148 Mr. Harold B. Dixon on ^ 



chlorine the energy of the hydrochloric acid formed will be 

 communicated by collision to molecules of hydrogen and 

 chlorine ; these Jieated molecules, moving forwards, will meet 

 unheated molecules moving backwards, when combination 

 will occur between those of opposite kind. Heated hydro- 

 gen will thus combine with cool chlorine, and heated chlorine 

 with cool hydrogen. If we assume that an exceedingly thin 

 layer of gas may be heated nearly to the temperature of the 

 neighbouring layer by exchange of energies on impact of 

 the molecules, we may say that, on the average, the heat of 

 combustion of each molecule of hydrochloric acid formed is 

 communicated to a molecule of hydrogen or chlorine which 

 shares //with a molecule of the opposite kind ; each molecule 

 formed in turn will therefore have a temperature correspond- 

 ing to the heat of chemical combination plus half the 

 heat of a molecule previously formed. According to this 

 view the temperature reached by each successive layer 

 would increase until it was double that due to the chemical 

 change alone. The temperature of the explosion would 

 then remain constant, and the wave would advance at a 

 uniform rate as long as it met the same mixture of gases. 



If we accept the hypothesis that the explosion is propa- 

 gated by molecular collisions, we have just seen that the 

 movement of the products of combustion is communicated 

 to the unburnt molecules in front. It will, therefore, follow 

 that the rate of the advancing explosion will depend not 

 only on the rate of translation of the products of combustion, 

 but also on the rate of translation of the heated but yet 

 imcombined molecules. If the burnt molecules communicate 

 their rise of temperature without loss to the molecules 

 in front, no difference in the mean rate of motion will be 

 caused by this transference so long as the burnt and the 

 unburnt gases are of the same average density. But if a 

 change in density is produced by the combustion, the 

 average velocity of the molecules in the burnt layer will 



