26 PBESSUKE OF GASES. 



various points of the chamber where the combustion of the 

 powder takes place. 



The pressure, then, is not uniform, and it may vary in an 

 almost discontinuous manner as well as the movement at first 

 communicated to the projectile. 



Second Section. Calculations. 



In order to give a better idea of the value of the results 

 calculated by the ordinary laws of gases, such as there are 

 known at about the atmospheric pressure and the ordinary 

 temperature, we will give the experimental measurements made 

 on certain explosive bodies, which do not give dissociable 

 products (at least to an appreciable extent), and which give by 

 their decomposition elementary bodies or gases formed without 

 condensation : for example, carbonic oxide, of which the specific 

 heat is comparable to that of the simple gases. Such are 

 nitrogen sulphide, decomposable into sulphur and nitrogen, and 

 mercury fulminate, decomposable into mercury and carbonic 

 oxide. They will supply us with types for calculations of this 

 kind. 



1. Take, for instance, 10 grms. of mercury fulminate de- 

 tonating in a capacity of 50 cms. (density of charge 0'2). The 

 heat liberated amounts to 114,500 cal. for the reaction, 



C 2 HgN 2 2 = 2CO + N 2 + Hg ; 



but the mercury being gaseous, the heat of vaporisation must 

 be deducted in calculating the pressure, say 15,400 ; which 

 reduces the heat available for increasing the pressure to 

 99,100 cal. 



Taking for the value of the specific heat at constant volume 

 of carbonic oxide as well as for that of nitrogen and mercury, 

 each at its molecular weight, the figure 4'8 (a figure, moreover, 

 which is in accordance with experiment for the two first bodies), 

 and neglecting the deviation which exists between this number 

 and the specific heat of liquid mercury, we find for the tempera- 

 ture produced 



The volume of the permanent gases (nitrogen and carbonic 

 oxide) given by the reaction and reduced to and 0'760 metres 

 will be 22-32 litres X 3. 



At a temperature t it becomes 



22-32 X 3 X (l + 2^). 



To this should be added, starting from 360, and at the pressure 

 0760 metres, a volume 22*32 lit. (l + ^73 j of mercurv vapour. 



