36 DURATION OF EXPLOSIVE REACTIONS. 



of the ground, which are propagated to a distance without great 

 local destruction, the pressures developed having been exerted 

 sufficiently slowly to give the rock or wall time to displace itself 

 very slightly " en masse." In this case the explosive substance 

 will have produced scarcely any useful effect. 



This question of the duration of reactions playing an essential 

 part in all questions relative to explosive substances, has led the 

 author to bring together here the principal considerations and 

 experiments to which it has given rise, experiments on which he 

 has been engaged for many years. 



2. The chemical transformation of the explosive substance 

 has therefore to be defined from the threefold point of view of 

 its origin, its duration, and its propagation. 



2. ORIGIN OF EEACTIONS. 



1. A reaction, once started, continues by itself, being propa- 

 gated either by simple progressive inflammation, or by almost 

 instantaneous detonation. 



2. In all cases relative to the usual explosive substances, to 

 develop the reaction requires preliminary work, 1 a sort of pre- 

 paration which is represented by the necessity of raising the 

 substance to a certain initial temperature, such as 315 for black 

 powder, 190 for mercury fulminate, etc. Indeed, if it were 

 otherwise, no explosive substance could be prepared beforehand 

 and stored in a magazine. 



But to what point are these notions applicable to the cases in 

 which the reaction results from a shock, a sudden pressure, or 

 any mechanical influence ? 



3. The author is of opinion that every explosive reaction 

 should be attributed to a preliminary heating, which is gradually 

 transmitted, directly or indirectly, raising successively all the 

 parts of the matter to the temperature of decomposition. 



Shock pressure, friction, or mechanical effects are only 

 efficacious by causing this preliminary heating, and sometimes 

 propagating it in virtue of the direct or alternative transforma- 

 tions of the energy into heat, and according to the various 

 mechanisms, to which reference will be made in 6. 



4. This being granted, let it be noted that the decomposition 

 of one and the same substance can take place at very unequal 

 temperatures and velocities; a substance slowly decomposable at 

 a certain temperature being able to exist at much higher tempe- 

 ratures, though during a gradually shortening interval. 



It is in this way that certain explosive substances are some- 

 times spontaneously decomposed with great slowness, from the 

 ordinary temperature, and only produce detonations if the 

 temperature be raised intentionally or by accident. 

 1 " Essai de M^canique Chimique," torn. ii. p. 6. 



