32 PRESSURE OF GASES. 



explained equally well by the new laws applicable to the calcu- 

 lation of pressures in very highly compressed gases. 



Instead of the above formula, the following may be em- 

 ployed : 



4030 

 Sl " 



which gives somewhat higher results, but which would appear 

 preferable in some respects. 1 



11. In the calculations in Book III., it has been thought 

 useful, notwithstanding the previous reservations, to give the 

 calculation of the theoretical pressure according to the laws of 

 Mariotte and Gay-Lussac. But care has been taken to define 



the result with reference to the density of charge , instead of 



n 



merely taking the density 1. 



This offers the advantage that the figure thus defined has 

 a physical signification for low densities of charge. For high 

 densities its value becomes more and more dubious. However, 

 it can still be employed in a certain number of comparisons, as 

 follows from what has preceded. 



12. The permanent pressure will also be given, that is, the 

 pressure exerted by the permanent gases, produced by the ex- 

 plosion in a completely closed and resisting vessel, and reduced 

 to 0. This pressure will always be calculated for a density of 



charge _. In fact, it cannot exceed the tension of liquefaction 



n 

 of the gases experimented upon. 



Fourth Section. " Characteristic Product" 

 1. Another simpler term of comparison deduced solely from 

 experimental data can be presented in the study of the pressure 

 developed by explosive substances, viz. the product of the 

 reduced volume of the gases, V , by the heat liberated, Q, this 

 product being divided by the specific heat, c. The latter is 

 calculated by referring it to the weight of matter capable of 

 producing this volume and quantity of heat. 

 By this means is obtained the expression 



which may be termed the " characteristic product." 



2. It is sufficient to divide it by the actual volume, n (ex- 

 pressed in cub. cms.), of the capacity in which the unit weight 

 of the explosive substance has been placed, in order to refer 



it to the density of charge - : 



n 



1 " Memorial de 1'Artillerie de Marine," torn. x. p. 187. 



