Instantaneous Pressures in the Explosion- Wave. 175 



pressure produced in this reaction is given as 25 atmospheres, 

 a number more in accordance with the first than the second 

 hypothesis. 



But the measurements made by MM. Berthelot and 

 Vieille* do not, we think, apply to the pressures in the 

 wave. They fired mixtures of gases in a bomb and 

 observed the movement of a piston working against a 

 spring in a tube attached to the bomb. From the accelera- 

 tion of the piston they calculated the pressure in the bomb. 

 The pressures so measured are called by Berthelot the 

 " effective pressures." Now, since the explosion-wave 

 travels faster than sound in the unburnt gas, the explosion- 

 wave is the first impulse which reaches the piston. It 

 follows that when the piston feels this impulse and begins 

 to answer to it, the explosion-wave has traversed the whole 

 of the gas and the true explosion is over. The piston receives 

 the blow of the wave and then the thrust of the expanding 

 gases, no doubt still combining, to a greater or less extent, 

 behind the wave front. In Berthelot's experiment, therefore, 

 the movement of the piston gives, in the main, the rate of 

 expansion of the heated gases after the explosion-wave has 

 passed through them : it does not give the instantaneous 

 pressure in the wave front itself. That higher pressures are 

 produced for a moment in the explosion of gases has been 

 proved by Mallard and Le Chatelier by the use of the 

 delicate indicator designed by Deprez. MM. Mallard and 

 Le Chatelier have also suggested a method of measuring 

 these pressures by the fracture of glass tubes of known 

 strength. This method we believe to give approximately 

 correct results : it depends on the principle that if a pressure 

 is produced in a glass tube greater than it can stand, the 

 glass will be broken although the pressure may only last 

 for a very small interval of time. 



* Ann. Chim. et Phys. [vi.] 4. p. 14 (1885). 



