MIXTURES OF AIR AND CHARCOAL. 401 



7. The proportion of volatile substances which coal dust can 

 supply also plays an essential part, for these substances, 

 reduced to vapour by combustion, in their turn promote the 

 propagation of the ignition. This dust, however, only burns in 

 an incomplete manner and by means of a kind of distillation 

 which deprives it of its hydrogen, leaving as a residuum portions 

 of coke adhering to the walls and wood-work. Owing to this 

 fact it is not the mixture of air and dust effected in theoretical 

 proportions which is the most combustible, but a mixture which 

 is richer in carbon, seeing that only the superficial layers of the 

 grains take part in the combustion. 



8. Finally, the propagation of the inflammation is effected all 

 the better if the air in the mine already contains a small 

 quantity of some combustible gas, such as methane, the propor- 

 tion of which is often too feeble for it to form by itself a 

 detonating mixture with the air in the mine. 



In mixtures of this kind, even an inert dust, such as magnesia, 

 lowers the limits of combustibility ; a mixture containing only 

 2'75 of fire-damp may thus burn, but in this case the combus- 

 tion does not propagate itself. This circumstance seems to be 

 due to the storage of heat by the magnesia, which then heats 

 the neighbouring gaseous particles, and consequently lowers 

 their limit of combustibility (p. 393). 



Combustible dusts are evidently more efficacious. They 

 increase, moreover, the violence of the explosion produced by 

 fire-damp, owing to the volume of the gases and the supple- 

 mentary heat they supply. Besides this, they tend to increase 

 the quantity of carbonic oxide which is so dangerous to the 

 mines. 



All these circumstances, observed by engineers and managers 

 of mines, have been made the object of methodical experiments 

 by Galloway and Abel in England, and also by Mallard and 

 Le Chatelier in France, 1 in an inquiry recently instituted by the 

 fire-damp commission. For further details the reader is referred 

 to the publications issued by that commission. 



1 " Annales des Mines," Janvier et FeVrier, 1882. 



2 D 



