oal burns, the chief ultimate products of its combustion are
carbonic acid, water, and ammoniacal products, which escape up the
chimney; and a greater or less amount of residual earthy salts, which
take the form of ash. These products are, to a great extent, such as
would result from the burning of so 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 mere 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, 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 yel