Coal and the coal mines

by Homer Greene

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.