263 



The arrangement of the respective particles of the mass, which 

 would make the nearest approach to the attainment of the required 

 object, would be that in which each individual particle of bitumen 

 would have its combustibility left unimpaired ; except so far as its 

 inflammability would have been corrected by the excess of its car- 

 bonaceous part : at the same time that, by the appropriate interpo- 

 sition of some nearly incombustible matter, such a check should be 

 given to the spread of the ignition, of one series of the bituminous 

 particles to another, as should render their combustion sufficiently 

 slow and gradual. Such an arrangement as might thus, cL priori, 

 have been supposed to be best calculated to produce this effect, 

 may be perceived, to have been actually employed, in the formation 

 of coal. 



If a piece of common coal be washed in water, so as to remove 

 from its surface all the minute shattered fragments, which adhere 

 to it, the fingers will be found to be no longer stained by it ; and 

 with a lens of very moderate power, or even with the naked eye, 

 it may be plainly seen, that the bituminous matter is disposed in 

 an irregularly laminated structure, and that the horizontal surface 

 of each small and thin stratum is finely coated with an extremely 

 thin film. This film, or pellicle, may also be seen frequently to dip, 

 and pass through the substance of the bitumen ; intersecting the 

 horizontal septa nearly at right angles, and thus placing the bitumen 

 in distinct divisions, the parietes of which are formed of an incom- 

 bustible pellicle. This film, in general, reflects tints of a greyish 

 white, but sometimes it appears with the hue of bronze. Fre- 

 quently it is of a pyritous nature, shining with a bright metallic 

 lustre, or, as in the peacock coal, manifesting a beautiful iridiscent 

 appearance : frequently also, when the pellicle itself is of a greyish 

 white, it will be variegated with bright yellow pyritous illinitions ; 

 and sometimes spathous, or pyritous laminae, will be interposed, a 

 quarter of an inch, or even more, in thickness. On inspecting the 



