WHY A FLAME EMITS LIGHT. 217 



vincing. By comparing ethylene, CgH^, with acetylene, C2H2 (where 

 for equal consumption the same number of carbon atoms were pres- 

 ent), and also with coal-gas, it was seen that the luminous portion of 

 the acetylene, flame is not as hot as that of either ethylene or coal-gas, 

 while the illuminating powers of the flames were : acetylene, 240.0 can- 

 dle power, ethylene, 65.5 c.p. and coal-gas, 16.8 c.p. Evidently the 

 heat of combustion does not account for the incandescence of the car- 

 bon ; for if it did the cooler acetylene flame would give less light, while, 

 as a matter of fact, it gives twice as much as the ethylene and about 

 fourteen times as much light as the very much hotter coal-gas flame. 

 It was evident that our temperature measuring instruments do not 

 detect the heat of the carbon particles themselves. 



To see if luminosity be even partly due to the latent heat of acety- 

 lene, Lewes exploded that gas in a closed tube. This was done by 

 wrapping a bit of fulminate of mercury in tissue paper and suspend- 

 ing it by copper wires joined by platinum in contact with the fulminate, 

 and passing an electric current. There followed a brilliant flash of 

 light and a complete decomposition of the gas, and of the eudiometer as 



3 



J't'g.JZ 



well. Pieces of glass were coated with carbon, and the tissue paper was 

 not scorched except in a small hole where the explosion of the fulmi- 

 nate had burst through. This experiment showed the formation of 

 carbon, the emission of a brilliant light and the localization of the 

 heat liberated. But as the decomposition in a flame can hardly be as 

 rapid as in this experiment, and as hydrogen and oxygen also give a 

 feeble light when exploded, he sought to detect the rise in temperature 

 at the moment of decomposition when this is caused by heat. He 

 arranged a thermo-couple in a small tube so that only the turn of wires 

 was exposed, and after sweeping out the air passed a slow current of 

 acet3dene through the tube, the arrangement being as shown in Fig. 3. 

 The heat was raised throughout the tube at a rate of about 10° C. per 

 minute, and almost as soon as the temperature of area a passed 800° 

 C. it took a sudden leap to 1000° C, the gas burst into a lurid flame 

 and streams of carbon passed on through the tube. Although the tem- 

 perature of area h was made considerably higher than a the carbon 



