a great series of coals, from different localities and seams, or even
from different parts of the same seam, be examined, this structure
will be found to vary in two directions. In the anthracitic, or
stone-coals, which burn like coke, the yellow matter diminishes, and
the ground substance becomes more predominant, and blacker, and more
opaque, until it becomes impossible to grind a section thin enough to
be translucent; while, on the other hand, in such as the "Better-Bed"
coal of the neighbourhood of Bradford, which burns with much flame,
the coal is of a far lighter colour, and transparent sections are very
easily obtained. In the browner parts of this coal, sharp eyes will
readily detect multitudes of curious little coin-shaped bodies, of a
yellowish brown colour, embedded in the dark brown ground substance.
On the average, these little brown bodies may have a diameter of about
one-twentieth of an inch. They lie with their flat surfaces nearly
parallel with the two smooth faces of the block in which they are
contained; and, on one side of each, there may be discerned a figure,
consisting of three straight linear marks, which radiate from the
centre of the disk, but do not quite reach its circumference. In the
horizontal section these disks are often converted into more or less
complete rings; while in the vertical sections they appear like thick
hoops, the sides of which have been pressed together. The disks are,
therefore, flattened bags; and favourable sections show that the
three-rayed marking is the expression of three clefts, which penetrate
one wall of the bag.

The sides of the bags are sometimes closely approximated; but, when
the bags are less flattened, their cavities are, usually, filled with
numerous, irregularly rounded, hollow bodies, having the same kind of
wall as the large ones, but not more than one seven-hundredth of an
inch in diameter.