78 EXPLOSIONS BY INFLUENCE. 



of a shock resulting from enormous and sudden pressures 

 produced by nitroglycerin or gun-cotton, the energy of which 

 shock is transformed into heat in the explosive substance (see 

 pp. 36, 57). 



2. In an extremely rapid reaction, the pressure may approach 

 the limit corresponding to the matter exploding in its own 

 volume; and the disturbance due to the sudden development 

 of pressures, nearly theoretical, may propagate itself either by 

 the mediation of the ground and of the supports, or through 

 the air itself, when projected en masse, as has been shown by 

 the explosions of certain powder mills, gun-cotton magazines, 

 and also by some of the experiments made with dynamite and 

 compressed gun-cotton. The intensity of the shock propagated 

 either by a column of air or by a liquid or solid mass, varies 

 according to the nature of the explosive body and its mode of 

 inflammation ; it is more violent the shorter the duration of the 

 chemical reaction and the more gas there is developed ; that is 

 to say, a stronger initial pressure and a greater heat, or, in 

 other words, greater work for an equal weight of explosive 

 substance (see pp. 40, 41). 



3. This transmission of the shock is more easily effected by 

 solids than by liquids, and more easily by liquids than by gases ; 

 in the case of gases it takes place all the more easily if they are 

 compressed. It is propagated all the more easily through solids 

 when these are hard ; iron transmits better than earth, and hard 

 earth better than soft soil. 



Any kind of junction has a tendency to weaken, especially if 

 any softer substance intervene. Hence the employment as a 

 receptacle of a tube formed of a goose quill, will stop the effect 

 of mercury fulminate, whereas a copper tube or capsule trans- 

 mits this effect in all its intensity. 



Explosions by influence propagate themselves all the more 

 easily in a series of cartridges, if the casing of the first deto- 

 nating cartridge is very strong ; this allows the gases to attain 

 a very high pressure before the bursting of the casing (p. 40). 



The existence of an air-space between the fulminate and the 

 dynamite, will, on the other hand, diminish the violence of the 

 shock transmitted, and consequently that of the explosion. As 

 a general rule, the effect of shattering powders is lessened when 

 there is no contact. 



4. In order to form a complete idea of the transmission by 

 supports of sudden pressures which give rise to shock, it is well 

 to bear in mind the general principle whereby pressures in a 

 homogeneous mass transmit themselves equally in all directions, 

 and are the same over a small surface, whatever may be the 

 direction. The explosions produced under water with gun-cotton 

 show, as has been said above, that this principle is equally ap- 

 plicable to sudden pressures produced by explosive phenomena. 



