In favourable specimens, again, almost the whole ground substance
appears to be made up of similar bodies--more or less carbonized
or blackened--and, in these, there can be no doubt that, with the
exception of patches of mineral charcoal, here and there, the whole
mass of the coal is made up of an accumulation of the larger and of
the smaller sacs.

But, in one and the same slice, every transition can be observed from
this structure to that which has been described as characteristic of
ordinary coal. The latter appears to rise out of the former, by the
breaking-up and increasing carbonization of the larger and the smaller
sacs. And, in the anthracitic coals, this process appears to have gone
to such a length, as to destroy the original structure altogether, and
to replace it by a completely carbonized substance.

Thus coal may be said, speaking broadly, to be composed of two
constituents: firstly, mineral charcoal; and, secondly, coal proper.
The nature of the mineral charcoal has long since been determined. Its
structure shows it to consist of the remains of the stems and leaves
of plants, reduced to little more than their carbon. Again, some of
the coal is made up of the crushed and flattened bark, or outer coat,
of the stems of plants, the inner wood of which has completely decayed
away. But what I may term the "saccular matter" of the coal, which,
either in its primary or in its degraded form, constitutes by far the
greater part of all the bituminous coals I have examined, is certainly
not mineral charcoal; nor is its structure that of any stem or leaf.
Hence its real nature is, at first, by no means apparent, and has been
the subject of much discussion.