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 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