400 EXPLOSIVE GASES AND DETONATING GASEOUS MIXTURES. 



flour : 1 cubic metre of air would burn 255 grms. of starch 

 (C 6 H 10 O 5 ), developing a theoretical pressure rather above that 

 which carbon would produce (owing to the aqueous vapour). 



(3) Mixtures of air and sulphur. Finally 1 cubic metre of air 

 would burn about 300 grms. of powdered sulphur, developing a 

 pressure of 11 atm. 



4. The limits we have just defined presuppose a uniform dis- 

 tribution of the dust in the air, which, however, can only be 

 realised under very special conditions of movement and division 

 of the dust. 



It is, moreover, difficult to reproduce them by experiment. 



Such systems, moreover, supposing them to be produced 

 instantaneously, cannot exist in the same state, without violent 

 agitation, since the action of gravity tends to separate the com- 

 ponents, contrary to what occurs with systems formed by the 

 mixture of two gases. 



In a system consisting of gas and dusts the relative propor- 

 tions are therefore continually modified by time, as are also the 

 combustible properties of the system which can only maintain 

 their maximum for a very short period. 



5. On the contrary, however, combustible dusts mixed with 

 air remain inflammable far beyond the combustible limits of 

 purely gaseous mixtures, and one single grain in a state of 

 ignition suffices to propagate the flame, either to the neighbour- 

 ing strata, or to the surface of the surrounding solid bodies. 



Such seem to be the most ordinary conditions of the accidents 

 produced by inflammable dusts at the bottom of mines. They 

 are due to a propagated inflammation rather than to a real ex- 

 plosion. Nevertheless, the expansion of the gases is sufficiently 

 sudden to produce violent mechanical effects, which are very 

 dangerous. 



6. The propagation of fire in a mixture of air and combustible 

 dust is intensified by the movements of expansions and the 

 projection of gaseous masses, inflamed at the very outset. 

 Hence it is as regards coal-mines that experience has led to 

 attributing a very dangerous part to carbonaceous dust, raised 

 like a whirlwind when a blast is fired, and which propagates 

 fire and asphyxia even to a great distance in the galleries. 

 Thus it has happened that a blast, the flame of which did not 

 extend beyond 4 metres, has propagated combustion through 

 the dust that was raised, to a distance of more than 14 metres, 

 and reached workmen who thought they were out of danger. 



Blastings which blow out are especially dangerous in this 

 respect. 



At the outset a real amplification of the flame is produced ; 

 afterwards it is a simple propagation of the ignition of the dust. 



The finer the dust is the more the volume of the initial flame 

 provoking the phenomenon can be limited. 



