The Story in a Lump of Coal
How Did the Coal Get Into the Coal Mines?
The heavy black mineral called coal, which we burn in our stoves and furnaces, and use to heat the boilers of our engines was formed from trees and plants of various sorts. Most of the coal was formed thousands of years ago at a time when the atmosphere that envelopes the earth contained a much larger proportion of carbonic acid gas than it does now, and the climate of all regions of the earth was much warmer than it now is. This period was known as the carboniferous age, that is, the coal-making age, and its atmospheric conditions, favored the growth of plants, so that the earth was covered with great forests, of trees, giant ferns, and other plants, many of which are no longer found on the earth. In the warm, moist, and carbon-laden atmosphere of that period the growth of all kinds of plants was rapid and luxuriant, and as fast as old trees fell and partially decayed, others grew up in their places. In this way, thick layers of vegetable matter were formed over the soil in which the plants grew. In many places, where these beds were formed, the surface of the earth became depressed and the water of the sea flowed over the beds of vegetable matter.
Sediment of various kinds was deposited over the vegetable matter, and in the course of centuries the sediment was transformed into rock.
After the formation of the covering of sediment, the decay of the vegetable matter was checked, but a slow change of another kind was brought about by the pressure of the sedimentary deposits and the heat to which the plant remains were subjected. The hydrogen and oxygen which constituted the greater part of the plant substance was driven off and the carbon left behind. This change took place very gradually, through periods so long that we can only guess at their duration, but we know that many beds of coal were formed from layers of vegetable matter that were covered up many thousand years ago.
The coal first formed and submitted longest to pressure is known as hard coal, or anthracite. It is pure black, or has a bluish metallic luster. Its specific gravity is 1.46; which is about the same as that of hard wood. Anthracite contains from 90 to 94 per cent. of carbon, the remainder being composed of hydrogen, oxygen, and ash.
Hard coal may be called the ideal fuel and is especially adapted to domestic heating purposes. It burns without smoke and produces great heat. There is no soot deposit upon the walls of chimneys, and in good stoves or furnaces the small amount of gas given off by it is consumed. Anthracite is the least abundant of all the varieties of coal and is much more costly than the other varieties. For this reason it is not much used in manufacturing.
The coal formed later is very different in composition and is called bituminous or soft coal. Its name is derived from the fact that it contains a soft substance called bitumen, which oozes out of the coal when heat is applied to it. Soft coal contains from 75 to 85 per cent. of carbon, some traces of sulphur, and a larger percentage of oxygen and hydrogen than anthracite. When soft coal is heated in a closed vessel or retort, the hydrogen and oxygen, in combination with some carbon, are driven off.
HOW A COAL MINE LOOKS INSIDE
Shaft gate. One of the two cages in the shaft has just brought the men to the surface; the other is at the bottom. Safety gate resting on top of cage covers top of shaft when cage is down, as shown at right.
Section showing Anthracite Seams. Coal is shown black; rock and dirt lighter; shaft tunnels and workings, white. Upper part of “Mammoth” seam is stripped and quarried.
Lignite mine in Texas. Loaded mine cars ready to go to surface.
Soft coal is black, and upon smooth surfaces it is glossy. It lacks the bluish luster sometimes seen in hard coal and is much softer and more easily broken. When handled it blackens the hands more than hard coal does. In this kind of coal are frequently seen the outlines of leaves and stems of plants that enter into its formation. Occasionally, trunks of trees with roots extending down into the clay below the bed of coal have been found.
Soft coal has a specific gravity of 1.27. It burns with a yellow flame which is larger than the flame from hard coal, but it does not emit so high a degree of heat. Combustion, generally imperfect, gives rise to offensive gases and to black smoke that concentrates in the air and falls to the ground as soot, which blackens buildings, and, in winter, noticeably discolors the snow.
The formation of lignite has been observed in the timbers of some old mines in Europe. In some of these mines wooden pillars have been supporting the rocks above for four hundred years or longer, and in that time the pressure of the rocks and other influences acting upon the wood of the pillars have caused it to become transformed into a brown substance resembling lignite. This fact tends to confirm the theory of coal formation stated at the beginning of this article. The proportion of carbon in lignite is never above 70 per cent., and the ash indicates the presence of considerable earthy matter. It is chiefly used in those forms of manufacture where a hot fire is not required. In Europe it is used, to some extent, in heating the houses of the poorer classes.
Peat is regarded as the latest of the coal formations. In it, the change in the vegetable matter has not extended beyond merely covering it, and subjecting it to slight pressure.
Peat is formed in marshy soils where there is a considerable growth of plants that are constantly undergoing partial decay and becoming covered by water. It consists of the roots and stems of the plants matted together and mingled with some earthy material. When freshly dug out of the bog or marsh in which it was formed there is always a quantity of water in it, the amount being greatest in the peat found nearest the surface and least in that at the bottom of the bed, where the peat is not very different in appearance from lignite.
Peat is used for fuel where wood is scarce and coal is high in price. Recent experiments in saturating peat with petroleum, have shown that in this way a form of fuel may be produced for which considerable value is claimed. Its manufacture is confined to Southern Russia, where peat is plentiful and petroleum is cheap.
Why Does Firedamp Explode in a Safety Lamp Without Producing an Explosion of the Gas With Which the Lamp Is Surrounded?
