12 CHEMICAL COMPOSITION. 



in the case of such enormous pressures as those observed in the 

 combustion of powder. With greatly compressed gases the 

 pressure varies with the temperature much more rapidly than 

 would follow from these laws ; it approaches the rate observed 

 by physicists in the study of vapours. For a given temperature 

 the pressure is therefore generally higher than that which would 

 be given by calculating according to the ordinary laws of gases. 

 This tends to compensate in the calculation of pressures the 

 contrary influences exercised by the variation in the specific 

 heats. 



Now, the phenomena of dissociation depend on the pressure, 

 as well as on the temperature. The state of combination of 

 elements, all things else being equal, is higher as the pressure 

 is greater a relation which is easily conceived a priori, and 

 which is confirmed by experiments relative to the decomposition 

 of acetylene into carbon and hydrogen at different pressures by 

 the electric spark. 1 But the pressures increase with the tempe- 

 ratures, and even much more rapidly, as has just been stated ; 

 the decomposing influence of the temperature can therefore be 

 compensated, either wholly or in part, by the opposite influence 

 of pressure. 



9. The inverse action of these two classes of phenomena 

 remains such that a substance undergoing transformation at 

 constant volume without loss of heat, will tend towards a 

 certain limiting state ; the transformation of the first portions 

 will at first raise the temperature and pressure to the point at 

 which dissociation will limit the phenomenon. This is also a 

 theoretical maximum, since the mass is continually cooled by 

 radiation and conduction. But the greater the mass operated 

 upon, the nearer will this result be approached. 



10. The phenomena of dissociation do not only exert their 

 influence on the maximum effort which the explosive substance 

 can develop, but they also come into play during the first period 

 of expansion. In proportion as the gases of the explosive 

 expand in acting on the projectile, they cool, in consequence of 

 which the elements enter into combination in a more complete 

 manner and with the formation of more complicated compounds. 

 From this there results a new disengagement of heat which 

 increases without ceasing during the whole of a period of 

 expansion. 



Therefore, in general, the transformation effected in the bore 

 of a cannon cannot be regarded as adiabatic. The temperature 

 of the gases will not be lowered by a quantity any way pro- 

 portionate to the exterior work done, even independently of the 

 losses of heat due to exterior causes of cooling; seeing that 

 restoration of heat takes place through the chemical reaction, 

 during a considerable period. 



1 " Annales de Chimie et de Physique," 4 e se'rie, torn, xviii. p. 196. 1869. 



