NATURAL PHILOSOPHY.

305

particle of bitumen would have its combustibility left unim-paired; except so far as its inflammability would have beenconnected by the oxygenization of its carbonaceous part;at the same time that, by the appropriate interposition ofsome nearly incombustible matter, such a check would begiven to the spread of the ignition of one series of the bitu-minous particles to another, as would render their combus-tion sufficiently slow and gradual. Such an arrangement asthis, which might d priori have been supposed to be bestcalculated to produce this effect, may be perceived to beactully employed in the formation of coal.

“ If a piece of common coal be washed in water, so asto remove from its surface all the minute shattered fragmentswhich adhere to it, the fingers will be found no longerstained by it/ and with a lens of very moderate power, oreven with the naked eye, it may be plainly seen that thebituminous matter is disposed in an irregularly laminatedstructure, and that the horizontal surface of each stratum isfinely coated with an extremely thin film, or pellicle, which mayalso be seen frequently to dip and pass through the substanceof the bitumen; intersecting the horizontal septa nearly atright angles, and thus placing the bitumen in distinct divi-sions, the parietes of which are formed of an incombustiblepellicle. This film in general reflects tints of a greyishwhite, but sometimes it appears with the hue of bronze.Frequently it is of a pyritous nature, shining with a brightmetallic lustre; or, as in peacock coal, manifesting a beauti-ful indistinct appearance ; frequently also, when the pellicleitself is of a greyish white, it will be variegated with brightyellow pyritous illinitions; and sometimes spatheous or pyri-tous laminae will be interposed, a quarter of an inch, or evenmore in thickness. On inspecting the edges of the coal,each separate lamina will be found to be composed of bitu-minous concretions exhibiting the conchoidal fracture, andshining with a considerable degree of lustre.

“ On letting a piece of coal fall on any hard body, thusbreaking it in such a manner as will not direct its fracture,but will allow it to take place in such directions as accordwith the natural divisions formed by the interposed films, thefragments will in general be found to assume the form ofrhomboids or of parallelopipeds; the very form which abody, whose parts were thus separated, might be expectedto exhibit on being fractured.

“ The separating pellicles or interposed septa, in thosespecimens which I have examined, appear to be formed ofsulphate of lime, containing a small proportion of alumine,