1895.] in the Flame of Hydrocarbon Gases. 465 



tube 2'5 inm. in diameter was passed up through the broad tube to 

 the apex of the inner zone, and a slow stream of cyanogen was 

 admitted, with the result that the flame at once became luminous, 

 and on surrounding the hydrogen flame with an atmosphere of 

 oxygen to increase the temperature, the luminosity was considerably 

 increased. 



This experiment at once explains the cause of the non-luminosity 

 of the cyanogen flame, and shows that it is purely a question of 

 temperature, and the probabilities are that, burnt in a flame which 

 gave sufficient heat to rapidly decompose it, nearly as high an 

 illuminating value as that of acetylene would be obtained. 



I think the explanation of the apparent anomaly of the cyanogen 

 flame having a higher temperature than the acetylene and ethylene 

 flames, is to be found in the fact that the molecules of cyanogen 

 are consumed without previous decomposition, so that the heat 

 absorbed during the formation of the cyanogen is added to the 

 heat of combustion, and raises the average temperature of the flame, 

 whereas with acetylene the instantaneous decomposition of the 

 molecule before combustion confines the heat evolved to the liberated 

 products, and the average temperature of the flame is but little more 

 than the heat of combustion. 



If the luminosity of a hydrocarbon flame is principally due to the 

 localisation, during intensely rapid decomposition, of the heat of 

 formation in the products, the illuminating values of such hydro- 

 carbon gases as contain two atoms of carbon in the molecule should 

 bear a simple ratio to their heat of formation. The gaseous hydro- 

 carbons are 



Heat formation at 

 Hydrocarbon. Composition. constant pressure. 



Ethane C 2 H 6 +25670 



Ethylene C 2 H 4 - 8000 



Acetylene C 2 H 2 -47770 



and although they may undergo many changes in the flame, they will 

 all ultimately be reduced to carbon and hydrogen again before the full 

 luminosity of the flame is developed. 



When the acetylene into which these hydrocarbons is converted 

 by heat is decomposed, the action takes place with such enormous 

 rapidity that one would expect the heat evolved to simply divide itself 

 amongst the liberated atoms, so that the question of specific heat at 

 high temperatures may be omitted. 



With exothermic compounds like ethane, considerable heat will 

 have to be developed by its own combustion before it is converted 

 into the acetylene, which, by its decomposition, endows the flame 

 with luminosity, and if we take the ethane and call its light pro- 

 ducing energy 1, we can then obtain a ratio of such energy for the 



VOL. LVII. 2 L 



