present (naturally or artificially) in the burning gas and heated to incandescence; and the shape of the flame is commonly that of a hollow cone. The simple flame occurring when a single gas, such as hydrogen, burns in another gas, such as air, shows two areas, or zones: an inner, cone-shaped area consisting of unburned gas; and an outer area in which the chemical reaction (the combination of hydrogen and oxygen to form water) is taking place. Furthermore, the flame is nonluminous and therefore very hot, since the chemical energy is nearly all transformed into heat energy. This reaction is illustrated in the flame of the oxyhydrogen blowpipe. The flame of the oxyacetylene torch is also extremely hot. A decrease in light with an increase in heat is brought about in the Bunsen burner flame (a more complex flame) by mixing the combustible gas with air before it is ignited. Flames become more complex as the combustible gas increases in complexity, since an increasing number of chemical reactions are involved. Three zones, for example, are apparent in the Bunsen burner flame: an inner zone of unburned gas; a middle zone called the reduction zone or reducing flame, since there the supply of oxygen is deficient and the oxygen is therefore removed from an oxide placed in it; and an outer, or oxidizing, zone. The candle flame is extremely complex. Several zones can be observed: a nonluminous inner portion where the melted wax produces gases; a middle area where the gases are decomposed to hydrogen, which burns, and carbon, which is heated to incandescence; and an outer, hardly visible region in which combustion is complete (carbon dioxide and water being formed). Flames are colored by the introduction of various substances, a fact utilized in the flame test for the identification of certain metals.