506 POWDERS WITH A NITRATE BASE. 



lated for the ordinary temperature may be very different from 

 the thermal maximum near 2000 or 3000, temperatures ap- 

 proaching that of the explosion of powder. 



(4) The rapidity of cooling is .too great for the products, 

 formed at the first instant to have time to react on one another 

 so as to reconstitute the most stable system. 



The specific rapidity of each reaction (p. 40) plays here a 

 most important part, both at the moment of the initial forma- 

 tions which take place at the highest temperature, and during 

 the successive reactions. 



It should further be noted that cooling is more rapid at the 

 point of contact with the walls of the vessels, when operating 

 in a closed vessel, than towards the centre of the mass. Hence 

 the composition is different at the various points of the mass, 

 apart from the reactions exercised by the substances of the walls 

 themselves, such as the formation of iron sulphide. 



17. The rapidity of cooling is very different according as 

 combustion takes place in a closed vessel strong enough not to 

 be broken ; or in a shell which bursts suddenly, the fragments 

 being projected and a portion of the heat being transformed 

 into mechanical work; or again in a firearm, where the 

 expansion of the gases takes place according as the projectile is 

 thrust forward and the gases themselves are continually expelled 

 towards the cold portions of the metallic tube. The variation 

 in the chemical reactions which may result from these different 

 circumstances would be very interesting to study, but it has not 

 been fully examined. 



18. We shall, however, note that according to thermo- 

 chemical principles the progressive reactions produced during 

 cooling must be such as to liberate increasing quantities of 

 heat. 



In principle, when operating without changing the condensa- 

 tion of the substance that is to say, at constant volume it 

 cannot be admitted, in the author's opinion, that endothermal 

 reactions, such as dissociations, succeed during the period of 

 cooling to a total combination produced at the instant of 

 explosion. The dissociation must, generally speaking, be 

 regarded as being at its maximum at the outset, that is, at the 

 moment when the temperature is highest, and diminishing as 

 cooling progresses. This applies principally to reactions effected 

 in closed and resisting vessels. 



It is only when expansion takes place at constant temperature, 

 owing to the increase in the volume of the gases, that dissocia- 

 tion, regarded as a function of the pressure, could increase ; the 

 possibility of this increase may even be conceived, strictly 

 speaking, in a case of this sort during a certain period of the 

 cooling. 



But these are quite exceptional cases, and endothermal 



