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XC1V. Intra-Molecular Energy during Combustion. By 

 W. T. David, M.A., D.Sc, Professor of Engineering, 

 University College, Cardiff^. 



QOMB early experiments of mine with mixtures o£ coal- 



gas and air ignited by means of a spark in a vessel of 

 constant volume showed that the rate at which radiation is 

 emitted is greater during the explosion period (when the 

 bulk of combustion takes place) than at the moment of 

 maximum temperature (when about 90 per cent, of the coal- 

 gas has been completely burnt). These experiments were 

 interpreted as supporting the view that the energy of com- 

 bustion first passes (either wholly or in a large measure) into 

 the form of intra-molecular energy of the freshly formed 

 molecules of C0 2 and water- vapour, and that the excess of 

 such energy over and above an equilibrium value dependent 

 mainly upon gas temperature is rapidly transformed during 

 molecular collisions into translational energy "|\ 



Later experiments on the analysis of the radiation emitted 

 during the explosion of mixtures of various strengths seemed 

 to indicate that the intra-molecular energy acquired by the 

 freshly formed molecules during combustion is distributed 

 over their internal degrees of freedom in a manner very 

 much depen lent upon the degree of violence with which 

 combustion takes place. In a previous paper % it was sug- 

 gested tentatively that when combustion is gentle the intra- 

 molecular energy acquired by these molecules is concentrated 

 in the rotational degrees of freedom and in such very low 

 frequency vibrations as the molecules may be capable of 

 executing, but as the combustion increases in vigour the 

 higher frequency vibrations begin to share in this energy § . 



1 stated in the paper referred to that such a theory would 

 serve to explain many of the phenomena of the pre-pressure 

 and explosion periods in coal-gas and air explosions, and 

 in this paper these phenomena are considered briefly in the 

 light of the theory. 



Phenomena of the pre-pressure period.-— During the pre- 

 pressure period, which is the interval elapsing between the 

 moment when the igniting spark passes and that when the 

 pressure begins to rise ||, a considerable volume of gentle 



* Communicated by the Author. 



t Phil. Trans. A. vol. 211. p. 410. 



t Phil. Mag. Jan. 1920, p. 95. 



§ Only those vibrations coi responding to infra-red radiation are 

 referred to. 



|| The pre-pressure period is not shown in fig. 1. The zero of time is 

 taken to be the moment when the pressure begins to rise. 



