ACTION OF GASES AND WATER AS TAMPING. 41 



retained the whole of their heat the pressure after some instants 

 would depend solely on the extent of the space. The pressure 

 would be greater the more limited the space; the maximum 

 pressure corresponding to the explosion of the substance in its 

 own volume, and the molecular rapidity of the reaction exerting 

 no influence. But this is an extreme limit, owing to the losses 

 of heat which the products of the explosion continually undergo 

 by contact, conduction, and radiation ; hence results a cooling, 

 which lowers the temperature, and therefore the pressure, as 

 well as the rapidity of the chemical reaction. The initial 

 pressure tends to approach this limit, the more rapid the powder, 

 the smaller the capacity enclosing the powder, and the more 

 resisting the walls of this capacity, which enables them to hold 

 the compressed gases during a longer period. 



7. The same will happen, not only when an explosive body is 

 placed in a fixed and resisting capacity, but also if it be placed 

 in a thin envelope, or under a stratum of water, or even in the 

 open air. In fact, when the duration of reaction decreases 

 beyond measure, the gases liberated develop pressures which 

 increase with an extreme rapidity, so rapidly, indeed, that the 

 surrounding bodies, solid, liquid, or even gaseous, have not time 

 to put themselves in motion in order to yield gradually to these 

 pressures ; these bodies, therefore, offer to the expansion of the 

 gases resistances comparable to those of a fixed wall. 



It is known that a film of water on the surface of nitrogen 

 chloride is sufficient to give rise to such effects. A drop of this 

 substance placed in a watch-glass may detonate without breaking 

 it, while if it be covered with a little water the glass is broken. 

 By operating on a slightly larger mass, even the plank upon 

 which the vessel is placed may be pierced under these conditions. 



The same result may sometimes be attained by increasing the 

 mass of the explosive substance; the gases first ignited not 

 having time to dissipate, are then acting as tamping. This effect 

 becomes greater and greater, as the temperature of the substance, 

 and therefore the rapidity of the reaction, increases. 



It is in this way that compressed dynamite and gun-cotton, 

 substances capable of being inflamed without danger by the aid 

 of an ignited body when operated upon in small quantities, have 

 sometimes caused terrible explosions, owing to the general 

 inflammation of a considerable mass. 



To sum up, the nearer the duration of the reaction approaches 

 being instantaneous, the nearer the initial pressure, even in an 

 open vessel, approaches the theoretical pressure, the latter being 

 calculated for the case of a decomposition effected in a constant 

 capacity entirely filled by the explosive substance. It is thus 

 that the extraordinary destructive effects produced by mercury 

 fulminate, nitroglycerin, or compressed gun-cotton can be 

 appreciated. 



