THE COMPOSITION OF COAL.
The first question that would naturally be asked concerning the subject
with which we are dealing is, What is coal?
In reply it may be said that it is a mineral. It is black or brown
in color, solid, heavy, and amorphous. The specific gravity of the
average Pennsylvania anthracite is about 1.6, and of the bituminous
coal about 1.4. There are four varieties of mineral coal, namely:
anthracite, bituminous, lignite or brown coal, and cannel coal. To
this list it would not be improper to add peat, since it partakes
of most of the characteristics of mineral coal, and would doubtless
develop into such coal if the process of transformation were allowed to
continue undisturbed. The principal element contained in each of these
different kinds of coal is carbon. An analysis of an average piece of
Pennsylvania anthracite would show the following chemical composition:――
Fixed carbon 86.4
Ash 6.2
Water 3.7
Volatile matter 3.1
Sulphur .6
―――――
Total 100
The composition of the bituminous coals of Pennsylvania, as represented
by the gas coal of Westmoreland County, is shown by analysis to be as
follows:――
Fixed carbon 55.
Volatile matter 37.5
Ash 5.4
Water 1.4
Sulphur .7
―――――
Total 100
An analysis of coal from the Pittsburgh region would show its percentage
of carbon to be from 58 to 64, and of volatile matter and ash to be
proportionately less.
There is no strict line of demarcation between the anthracite and the
bituminous coals. They are classed generally, according to the amount
of carbon and volatile matter contained in them, as:――
Hard-dry Anthracites,
Semi-Anthracite,
Semi-Bituminous,
Bituminous.
Coals of the first class contain from 91 to 98 per cent, of carbon, and
of the second class from 85 to 90 per cent. The volatile matter in the
third class is usually less than 18 per cent., and in the fourth class
more than 18 per cent. of its composition.
The anthracite coal is hard and brittle, and has a rich black color
and a metallic lustre. It ignites with difficulty, and at first burns
with a small blue flame of carbonic oxide. This disappears, however,
when ignition is complete. No smoke is given off during combustion.
Semi-anthracite coal is neither so hard, so dense, nor so brilliant in
lustre as the anthracite, though when once fully ignited it has all
the characteristic features of the latter in combustion. It is found
principally at the western ends of the anthracite coal basins.
Bituminous coal is usually deep black in color, with little or no
lustre, having planes of cleavage which run nearly at right angles with
each other, so that when the coal is broken it separates into cubical
fragments. It ignites easily and burns with a yellowish flame. It gives
off smoke and leaves a large percentage of ashes after combustion. That
variety of it known as caking or coking coal is the most important.
This is quite soft, and will not bear much handling. During combustion
it swells, fuses, and finally runs together in large porous masses.
Following the question of the composition of coal comes the question
of its origin, of which, indeed, there is no longer any serious doubt.
It is generally conceded that coal is a vegetable product, and there
are excellent reasons for this belief. The fragments of which coal is
composed have been greatly deformed by compression and decomposition.
But when one of those fragments is made so thin that it will transmit
light, and is then subjected to a powerful microscope, its vegetable
structure may readily be distinguished; that is, the fragments are seen
to be the fragments of plants. Immediately under every separate seam of
coal there is a stratum of what is known as fire clay. It may, under
the beds of softer coals, be of the consistency of clay; but under the
coal seams of the harder varieties it is usually in the form of a slaty
rock. This fire clay stratum is always present, and contains in great
abundance the fossil impressions of roots and stems and twigs, showing
that it was once the soil from which vegetation grew luxuriantly. It
is common also to find fossil tree-stems lying mashed flat between the
layers of black slate which form the roof of the coal mines, also the
impressions of the leaves, nuts, and seeds which fell from these trees
while they were living. In some beds of cannel coal whole trees have
been found, with roots, branches, leaves, and seeds complete, and all
converted into the same quality of coal by which they were surrounded.
In short, the strata of the coal measures everywhere are full of the
fossil impressions of plants, of great variety both in kind and size.
