It has been found useful in some cases to examine microscopically the thin film of coal that often covers the pinnae of fossil fronds, in order to determine the form of the epidermal cells which may be preserved in the carbonized cuticle; rectilinear epidermal cell-walls are usually considered characteristic of Cycads, while cells with undulating walls are more likely to belong to Ferns.
In repairing the modern road just outside the south entrance to the tunnel, a stratum of carbonized corn, beans, &c., and a quantity of burnt wood, stones, tiles, pottery, &c., was found under and above the modern road, for a distance of some Soo yds.
Here it need only be said that the masses of vegetable substance, more or less carbonized and chemically altered, of which coal is composed, frequently contain cells and fragments of tissue in a condition recognizable under the microscope, as for example spores (sometimes present in great quantities), elements of the wood, fibres of the bark, &c. These remnants, however, though interesting as revealing something of the sources of coal, are too fragmentary and imperfect to be of any botanical importance.
The retorts in which the coal is carbonized are almostluniversally made of fire-clay, and in all but small country works the old singleended retort, which was about 9 ft.
As much as one-third on each ton of coal carbonized is saved by the use of machinery in the retort-house.