much wood.

These properties of coal may be made out without any very refined
appliances, but the microscope reveals something more. Black and
opaque as ordinary coal is, slices of it become transparent if they
are cemented in Canada balsam, and rubbed down very thin, in the
ordinary way of making thin sections of non-transparent bodies. But
as the thin slices, made in this way, are very apt to crack and break
into fragments, it is better to employ marine glue as the cementing
material. By the use of this substance, slices of considerable size
and of extreme thinness and transparency may be obtained.[1]

[Footnote 1: My assistant in the Museum of Practical Geology, Mr.
Newton, invented this excellent method of obtaining thin slices of
coal.]

Now let us suppose two such slices to be prepared from our lump of
coal--one parallel with the bedding, the other perpendicular to it;
and let us call the one the horizontal, and the other the vertical,
section. The horizontal section will present more or less rounded
yellow patches and streaks, scattered irregularly through the dark
brown, or blackish, ground substance; while the vertical section will
exhibit more elongated bars and granules of the same yellow materials,
disposed in lines which correspond, roughly, with the general
direction of the bedding of the coal.

This is the microscopic structure of an ordinary piece of coal. But if
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