WHY A FLAME EMITS LIGHT 99 



cent, is lost in the lower half of the luminous part. In the 

 dark part there occurs a transformation of saturated into 

 unsaturated hydrocarbons, along with a general breaking 

 down of all to yield products less rich in hydrogen and the 

 oxides of carbon. At the point where luminosity just 

 begins, seventy to eighty per cent, of the unsaturated com- 

 pounds is acetylene, although less than one per cent, was 

 originally present. No acetylene could be found in the 

 flame when it was made non-luminous. 



By causing pure gases to pass through tubes heated to 

 known temperatures and analyzing the products formed, 

 Lewes studied the effects of heat upon both saturated and 

 unsaturated hydrocarbons. At 800 C. an unsaturated 

 compound, like ethylene, C 2 H 4 , breaks down into hydrogen 

 and the still more unsaturated acetylene, C 2 H 2 . At 1200 

 C. the very stable saturated hydrocarbons decompose into 

 acetylene and hydrogen, and the acetylene in turn decom- 

 poses into carbon and hydrogen. Even very dense hydro- 

 carbons decompose at 1200 C. These results strengthened 

 Lewes' conviction that under the baking action of the 

 flame walls in the lower portions acetylene is produced in 

 relatively large quantities and that this is the source of the 

 carbon. 



The question which immediately presented itself was, 

 Does there exist in an ordinary flame such conditions of 

 temperature as may bring about the formation of acetylene 

 from the very stable constituents of the illuminants? On 

 measuring the temperatures at various places the necessary 

 temperatures were found to exist. 



The work was complete and conclusive and forced a 

 general acceptance of the theory that acetylene is the 

 immediate source of the carbon. 



But a yet harder problem presented itself, What gives 

 rise to heat sufficient to make the carbon become incandes- 

 cent? a burning question certainly and one not easy to 

 answer. 



From the time of Davy to the year 1892 the only opinion 



