176 



PRINCIPLES OF CHEMISTRY 



brilliancy, according to whether solid incandescent particles occur in 

 the combustible gas or vapour, or not. Incandescent gases and 

 vapours emit but little light by themselves, and therefore give a paler 



flame. 43 If a flame does not 

 contain solid particles it is 

 transparent, pale, and emits 

 but little light. 44 The flames 

 of burning alcohol, sulphur, 

 and hydrogen are of this kind. 

 A pale flame may be rendered 

 luminous by placing fine par- 

 ticles of solid matter in it. 

 Thus, if a very fine platinum 

 wire be placed in the pale 

 flame of burning alcohol or, 

 better still, of hydrogen then 

 the flame emits a bright light. 

 This is still better seen by sift- 

 ing the powder of an incom- 

 bustible substance, such as 

 fine sand, into the flame, or 

 by placing a bunch of asbestos 

 threads in it. Every brilliant 

 flame always contains some 

 kind of solid particles, or at least some very dense vapour. The flame 

 of sodium burning in oxygen has a brilliant yellow colour, from the 

 presence of particles of solid sodium oxide. The flame of magnesium 

 is brilliant from the fact that in burning it forms solid magnesia, which 

 becomes white hot, and similarly the brilliancy of the Drummond light 

 is due to the heat of the flame raising the solid non-volatile lime to a 

 state of incandescence. The flames of a candle, wood, and similar sub- 

 stances are brilliant, because they contain particles of charcoal or soot. 

 It is not the flame itself which is luminous, but the incandescent soot 

 it contains. These particles of charcoal which occur in flames may be 

 easily observed by introducing a cold object, like a knife, into the 



Fw. 35. Faraday's experiment for investigating the 

 different parts of a caudle flame. 



43 All transparent substances which transmit light with great ease (that is, which 

 absorb but little light) are but little luminous when heated ; so also substances which 

 absorb but few heat rays, when heated transmit few rays of heat. 



44 There is, however, no doubt but that very heavy dense vapours or gases under 

 pressure (according to the experiments of Frankland) are luminous when heated, be- 

 cause, as they become denser they approach a liquid or solid state. Thus detonating 

 gas when exploded under pressure is brightly luminous. 



