40 DURATION OF EXPLOSIVE REACTIONS. 



double that of the heat liberated by an explosive mixture formed 

 of electrolytic gas at the same volume, viz. 33*6 litres, or 

 112,000 cal. instead of 59,000, when the cyanogen is decom- 

 posed into free carbon and nitrogen by the electric spark. 

 Nevertheless, even though the carbon immediately commences 

 to be precipitated, the cyanogen does not detonate under the 

 influence of the spark, nor even of the voltaic arc, which is a 

 proof of the slowness of the reaction thus caused. 



Under other conditions, however, the cyanogen and acetylene 

 can be decomposed with detonation into their elements, but it is 

 neither by simple heating nor by the action of the electric arc 

 or spark (see p. 67). 



Instances might be multiplied of these facts l which comprise 

 the explosive bodies, properly so called, when maintained at a 

 temperature slightly less than that which determines the ex- 

 plosion. Silver oxalate, for instance, is slowly decomposed at 

 100, while it detonates sharply at a slightly higher temperature. 



4. In a word, every molecular reaction effected by simple 

 heating at a constant temperature in a homogeneous body and 

 submitted to conditions which appear identical for all its parts, 

 has a characteristic coefficient relative to the duration. This 

 coefficient depends on the temperature, the pressure, and the 

 relative proportions; it plays a very important part in the 

 study of the unequally destructive properties of explosive 

 compounds. 



This will be exemplified by a few applications. 



5. The longer or shorter duration of a reaction does not change 

 the quantity of heat liberated by the total transformation of a 

 given weight of explosive matter. But if the gases formed 

 gradually expand, for instance, as in a cannon, owing to the 

 change of capacity, increased by the flight of the projectile, or 

 owing to cooling due to contact with the walls of the vessel ; 

 under these circumstances the initial pressures will be less the 

 longer the transformation of a given weight of matter lasts. 



On the contrary, when a very rapid transformation of the 

 whole mass in a closed vessel, allows the initial pressures to 

 attain the immensity of their theoretical limits, or to approach 

 it, it is difficult to construct vessels strong enough to contain 

 the gases of explosion. 



6. This explains the influence of resisting envelopes and of 

 tamping, an influence which is especially apparent with slow 

 powders, but which is also observed with rapid powders, par- 

 ticularly in detonators. 



At the moment of the explosion the pressure at first developed 

 around the ignited point tends to diminish, owing to the expan- 

 sion of the gases, and in proportion as the products are dis- 

 tributed throughout a more considerable space. If the gases 

 1 " Annales de Chimie et de Physique," 4 e se'rie, torn, xviii. p. 142. 



