1 68 CALORIFIC POWER OF FUELS. 



FLAME AND FLAME TEMPERATURES. 



Whenever the temperature is sufficiently high to raise a 

 portion of the carbon, hydrogen, or other gaseous com- 

 bustible to incandescence, flame is produced. The tempera- 

 ture at which this phenomenon occurs varies with the sub- 

 stance burnt. Usually it requires a red heat or higher, but 

 in some cases a much lower temperature suffices : bor-methyl 

 B(CH 9 ) 3 is an example, the flame temperature of which is not 

 high enough to scorch the finger placed in it. It is not neces- 

 sary that the flame should have solid particles in it, as flame 

 is produced by hydrogen burning under pressure in oxygen ; 

 neither is incandescence alone sufficient, as the fire of pure 

 carbon, magnesium, or iron glows but does not flame. 

 Flame is hollow, the combustion occurring on the surface, 

 and this may be easily demonstrated, by drawing off some of 

 the interior unconsumed gases with a tube and burning them. 



Bunsen's researches led to the conclusion that the tem- 

 perature of burning carbonic oxide rapidly rose to 3000 C., 

 and remained stationary till one third of it was consumed ; 

 the temperature then fell to 2500 C., at which more burnt; 

 and finally fell to about 1200 C., which temperature was 

 maintained till all the remainder was consumed. Actually 

 the last temperature is soon reached in practice. Berthelot 

 confirms this, but is in doubt whether the loss of temperature 

 is due to dissociation or to change in specific heat. Some 

 hold that part of this loss of heat is caused by its absorption, 

 due to the production of incandescence and its accompanying 

 flame phenomena. A gas raised to incandescence gradually 

 manifests each increment of heat till that point is reached, 

 and beyond this no increase is noticed, all such further 

 increase being consumed by the flame production. 



The rate of propagation of flame varies with the pressure 

 and with the material burning. The most rapid rate with 

 coal gas is when it is mixed with 5 parts of air; with marsh 



