376 GENEKAL DATA. 



In fact and in practice these data are less numerous than 

 might be inferred from the preceding statements. In the present 

 state of our knowledge they are reduced practically to the 

 following : 



(1) Chemical equation of the transformation. 



(2) Heat developed by this transformation, 



(3) Volume reduced to and 760 mm. of the gases and 

 bodies capable of being rendered gaseous in the conditions of 

 the transformation. 



(4) Pressures developed. 



(5) More or less crude empirical indications referring to the 

 work effected. 



These five orders of data regulate our knowledge of the force 

 of explosive substances. 



Let us remark here that the first three measurements are 

 deduced simply from the chemical equation of the phenomenon, 

 and the thermo-chemical tables ; the fourth and fifth would be 

 calculated by the preceding if the laws respecting the thermo- 

 dynamics of gases and those of the resistance of substances were 

 sufficiently well known, 



2. PRACTICAL QUESTIONS RESPECTING THE EMPLOYMENT OF 

 EXPLOSIVE SUBSTANCES. 



1. In practice an explosive substance must satisfy a certain 

 number of conditions which we will now summarise. These 

 conditions refer to the employment, manufacture, preservation, 

 and stability of the explosive substance. Let us commence with 

 the employment. 



2. The explosive substance placed in a small volume and under 

 a moderate weight should develop a considerable quantity of gas 

 and a great amount of heat, circumstances which exclude ex- 

 plosive gases and detonating gaseous mixtures, at least in most 

 applications. 



3. The chemical transformation which the substance under- 

 goes should be produced in a very short space of time, so that the 

 heat may not be gradually dissipated, which would greatly 

 reduce the pressure. 



Let us remark, moreover, that the effort of a sudden pressure 

 produces very different effects of rupture on a given substance 

 to what would have been the case if the same pressure had been 

 exercised slowly. 



In mining works, or with firearms, a slow reaction would tend 

 to let the gases escape little by little through the interstices of 

 the earth or the charge. 



4. The empirical measurement of the force of an explosive 

 substance will be effected by means of a system of tests approach- 

 ing as far as possible the conditions of its practical employment. 



