532 CONCLUSIONS. 



phenomenon at one point to reproduce themselves successively 

 at all points of the mass. This is what has been illustrated by 

 the works of artillerists on the velocity of the combustion of 

 ordinary powder, a velocity which is variable with the physical 

 structure of powders and their chemical composition. This 

 velocity varies exceedingly with the pressure ; gunpowder, for 

 instance, does not explode in a vacuum, because the heated 

 gases which combustion has caused, escape, and are dispersed 

 before having had time to communicate the heat to the adjacent 

 particles. 



Here considerations of an entirely novel character intervene. 



Formerly, it was thought it was sufficient to inflame an 

 explosive substance, no matter how, since the effects of the 

 ensuing explosion did not appear to depend on the initial pro- 

 cess of inflammation. But nitroglycerin and gun-cotton have 

 manifested a peculiar diversity in this respect. Thus, for 

 instance, according to the process employed in ignition, dynamite 

 can decompose quietly and flamelessly, or it may burn with a 

 flame, or again, it may give rise to explosion properly so called ; 

 this explosion may further be either moderated or accompanied 

 by shattering effects. Mercury fulminate used as a priming is 

 particularly apt to cause these latter effects ; it is the detonating 

 agent par excellence. 



11. It has been shown how thermo-dynamic theories and the 

 suitable analysis of the phenomena of shock will explain this 

 diversity ; the energy of the shock transforming itself into heat 

 at the point acted on, and raising the temperature of the parts 

 first struck, up to the degree of explosive decomposition, their 

 sudden decomposition produces a fresh shock more violent than 

 the first on the adjacent parts ; and this regular alternation of 

 shocks and of decompositions transmits the reaction from layer 

 to layer throughout the whole mass, developing a real explosive 

 wave, which progresses with a velocity incomparably greater 

 than that of simple inflammation. 



12. By this we see the all-importance of primings, hitherto 

 looked upon as simple igniting agents. Here also we note the dis- 

 tinction between progressive combustion and the almost instan- 

 taneous detonation of explosive substances, extreme phenomena 

 among which we observe a series of states and of intermediate 

 reactions, which explain the variety of the effects produced by 

 the same agent. In fact, there exists in chemistry a certain 

 number of endothermal combinations, that is to say, those 

 which are susceptible of liberating heat by their decomposition ; 

 these are acetylene, cyanogen, and arseniuretted hydrogen, etc. 

 Yet these gases do not detonate either by heating or by the 

 electric spark. The author has now shown that these same 

 gases do, on the contrary, detonate and resolve themselves into 

 elements, and with peculiar violence, under the influence of 



