MIXTURES OF AIR AND CHARCOAL. 399 



5. GAS AND COMBUSTIBLE DUSTS. 



1. A gas may form explosive mixtures, not only by its asso- 

 ciation with another gas, but also with a solid or liquid dust. 

 Hence we obtain systems of a very special order. Their explo- 

 sive nature may easily be conceived, seeing that these systems, 

 when once ignited, give rise to sudden expansion, accompanied 

 by an increase of pressure. However, the explosion of such a 

 system is necessarily slower than that of a purely gaseous 

 mixture, since the propagation of the reaction only takes place 

 as each solid particle is reached by the incandescent gases 

 arising from the combustion of the neighbouring particles. 

 Hence we may conceive the influence exercised .by the slightest 

 trace of combustible vapour or gas already mixed with air in 

 facilitating ignition. 



2. Explosions of this kind have been observed in coal mines, 

 in flour mills and warehouses, and in places containing sulphur 

 in the form of an impalpable powder. 



The clouds formed by petroleum vapours and other volatile 

 hydrocarbons have also given rise to similar explosions in 

 cellars or magazines, or even in the open air, but in this case 

 the effects are of a mixed character, owing to the peculiar 

 vapour tension of these hydrocarbons, a portion of which should 

 be considered as gaseous in these mixtures. 



3. Eeference will only be made to mixtures formed by air 

 associated with a combustible dust. Let us first define the 

 limits which correspond to the maximum effect with mixtures 

 of air and combustible dust, supposed to be effected in suitable 

 proportions at the moment of explosion. 



(1) Mixtures of air and charcoal. One cubic metre of air 

 may by its oxygen generate 208 litres of carbonic acid reduced 

 to and 760 mm. The same volume of air would burn 

 112 grms. of pure carbon. Now such a system, namely, an 

 intimate and as uniform a mixture as possible of air and 

 carbon in the form of powder would develop a theoretical 

 pressure of 15*5 atm. if it were burnt at constant volume. If 

 the quantity of charcoal were doubled (224 grms.) and the 

 whole could be changed into carbonic oxide, we should obtain 

 416 litres of the latter gas, and the pressure developed would 

 be 6 '7 atm. 



If necessary, carbonaceous dusts may be assimilated to carbon 

 for similar effects. 



At any rate we see that the maximum limit of theoretical 

 pressures which can be developed by the combustion of a 

 carbonaceous dust is similar to the pressures developed by fire- 

 damp itself. 



(2) Mixtures of air and starch. Let us take it to be starch 

 dust which, to facilitate calculation, we may substitute for 



