230 ELEMENTARY GENERAL SCIENCE. CHAP. 



observe the change in the appearance of the flame. Hold in 

 the flame a piece of glass tubing, and observe what happens. 



It is thus found that closing the air supply at the base of the 

 burner greatly increases the luminosity of the flame, while also 

 soot (or carbon) is deposited on a glass rod held in the luminous 

 flame. The increased luminosity appears, therefore, to be due 

 to the presence of solid carbon, which becomes white hot or 

 incandescent, owing to the high temperature of the flame. In 

 general, also, when solids are present among the products of 

 combustion the flame becomes intensely luminous, owing to the 

 incandescence of the solid, whereas if the products are entirely 

 gaseous the flame is as a rule not very luminous. Solids raised 

 to a white heat are largely used for illuminating purposes. The 

 incandescent electric lamps consist of a filament of carbon in a 

 vacuous globe raised to a white heat and made incandescent by 

 means of the electric current. It is important to notice that 

 here there is no burning or combustion, since no oxygen is 

 present. The limelight is produced by the incandescence of a 

 block of hard lime, which is made white hot by the flame obtained 

 by burning a mixture of coal gas and oxygen. In the incandescent 

 gas light a network is formed of some white incombustible oxide 

 (the mantle), and this is heated by the flame of what is practically 

 a Bunsen burner, and so made white hot. It is similarly im- 

 portant to notice that in the last two cases the lime and mantle 

 do not burn, but are really heated by the burning gases. 



It must be emphasised, however, that although the presence 

 of incandescent solid particles is a frequent and important cause 

 of luminosity of flames, yet it is not the sole cause, as some 

 flames are intensely luminous, although 110 solids are present. 

 Many factors, indeed, influence the luminosity, as, for example, 

 the temperature and the density of the gases. 



Oxidising and Reducing Flame. It has been already seen 

 that in the interior of the flame hot combustible gases are 

 present which are capable of burning, i.e., of combining with 

 oxygen, and which can thus abstract oxygen from oxides and 

 other oxygen compounds. It is said to be the reducing part of 

 the flame. At the top apex of the flame, however, no com- 

 bustible gases are present, as all have been burnt, but there is 

 present a quantity of strongly heated air which can readily give 

 up oxygen. This part may be hence said to be the oxidising 

 part of the flame. 



