The Rate of Explosions in Gases. 147 



of the calculated with the observed numbers supports his 

 explanation of the phenomena. 



Again, the fact that each layer is fired by compression 

 involves the preliminary heating of that layer, and this 

 heating must be added to the heat developed by its burning. 

 The temperature of the burning layer must, therefore, be 

 greater than that obtained by dividing the heat developed 

 in the chemical change b}^ the specific heat of the products 

 of combustion. 



When a sound wave alone is transmitted through a 

 gas its velocity shows that each layer of gas forming the 

 wave-front is heated by compression, and there seems no 

 reason why this should not happen when the compression 

 is accompanied by a chemical change. 



To what extent is each layer heated before combustion ? 

 MM. Mallard and Le Chatelier* state that the explosion- 

 wave will be propagated when each layer is brought by 

 compression to its own temperature of inflammation. For 

 hydrogen and oxygen, they find this temperature to 

 be about 550°C., and calculate that a pressure of 30 

 atmospheres must be exerted upon the gas to raise it to 

 this point. This temperature may be regarded as the 

 loivest limit of the preliminary heating of each layer before 

 combustion. 



If we regard the transmission of the explosion to be 

 due to the collisions of the molecules, and assume that 

 molecules which are chemically inactive towards each other 

 act as elastic bodies when they come into collision, and 

 that molecules which combine chemically lose energy of 

 translation and gain energy of vibration, then it must 

 happen that unburnt molecules come into collision with 

 burnt molecules and take up their energy in the form of 

 motion. For instance, in the explosion of hydrogen and 



* Kecherches stir la Combustion des Melanges Gazeux Explosifs, p 88 — 91 



