tes, but in some cases
zeolitic minerals and galena have been observed. Salt, in the form of
brine, is sometimes present in coal. Hydrocarbons, such as petroleum,
bitumen, paraffin, &c., are also found occasionally in coal, but more
generally in the associated sandstones and limestones of the
Carboniferous formation. Gases, consisting principally of light
carburetted hydrogen or marsh gas, are often present in considerable
quantity in coal, in a dissolved or occluded state, and the evolution of
these upon exposure to the air, especially when a sudden diminution of
atmospheric pressure takes place, constitutes one of the most formidable
dangers that the coal miner has to encounter.

Classification.

Anthracite.

The classification of the different kinds of coal may be considered from
various points of view, such as their chemical composition, their
behaviour when subjected to heat or when burnt, and their geological
position and origin. They all contain carbon, hydrogen, oxygen and
nitrogen, forming the carbonaceous or combustible portion, and some
quantity of mineral matter, which remains after combustion as a residue
or "ash." As the amount of ash varies very considerably in different
coals, and stands in no relation to the proportion of the other
constituents, it is necessary in forming a chemical classification to
compute the results of analysis after deduction of the ash and
hygroscopic water. Examples of analyses treated in this manner are
furnished in the last column of Table I., from which it will be seen
that the nearest approach to pure carbon is furnished by anthracite,
which contains above 90%. This class of coal burns with a very small
amount of flame, producing intense local heat and no smoke. It is
especially used for drying hops and malt, and in blast furnaces where a
high temperature is required, but it is not suited for reverberatory
furnaces.

Bituminous coals.

The most important class of coals is that generally known as bituminous,
from their property of softening or undergoing an apparent fusion when
heated to a temperature far below that at which actual combustion takes
place. This term is founded on a misapprehension of the nature of the
occurrence, since, although the softening takes place at a low
temperature, still it marks the point at which destructive distillation
commences, and hydrocarbons both of a solid and gaseous character are
formed. That nothing analogous to bitumen