376 ME. J. E. PETAVEL ON THE PKESSURE OF EXPLOSIONS. 



towards the zero of time and diameter. We may, therefore, conclude that the 

 combustion of finely divided cordite is nearly instantaneous. Under such conditions 

 the result of an explosion would be very destructive, and it is possible that some 

 abnormal effects which have on certain occasions been observed may be due to the 

 pulverisation of the explosive at any early stage of the combustion. 



However rapid an explosion may be, it remains, in principle, very distinct from a 

 detonation. In an explosion the combustion is propagated from layer to layer without 

 discontinuity. In a detonation the chemical reaction is practically instantaneous and 

 simultaneous throughout the entire mass. The determining cause is, in this case, a 

 compression wave of sufficient intensity to raise the material to its temperature of 

 ignition. 



Let us take for the sake of illustration a numerical example, although the values 

 employed can only be rough estimations, and suppose a sphere of cordite 1 centim. in 

 diameter under a gravimetric density of O'l. If this were ignited in the ordinary 

 way, the combustion would travel towards the centre of the sphere at an average rate 

 of 8 centims. per second and the maximum pressure would therefore be reached in 

 0'063 second. If, on the other hand, the material were to detonate, the detonation 

 wave would travel through the mass at a speed of something like 800,000 centims. 

 per second,* and the total time occupied would be one hundred thousand times 

 less. 



In an explosion we have usually to deal with pressures which may be considered as 

 statical as far as their action is concerned ; in a detonation with a dynamical pressure 

 or impact. The impact of the products of combustion travelling with enormous 

 velocity may correspond in effect to an instantaneous pressure five or ten times 

 greater than the normal pressure calculated from the composition of the explosive and 

 its heat of reaction. 



A typical case of this kind occurred when working with a compressed mixture of 

 coal gas and oxygen. The total pressure of the explosion should have been some 4 or 

 5 tons per square inch. The mixture, however, detonated, and the solid steel piston 

 of the recorder, though encased in a steel cylinder over 2 inches thick, was expanded 

 outwards like the head of a rivet, t It is not easy to estimate exactly the statical 

 pressure required to produce a corresponding effect, but it cannot be less than 25 tons 

 per square inch. 



To return now to the work on cordite, the results obtained with one of the smallest 

 diameters in use are shown in fig. 14. It will be seen that, though the time occupied by 

 the combustion is small, amounting to less than O'OOS of a second, the shape of the 



* ABLE found that the rate of detonation of a train of dynamite or guncotton was about 608,000 

 centims. per second. See also SEBERT, BERTHELOT and METTEGANG. The latter (' Ber. 5. Int. Kong. Ang. 

 Chem., Berlin, 1903,' vol. II., p. 322) gives 700,000 centims. per second as the detonation rate of dynamite. 



t A similar effect is recorded hy NOBLE ('Proc. R. I.,' 1900), as having been produced on the copper of 

 a crusher gauge by a charge of lyddite. 



