112 THE EXPLOSIVE WAVE. 



This result is also obtained in the experiments with the 

 oxyhydrogen mixture, in which the same velocity was found 

 either for the propagation of the flame (as attested by the 

 destruction of the solid fulminate interrupters) or for the propa- 

 gation of the pressure (as shown by the piston). The tracings 

 also show that the pressure attains its maximum instantly 

 upon the contact of the ignited layer with the layer immediately 

 in front of it. 



9. Several conditions contribute to the production of these 

 effects. In the first place, it is necessary that the mass ignited 

 at the commencement should not be too small, in order that 

 radiation and conduction may not be given time to deprive this 

 mass of an amount of heat, i.e. of energy, greater than that 

 which is indispensable for the propagation of the wave. In 

 fact, if the radius of the sphere ignited is equal to the thickness 

 of the radiating layer, the loss of heat is proportionately 

 greater than if the radiating layer is merely a fraction of this 

 radius. 



Moreover, when the number of molecules surrounding the 

 point first ignited is too small they may not contain the com- 

 bustive and the combustible elements in the exact ratio that 

 corresponds to the average composition of the mixture; this 

 would lower the temperature of this section, and consequently 

 the energy of the molecules. 



Another circumstance, no less important, is, that the ele- 

 mentary velocity of the chemical reactions, at the temperature 

 of the combustion, should be sufficiently great for the heat 

 given off in a given time to maintain the system at the point 

 required ; a condition which is all the more important when 

 the elementary velocity of the reactions increases rapidly with 

 the temperature. It can even be conceived that the explosive 

 wave is only propagated if its theoretical velocity (rate of 

 translation of the molecules) is below, or at the most equal to, 

 the elementary velocity of the reaction. 



10. Thus there is a limit in the condition that* corresponds 

 to the propagation of the explosive wave ; this is the regime of 

 detonation. 



But it is easy to conceive quite a different limit, in which 

 the excess of pressure of the ignited section upon the following 

 one tends to fall to zero, and consequently the excess of velocity 

 in the translation of the molecules, i.e. the excess of their 

 energy, or, what is the same thing, the excess of heat which 

 they contain, has the same tendency. In such a system the 

 heat will be almost entirely lost by radiation, conduction, the 

 contact of surrounding bodies and of inert gases, etc., with 

 the exception of the very small quantity that is required for 

 raising the adjacent portions to the temperature of combustion ; 

 this is the regime of ordinary combustion, to which the measure- 



