172 Mr. E. Threlfall on the 



and is summed up by M. Berthelot somewhat as follows : — 

 The kinetic energy of the shock of the explosion (by the 

 detonator) is transformed into heat at the point struck ; the 

 temperature of this point is thus raised to the temperature of 

 explosion ; a new shock is produced which raises the tempera- 

 ture of the neighbouring portions to the same degree; they 

 then explode and the action is thus propagated with an ever- 

 increasing velocity. 



Many experiments tend to show the justness of this view. 

 To begin with, Abel found that almost any variety of effect 

 could be obtained by burning explosives under diminished 

 pressure. For the lower the pressure the more easily do the 

 products of decomposition escape, and carry with them the 

 energy due to their liberation. By this means the tempera- 

 ture of explosion is constantly kept down, and the chemical 

 character of the products modified in such a way that they 

 correspond to the temperature. In other words, the com- 

 pounds liberated are as a rule more complex than those which 

 would be set free at a higher temperature, and therefore the 

 energy run down is less. 



Again, it will come to the same thing, so far as the propa- 

 gation of an explosion is concerned, whether the products of 

 decomposition are facilitated in their escape by conducting 

 the experiment in a partial vacuum, or whether the decompo- 

 sition is itself so slow that the products are enabled to escape 

 without marked hindrance under ordinary pressure. Now 

 the resistance of the air to the escape of the products of com- 

 bustion will depend on the rate at which they are liberated. 

 And the shock given to neighbouring portions of the exp]o- 

 sive will be proportional to the pressure of the explosion 

 gases at these points ; and, therefore, ultimately to the re- 

 sistance of the air, and hence to some function of the velocity 

 of decomposition. But, in order to convert an explosion by 

 combustion into an explosion by detonation, what is required 

 is, that the temperature of any point shall be raised suffi- 

 ciently to determine its complete, as distinguished from its 

 incomplete, decomposition, The raising of the temperature 

 of any point, however, will depend on the violence of the 

 shock to which it is subjected ; and this, as before stated, 

 will be proportional to some function of the velocity of the 

 decomposition producing it If the necessary temperature is 

 anywhere attained, we shall have detonation thereafter; if 

 not, an explosion by combustion will result. It appears, 

 inereiore, that in order to produce a detonation, we require 

 the initial velocity of decomposition to rise above a certain 

 minimum value ; that there is in fact a " critical velocity " of 



