The Coal Series
Disregarding minor constituents, the plants are largely made up of cellulose, which is a combination of carbon, hydrogen, and oxygen, (C₆H₁₀O₅). If this is heated in the air, where there is plenty of oxygen, it disintegrates, or burns, making carbon dioxide and water; but if the heating is done where the oxygen is excluded, as in a kiln, the hydrogen and oxygen will be driven off and the carbon will remain behind as charcoal. In Nature similar reactions go on, but more slowly. Vegetable matter, exposed to the air, disintegrates into carbon dioxide and water, and there is no solid residue. However, if the vegetable matter is under water, which excludes the air more or less completely including the oxygen in it, then disintegration still takes place, but the products formed are water, (H₂O) marsh gas (CH₄), and some carbon dioxide (CO₂), but a considerable part of the carbon remains behind and accumulates.
Thus in bogs, swamps and ponds, where dead vegetation, especially that growing in the water, piles up, the oxidation is incomplete; so that there gradually accumulates on the bottom a layer of brown to black mud, known as peat. More plant remains are constantly being added, and the layer may increase to several feet in thickness. The decomposition is incomplete and some oxygen and hydrogen remain, but the carbon is in a constantly increasing ratio and in proportion far above that in cellulose. In the cold northern climates sphagnum moss is the most efficient peat producing plant, but in temperate and tropical climates the moss is replaced by the leaves, twigs, trunks, etc., of trees, bushes, and vines.
If these peat beds are buried beneath a layer or layers of sediment, especially clay, the peat is sealed up and oxidation stops almost entirely. With the pressure of the superincumbent beds, the peat becomes more and more compact, and changes to a dark-brown or black color. It is then known as lignite. If this lignite is buried still deeper, with consequently more pressure and more time, it changes into the still denser black bituminous coal. This is as far as it will go unless some new agent is added to the forces already working.
The next step in the series of changes forming coal is associated with mountain making. In case the layers of rock containing beds of coal are folded, and that presupposes at least a moderate increase in heat, the bituminous coal is altered to anthracite, which is still denser, and so hard that it breaks with a conchoidal fracture. Alteration may be carried a step still farther, in case the rocks between which lie beds of coal are effected by such high temperatures as accompany metamorphism. Then all the associated hydrogen, oxygen and moisture are driven off, and only the carbon remains, which is then known as graphite. All steps between the stages especially designated occur. The following represent steps only in the series of changes.
[Peat]
Peat is a mass of unconsolidated vegetable matter, which has accumulated under water, and in which the original plant remains are still, at least in part, discernible. It contains a large amount of water, so that before it can be used as a fuel, it is cut out in blocks, which are piled up and left for a time to dry before using. It burns with a long flame and considerable smoke. This country is so well supplied with other fuels, that so far peat has been but little used.
[Lignite]
brown coal
Lignite is more compact than peat, and is found buried to some depth under layers of clay or sandstone. It is dark brown to black in color, and still retains pretty clear traces of the plants from which it was derived. It also usually contains a considerable amount of moisture, and when this is dried out, it tends to crumble badly, so that it is undesirable to handle it much, or to ship it far, before using. It has a fair fuel value and is fairly widely used; but it is very desirable that some method be found, by which lignite could be treated to obtain its by-products, and at the same time make it more compact, so it would not crumble with the handling incident to using it in furnaces. There are extensive lignite deposits in this country in North and South Dakota, Montana, Wyoming, Colorado, New Mexico, Texas, Louisiana, and Mississippi.
[Bituminous Coal]
soft coal
This type of coal is compact, black in color, and breaks readily, but does not crumble as badly as lignite. It contains considerable water, and still has some hydrogen and oxygen compounds in it. Bituminous coal is the product of plant remains which have been preserved for long periods, (millions of years), sealed from the air by the overlying beds of rock. The pressure has made it compact, and nearly all traces of the original plants have disappeared.
Bituminous coal is our most abundant fuel, occurring the world over in seams from less than an inch in thickness to some over fifteen feet thick. The United States is peculiarly fortunate in the abundant and easily accessible deposits of this type of coal, in Pennsylvania, West Virginia, Ohio, Kentucky, Tennessee, Indiana, Illinois, Michigan, Iowa, Missouri, Kansas, Nebraska, Texas, Utah, and Colorado.
