20 PRESSURE OF GASES. 



3. Attempts have been made to supplement it by an hypo- 

 thesis more arbitrary, doubtless, but convenient for calculations. 

 This hypothesis consists in regarding all compound bodies as 

 possessing, at a high temperature, a constant specific heat, inde- 

 pendent of temperature and pressure, and equal to that of the 

 sum of their gaseous elements, of course at constant volume. 

 This specific heat will be the same, and equal to 4' 8 for every 

 gaseous element of a weight such that it occupies the molecular 

 volume taken as unity. 



4. The sum of the specific heats can therefore be found by 

 multiplying the sum of the molecular volumes concerned in the 

 reaction by 4*8, and dividing by the unit volume. 



4. PRESSURE. 



The pressure developed at the moment of the explosive 

 reaction can either be calculated a priori, or directly measured. 

 This subject will be divided into four sections, viz. : 



Direct measurements (1st section). 



Calculations (2nd section). 



Density of charge and specific pressure (3rd section). 



Lastly, the " characteristic product," a term of comparison 

 wholly deduced from purely empirical data. 



First Section. Direct Measurements. 



1. Direct measurements are made with the aid of various 

 apparatus, some based on the static, others on the dynamic 

 method, that is, on the study of the law of the movement 

 imparted to a heavy body. 



2. The earliest and simplest of all the apparatus is that of 

 Eumford (1792), who experimentally ascertained the weight 

 capable of keeping in equilibrium the pressure of powder gases. 1 

 The results obtained by this instrument for densities of charge 

 comprised between 01 and 0*3 do not greatly deviate from the 

 most recent figures observed by Noble and Abel. Above these 

 densities Kumibrd's figures are excessive. 



3. The Eodman punch (1857) and its modifications, as well 

 as the Uchatius eprouvette (1869), are based on the size of an 

 indent made on a copper disc by a steel punch fitted to a piston 

 acted on by the gases of the explosive substance. In the 

 apparatus of Meudon, successively improved by Colonels Mont- 

 luisant and Keffye, the " flowing " of a cylindrical mass of lead, 

 thrust by the gases into a conical channel of smaller dimensions, 

 is observed. 



The crusher gauge of the English Commission on explosive 

 substances, deduces the pressure from the crushing of a copper 



1 " Trait^ sur la poudre par Uppman et Meyer traduit et augmente par 

 Desortiaux," p. 562. 1878. 