The passing of the flame from the lamp to the outside air is prevented by the gauze. This splits the burning gas into little streamlets (784 to each square inch of gauze), which are cooled below the point of ignition, that is, are extinguished by coming in contact with the metal of the gauze, so that the flame does not pass outside the lamp. In some cases the explosion may be so great as to force the flame through the gauze and thus ignite the gas outside.
Are There Any Conditions Under Which It Would Not Be Safe to Use a Safety Lamp?
THE DANGERS TO THE MINERS
The underground conditions affecting the safety of the lamp are exposure in air-currents of high velocity by reason of which the flame may be blown through or against the gauze, or exposure for too great a time to mixtures of air and gas which will burn within the lamp and thus heat the gauze. The dangerous velocity of air-currents begins at about 500 feet a minute, but varies with the type of lamp, some being much less sensitive to air-currents of high velocity than others. Other conditions under which the lamp is not safe concern the lamp itself or the one using it. The lamp is dangerous in the hands of inexperienced persons or when the gauze is dirty or broken. If the gauze is dirty, that portion absorbs the heat and may become hot enough to ignite the outside gas; naturally any holes in the gauze will pass the flame.
The safety lamp when left too long in air containing much explosive gas may cause an explosion, and it is extinguished by certain unbreathable gases. The electric lamp burns safely regardless of the atmosphere, but gives no warning of poisonous or explosive gases. It is often used by rescue men wearing oxygen helmets to enter mines full of poisonous gases after explosions.
The safety lamp is dangerous when there is a hole in the gauze that will permit the passage of flame to the outside, or when the gauze is dirty, so that any particular spot may be overheated, or when the velocity of the air is so great that the flame is blown through the gauze, or (generally) when in the hands of an inexperienced person. The unbonneted Davy lamp is not safe where the velocity of the air exceeds 360 feet per minute. The velocity with which the air strikes a lamp carried against it is increased by the amount equal to the rate at which the fireboss travels. If he walks at the rate of, say, 4 miles an hour or 352 feet a minute (on the gangways he will usually have to move faster than this to make his rounds on time) he will create by his own motion (and in still air) a velocity practically the same as that at which the unbonneted Davy is considered unsafe.
History of the Safety Lamp
The safety lamp, the miner’s faithful and indispensable companion at his dangerous work, has been, heretofore, considered as the invention of the famous English scientist, Humphrey Davy, though the name of George Stephenson, of locomotive fame, has also been mentioned in this connection. Both came out with their inventions about the same time, but neither of them is the real inventor of the safety lamp; for there was, as proven by Wilhelm Nieman, a safety lamp in existence two years before Davy’s invention became known. It was not inferior to the latter, but rather surpassed it in illuminating power. Previous to this, all the precaution employed for the prevention of the threatening dangers of firedamp had been quite incomplete. One tried to thoroughly ventilate the mines by fastening a burning torch to a large pole, which was pushed ahead and exploded the gases. This was extremely dangerous work which, in the Middle Ages, was generally done by a criminal, in order that he might atone for his crimes, or by a penitent for the benefit of mankind. The attempt to substitute for the open light phosphorescent substances, encased in glass, was not much of a success. An improvement was the so-called steel mill, invented about 1750 by Carlyle Spedding, manager of a mine. This steel mill consisted of a steel wheel which was put into rapid motion by means of a crank. By pressing a firestone against the fast revolving wheel, an incessant shower of sparks was produced giving a fairly good and absolutely safe illumination. However, the running expenses of his apparatus, which necessitated the continual services of one man, were very high; for instance, the expenditure for light in a coal mine near Newcastle in the year 1816 amounted to about $200 per week. Nevertheless, the steel mill was very much appreciated and in use for a long time, only to be slowly supplanted by the safety lamp.
THE MAN WHO INVENTED THE SAFETY LAMP
At the beginning of the nineteenth century the existing coal mines were worked to the limit and the catastrophies, caused by firedamp, increased in an alarming manner. In fact the distress was so great that in 1812 a society for the prevention of mine disasters was formed at Sutherland, and the origin of the safety lamp can be traced back to the efforts and labors of this organization. Dr. William Reid Clanny, a retired ship’s surgeon, was probably the first to undertake the task (in the year 1808), which he successfully finished with energy and skill. He concentrated his efforts at first on the separation of the flames from the surrounding atmosphere, but he did not succeed till the latter part of 1812, when he constructed a lamp that seemed to meet all requirements. The report of this invention was submitted to the Royal Society of London, May 20, 1813, and was printed in the minutes of that academy. The casing of this original safety lamp was closed at the top and bottom by two open water tanks; the air was pumped in by means of bellows and, passing in and out, had to go through both these reservoirs which acted as valves, so to speak. The lamp proved to be absolutely safe and was successfully introduced by the management of Herrington Mill pit mine. The clumsy parts of this apparatus were eliminated by its inventor by various improvements. The so-called steam safety lamp was completed in December, 1815, and installed in several mines. In the meanwhile, two competitors made their appearance. George Stephenson had finished his lamp October 21, 1815, and Davy published his first experiments November 9, 1815, in the Transactions of the Royal Society of London. Clanny’s lamp, nevertheless, stood the test in the face of this competition, through its much superior illuminating power, and more particularly as it still continued to burn when the Davy and Stephenson lamps had gone out. To Clanny, therefore, belongs the distinction, in the history of invention, of having constructed the first reliable safety lamp.