WATTS : COAL-DUST AND EXPLOSIONS IN COAL-MINES. 1 47 



is found usually in very thin layers, but sometimes the layers are an 

 inch thick. More rarely it appears in pockets of considerable size. 

 The thin deposits are very numerous and come between the layers of 

 bright coal in planes parallel to those of the stratification of the 

 adjoining rocks. Its softness, abundance, and appearance along 

 these planes makes it form a considerable percentage of all pit -dust, 

 and especially of the more dangerous kinds. The readiness with 

 which ordinary charcoal absorbs oxygen may be significant as regards 

 this dant or mineral charcoal. The presence of spiral vessels and 

 strings of cells, entire or fractured, clearly prove the vegetable origin. 

 Pitted cells, uncommonly like those characteristic of conifers, are 

 occasionally met with. 



Ordinary pit-dust mainly consists of these two substances-— coal 

 and dant, varying widely in their ratios, and also of foreign matter, 

 which varies over a far wider range. I have conducted many 

 experiments with coal and dant, and they prove conclusively that 

 dant ignites most readily, and that coal burns with most energy. 

 The finer either of these ingredients is, the more easily does it 

 ignite, and the more perfecdy is the mass consumed before going out. 

 Conduction of heat is rendered easy by minute subdivision. Thus, 

 coal-dust, collected from the screens at the pit-mouth, will not take 

 fire from a match, when piled in a little heap on a slab of wood ; 

 it is comparatively coarse ; but when pounded in a mortar it will do 

 so, and burns with much smoke. This dust consists mainly of coal, 

 but there is an admixture of dant. Coal, carefully selected, so as to 

 be as free from dant as possible, will burn in like manner, but only 

 when crushed very fine. With dant, the degree of fineness is of less 

 moment, since it lights readily whether roughly scraped from the 

 coal by a knife or pounded fine in a mortar ; still, the ultimate com- 

 bustion of the heap is perfect in proportion to the fineness. 



The ' material ' is the other factor, and remains yet to be con- 

 sidered. The microscope shows that it consists of coal, dant, shale, 

 or other stone dust, and what may be called incidental matter, as 

 brick-dust, whitening, lime-plates, animal and vegetable refuse, and 

 the results of the wear and tear of the haulage and winning apparatus. 

 We vv'ill consider these — first, separately ; and second, in the natural 

 combinations in which they exist in coal-mines. 



The incidental matter varies much, is mostly small in amount, and 

 so far as this question is concerned, is of no importance, so may be 

 briefly dismissed. Its influence, whatever it may be, is almost in- 

 variably deterrent. The shale and other stone-dust may, however, 

 in some cases be found assisting exi)losions. Usually its amount is 

 small, often very small, except in the bottom dust of return air-ways 



M.'iy 18S8. 



