50 DURATION OF EXPLOSIVE REACTIONS. 



mined its rate of flow through a narrow orifice, ignited the jet 

 and endeavoured to discover what was the minimum rate of 

 liow at which the flame would remain stationary at the orifice 

 without going back into the interior. Mallard 1 made several 

 experiments on various mixtures of air and of marsh gas, or of 

 coal gas; he found that the velocity of combustion defined 

 as above rapidly diminishes in proportion as the ratio of the 

 gases which do not contribute to the combustion is increased, the 

 maximum speed being 0'560 metre per second in the case of 

 a mixture of eight parts by volume of air and one part of 

 marsh gas. If a mixture contain twelve parts of air and one 

 part of marsh gas it will descend to O04 metre. With coal 

 gas and air the maximum velocity obtained has been almost 

 double. Mallard and Le Chatelier have recently returned to 

 this question by other processes which have given them very 

 varied results according to the mode of combustion. This will 

 be referred to later on, and the existence of detonating velocities 

 for the same gaseous mixtures reaching almost up to 3000 

 metres per second, and the causes of these differences will be 

 shown. 



9. The study of new explosive substances has in fact led to 

 a fuller knowledge of the mode of propagation of chemical 

 reaction in the interior of a mass in combustion ; and it has 

 radically modified the ideas which prevailed on this question. 

 At one time, when black powder was the only known explosive, 

 the only point studied was how to ignite it, the effects of con- 

 secutive explosion not appearing to depend on the process of 

 ignition. But nitroglycerin and gun-cotton have evinced a 

 singular diversity in this respect. 



10. To form a correct conception of them, mention must first 

 be made of the phenomena of shock and of other analogous 

 causes capable of producing deflagration. 



Shock could hardly of itself affect the decomposition of a 

 substance which absorbs heat, unless recourse be had to colossal 

 masses animated by enormous energy and concentrating all their 

 action on a very small quantity of matter, which is very difficult 

 to effect. For instance, the energy of a weight of 1630 kgms. 

 falling from a height of 1 metre, would be necessary in order to 

 decompose 1 grm. of water, assuming that by any artifice the 

 totality of this energy could be transmitted to a gramme of 

 water. 



On the other hand, if the decomposition of the substance 

 disengages heat, we can conceive that a limited energy would 

 suffice to provoke it, provided it were applied in its entirety to 

 a very small quantity of matter whose temperature it raised to 

 the desired degree for determining reaction. 



Thus a few heavy strokes of a hammer on potassium chlorate 

 1 " Annales de Mines," torn. viii. 3 e livraison. 1871. 



