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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 inter- 
position of some nearly incombustible matter, such a check should 
be given to the spread of the ignition, of one series of the bitu- 
minous particles to another, us should render their combustion 
sufficiently slow and gradual. Such an arrangement as might thus, 
a 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 
edges of the coal, each separate lamina will be found to be com- 
posed of bituminous concretions, exhibiting the conchoidal fracture, 
and shining with a considerable degree of lustre. 
