146 WATTS : COAL-DUST AND EXPLOSIONS IN COAL-MINES. 



yet, in the other case, locked up in its coaly prisons, the dust 

 particles. In the one case it has obtained its freedom from the 

 unbroken seam by changes of atmospheric pressure and other causes 

 than local heat, whilst in the other case combustion opens the 

 prison doors at the moment when the coal-dust becomes coked. 

 Gas is ready-made in the one case, and has to be made at the 

 time in the other. The danger from the presence of the free gas 

 is fully recognised ; not so that of the gas imprisoned in the particles 

 of coal-dust. The kinetic energy is feared and watched; the potential 

 has been hitherto largely ignored or despised. To the latter the 

 present inquiry is directed. 



There are evidently two factors in the combustibility of the dust 

 of coal-pits, viz., material and degree of fineness. These will be 

 considered separately. The degree of fineness of the dust exercises 

 both a direct and an indirect influence on that combustion which 

 results in an explosion. Direct, in rendering ignition easier, by 

 bringing a larger surface under the influence of the agent ; indirect, 

 in favouring what we may call the saturation of the air-current with 

 inflammable material. The direct influence is in favouring the 

 transmission of heat ; the indirect, in favouring the dispersion of the 

 particles both before and after being heated. This indirect influence 

 is very important, and especially so far as the lighter material is 

 concerned, for the finer it is, the slighter need the shock be that 

 causes its primary displacement, thus loading the air-current with 

 combustible material, and making that prolongation of flame possible, 

 which alone, in the absence of fire-damp, can make an explosion 

 possible. 



As regards composition, ordinary coal-dust consists partly of coal, 

 partly of dant, and partly of extraneous and incidental matters. 

 The coal, however fine, retains its cubical character roughly, and so 

 can be identified with tolerable certainty under the microscope. 

 The dant demands a little more care and attention. Among the 

 rough cubes are seen, in larger or smaller numbers, many needle- 

 shaped bodies, varying much in size, but all forming oblongs or 

 rhomboids, and also certain triangular and polygonal bodies of slight 

 thickness. These find their origin in that substance variously known 

 by miners as ' Dant,' ' Mother of coal,' and ' Mineral charcoal' 

 Dant is of a deep dull black to the naked eye, soft to the touch, and 

 is that portion of the coal which soils the fingers when coal is handled. 

 Its dulness disappears under the microscope, and its long splinters, 

 often delicate as needles, render it easily recognised. A piece of 

 charcoal is so like it in appearance and physical condition that the 

 name ' Mineral charcoal' seems peculiarly appropriate. This ' Dant' 



Naturalist, 