If a piece of wood be subjected to heat and great pressure, a substance
is obtained which strongly resembles mineral coal.
That coal contains a very large proportion of carbon in its composition
has already been noted. If, therefore, it is a vegetable product, the
vegetation from which it was formed must have been subjected to some
process by which a large part of its substance was eliminated, since
wood or woody fibre contains only from 20 to 25 per cent. of carbon.
But wood can be transformed, by combustion, into charcoal, a material
containing in its composition 98 per cent, of carbon, or a greater
percentage than the best anthracite contains. This cannot be done,
however, by burning wood in an open fire, for in that case its carbon
unites with atmospheric oxygen and passes invisibly into the air. It
must be subjected to a process of smothered combustion; free access
of air must be denied to it while it is burning. Then the volatile
matter will be freed and expelled, and, since the carbon cannot come
in contact with the oxygen of the air, it will be retained, together
with a small percentage of ash. The result will be charcoal, or coal
artificially made. The principle on which this transformation is based
is combustion or decomposition out of contact with atmospheric air.
But Nature is as familiar with this principle as is man, and she may
not only be discovered putting it in practice, but the entire process
may be watched from beginning to end. One must go, for this purpose,
first, to a peat bed. This is simply an accumulation of the remains of
plants which grew and decayed on the spot where they are now found. As
these remains were deposited each year, every layer became buried under
its succeeding layer, until finally a great thickness was obtained.
When we remove the upper layer we find peat with its 52 to 66 per
cent. of carbon, and the deeper we go the better is the quality of the
substance. It may be cut out in blocks with sharp spades, the water may
be pressed from the blocks, and they may be stacked up, covered and
dried, and used for fuel. In most peat bogs the process of growth is
going on, and may be watched. There is a certain kind of moss called
_sphagnum_, which in large part makes up the peat-producing vegetation.
Its roots die annually, but from the living top new roots are sent out
each year. The workmen who dig peat understand that if this surface
is destroyed the growth of the bed must stop; consequently in many
instances they have removed the sod carefully, and after taking out
a stratum of peat have replaced the sod in order that the bed may be
renewed. There is little doubt that if these beds of peat could lie
undisturbed and covered over through many ages they would take on all
the characteristics of mineral coal.
A step farther back in geological history we reach the period of the
latest formations of lignite or brown coal. This coal is first found in
the strata of the glacial period, or first period in the age of Man.
But it is found there in an undeveloped state. The woody fibre has not
yet undergone the complete transformation into coal. The trunks and
branches of trees have indeed become softened to the consistence of
soap, but they still retain their natural color. Going back, however,
to the strata of the Miocene or second period of the Tertiary age or
age of Mammals, we find that this wood has become black, though it
has not yet hardened. But when we reach the upper cretaceous or last
period of the age of Reptiles, the transformation into coal has become
complete. The woody fibre is now black, hard, and compact, though it
may still be easily disaggregated by atmospheric action, and we have
the true lignite, so called because of its apparent woody structure.
The next step takes us back to the bituminous coal of the Carboniferous
age, the character and consistency of which has already been noted,
and finally we reach the complete development in anthracite. It is,
however, the opinion of the best geologists that the bituminous and
anthracite coals are of the same age, and were originally of the same
formation and character. That is, they were all bituminous; but during
the violent contortions and upheavals of the earth’s crust at the time
of the Appalachian revolution at the close of the Carboniferous age,
the bituminous coals involved in that disturbance were changed by heat,
pressure, and motion, and the consequent expulsion of volatile matter,
from bituminous to anthracite.
Cannel coal is a variety of bituminous coal, burning with great
freedom, the flame of which affords considerable light. It was called
“candle coal” by the English people who first used it, as it often
served as a substitute for that household necessity. But the name soon
became corrupted to “cannel,” and has so remained. It is duller and
more compact than the ordinary bituminous coal, and it can be wrought
in a lathe and polished. A certain variety of it, found in the lower
oölitic strata of Yorkshire in England, is manufactured into a kind of
jewelry, well known by its popular name of _jet_.