"
Occasionally one of the faces of a lump of coal will present
impressions, which are obviously those of the stem, or leaves, of a
plant; but though hard mineral masses of pyrites, and even fine mud,
may occur here and there, neither sand nor pebbles are met with.
When the coal 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.
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