The volatile constituents, hydrogen and oxygen compounds, of bituminous coal may be driven off by heating the coal in closed ovens, and the residual mass is known as coke, almost pure carbon. This is distillation, and the ovens in which this is done, without trying to save the volatile products, are called bee-hive ovens, while the more modern ovens which save the by-products are called by-products ovens. A ton of bituminous coal treated in the typical by-products oven, will yield on the average 1410 lb. of coke, 7.1 gallons of tar, 18.9 pounds of ammonia sulphate, etc., 2.4 gallons of light oils, 10440 cubic feet of illuminating gas, about half of this last being used to furnish the heat for the distillation. The coal-tar dye industry is built on the tar thus produced. Toluol, benzol, etc., come from the light oils; and half the gas produced is available for household illumination, etc. Coke is demanded, as it is a superior fuel for melting iron ores, iron and steel, and is made regardless of whether the by-products are used. The coke thus produced is hard, clean, and vesicular; but for some reason as yet unknown, by no means all bituminous coal will produce a coke which has this porous structure. These latter are known as “non-coking,” and are of little use to the steel industry.
[Cannel Coal]
This is a compact variety of non-coking bituminous coal, with a dull luster and a conchoidal fracture. It contains the largest proportion of volatile hydrocarbon compounds of any variety of coal; so that when the supply of petroleum and natural gas gives out, this will be one of the important sources of obtaining substitutes. Cannel coals occur in Ohio, Indiana, and eastern Kentucky. This cannel coal owes its peculiar fatty nature to the material from which it is derived, it being supposed to have resulted from the accumulation of the spores of lycopod trees, and their conversion to jelly-like masses by bacteria in the fresh-water marshes of those ancient days.
[Anthracite]
hard coal
Anthracite coal is hard, black, has a luster, and breaks with a conchoidal fracture. It contains but a low percentage of volatile matter, and so burns with a short flame, and less smoke, than is the case with the other coals. It is always associated with folded rocks, and appears to have been formed as a result of the combined pressure and the higher temperatures, which accompanied mountain making. Still the temperature was not high enough to metamorphose the adjacent rocks. Most of our anthracite comes from northeastern Pennsylvania.
[Carbonite]
Carbonite is natural coke. It occurs in coal seams which have been cut by dikes or intrusions of igneous rocks, the coal having been thus coked by natural processes. It is not vesicular like artificial coke, for which reason it is not useful as a fuel. Some carbonite is found in the Cerillos coal field of New Mexico, in Colorado, and Virginia.
[Jet]
Jet is a dense variety of lignite, a fossil wood of black color, which takes a high polish and cuts easily into various ornamental shapes. It has been used for ornaments since early ancient times, beads of jet being found in the early bronze period in England, the supply probably coming from the Yorkshire coast, whence the principal supply comes even to the present day. In Switzerland and Belgium it was used still earlier, even as far back as the Palæolithic age. Jet seems then to have had a talismanic value, and to have been worn to protect the owner. About 700 A.D. crosses and rosaries began to be made of jet, the custom starting at Whitby Abbey, the material being obtained nearby, so that it came to be known as “Whitby jet,” and in the eighteenth century became very popular. In recent times it has been used mostly as jewelry suitable for mourning.
[Amber]
[Pl. 61]
Amber is a gum which oozed from coniferous trees and was petrified. It is associated with lignite beds of middle Tertiary age. It is usually pale-yellow in color, but at times has a reddish or brownish tinge, and is more or less transparent. It occurs in rounded irregular lumps, up to ten pounds in weight, though most pieces are smaller; and is mostly picked up along certain coasts where it is washed ashore by the waves. Since the earliest records amber has been cast up on the shores of the Baltic, and it was used by peoples as early as in the stone age for ornaments and amulets. It has been found among the remains of the cave dwellers of Switzerland, in Assyrian and Egyptian ruins of prehistoric age, and in Mycenæ in the prehistoric graves of the Greeks, the first recorded reference to it being in Homer, and the Greek name for amber being elektron from which our word electricity comes. All these finds were of Baltic amber which was doubtless gathered and traded by those early men. Even down to the present many men make their living, riding along the shore at low tide and hunting for the amber washed ashore by the waves. As early as 1860 the German geologists concluded that the source of the amber must be lignite beds outcropping beneath the sea level, and started mining for the amber with fair success, so that today two types of Baltic amber are distinguished, “sea stone” which is washed ashore, and “mine stone” taken from the mines. Beside the Baltic locality, it is found along the shores of the Adriatic, Sicily, France, China, and occasionally of North America.
Some pieces of amber are found with insects inclosed and preserved almost as perfectly as if collected yesterday. They were apparently entangled in the gum while still viscid and completely embedded, before fossilization.