Scientific American Supplement, No. 447, July 26, 1884

by Various

I dare say many of you have had occasion to walk across peat bogs. The peat bog is a great mass of vegetable matter, which is every year growing thicker and thicker; and underneath it there is almost always a bed of thin clay, in look very much like the underclays, and this thin clay is penetrated by the rootlets of the moss forming the peat, exactly the same way as the underclays of the coal measures are penetrated by the stigmaria and its rootlets. But you must not suppose that the plants out of which coal was formed were exactly the same low type of moss which forms our present peat bogs. However, it is pretty certain that they were for the most part of a loose, succulent texture, and that they grew very rapidly indeed.

You will have noticed that there is one step more wanted to make good this theory of the growth of coal on the spot where we now find it. The coal is found, as already described, interbedded with shales and sandstones. These shales and sandstones, as shown, were formed beneath the water of the sea, and as long as they remained there of course no plants could grow upon them. The question is, How was the land surface formed for the growth of plants? It must have been formed in some way or other by the sea bottom having been raised above the level of the water. Now, we have distinct proof in many cases that elevation of the sea bottom and depression of the land is now going on in many parts of the earth's surface. And, therefore, we shall be assuming nothing beyond the range of experience if we say that such elevations and depressions went on during coal measure times. The coal measure times must have been times during which the same spot was now below the sea, and now dry land, over and over again. There was a land surface on which plants grew fast and multiplied rapidly, and as they died fell and accumulated in a great heap of dead vegetable matter. After a time this layer of vegetable matter was slowly and gently let down beneath the waters of the sea--so slowly that the water flowing over it did not, as a rule, disturb the loose, pasty mass; and then, by the method I have described to you, shales and sandstones were deposited on the top of this mass of dead vegetable matter. By their weight they compressed it, and by certain chemical changes (which we have not time to go into this evening) this dense mass of vegetable matter became converted into coal. After a time the shales and sandstones which had been piled above this stuff, which was to form coal for the future, were again elevated to form a land surface; upon this another forest sprang up, and by its decay produced another mass of vegetable matter fit to form coal. This again was let down below the water, more shales and sandstones were deposited on the top, and this process went on over and over again till the whole mass of our present coal measures was formed. You will now see how it is that trees are so seldom found in an upright position in the coal beds. As the land went down, they would in very many cases be toppled over by the water as it flowed against them, or their base would be rotted, and they would then either fall or be blown over; that is the reason why in most cases they are found lying flat on the roof of the coal bed. But in a few cases, when the depression was very gentle and gradual, the trees were not overthrown, and the shales and sandstones accumulated round them and preserved them in the position in which they grew.

I do not know that I can point out to you anything nowadays that exactly resembles the state of things that must have gone on during the times these coal measures were being formed; but there are a great many cases strikingly analogous to them. I shall not attempt to describe them to you, but may just mention the mangrove swamps that very often fringe the coasts in the tropics, and the cypress swamps of the Mississippi, which are so well described by Sir Charles Lyell in his recent works; also the great Dismal Swamp of Virginia, which appears to me to furnish the nearest analogue to the state of things that existed during coal measure times.

Having explained the way in which coal measures have been formed, we will now take a brief sketch of its uses and products. The year 1259 is memorable in the annals of coal mining. Hitherto the mineral had not been raised by authority, but in that year Henry III. granted a charter to the freemen of Newcastle-on-Tyne for liberty to dig coal, and a considerable export trade was established with London, and it speedily became an article among the various manufacturers of the metropolis. But its popularity was but short lived. An impression became general that the smoke arising therefrom contaminated the atmosphere and was injurious to public health. Years of experience have proved the fallacy of the imputation; but in 1306 the outcry became so general that a proclamation was issued by Edward I forbidding the use of the offending fuel, and authorizing the destruction of all furnaces, etc., of those persons who should persist in using it. Prejudice gradually gave way as the value of the fossil fuel became better known, and from that time downward its use has become more and more extended down to the enormous extent of our present trade. The annual increase in the production of coal in the British Isles since the year 1854 is over 2½ million tons. In that year the coal produce was about 65 million tons, and it has grown up to the year 1880 to the grand total of 135 million tons.

We will now deal with some of the uses that this valuable black diamond is now being put to. It is, in the first place, the center of all our enterprise and prosperity, and upon it depends our chief success as a manufacturing nation for the future. When it is exhausted we shall have to look forward to the condition of things which now obtains in those regions where there is no coal--that is to say, instead of our being a nation full of manufacturing and mercantile enterprise, a great nation to which all the people of the earth resort, we shall be merely a people who live for ourselves by the cultivation of the ground. The duration of our coal fields has been ascertained within certain limits. Mr. Hall, an accomplished geologist, tells us that in England at the present time we have a stock of coal sufficient for our consumption for no less than 1,000 years. On the other hand, Professor Jevons, whose opinion is worthy of the very greatest weight on such questions, calculates that 100 years is about the tenure of our coal fields, according to the present rate of increase in the consumption. Whichever view we take, sooner or later the end must ultimately come when the coal will be exhausted; when the great mainspring of our commercial enterprise will be gone, and we shall revert to that condition in which we were before the coal fields were worked. In this point of view, therefore, coal has an especial interest to us as engineers. If coal is important in this direction, it is no less important in a purely scientific point of view, apart from any mercantile end.

The chemist or physicist will tell you the wondrous story that the black substance which you burn is simply so much light and heat and motion borrowed from the sun and invested in the tissues of plants. He will tell you that when you sit round your firesides the flame which enlivens you, and the gas which enables you to read, and which civilizes you, is nothing in the world but so much sunlight and so much sunheat bottled up in the tissues of vegetables, and simply reproduced in your grates and gas burners. Very few persons, I am afraid, realize this, which is one of the many stories which science in its higher teachings shows us--one of those fairy tales which are the result of the most careful scientific investigation. Of the hundred and odd million tons of coal which we in this country burn in the course of a year, about 20,000,000 tons are thrown on our house fires; 30,000,000 tons find their way into our blast furnaces, or are otherwise used in the smelting and manufacture of metals; about 48,000,000 are burnt under steam boilers; 6,000,000 are used in gas-making; while the remainder is consumed in potteries, glass works, brick and lime kilns, chemical works, and other sundries which I need not speak of.

To go into the chemistry of coal is quite sufficient to take up more time than I have at my disposal this evening, therefore I will briefly touch on a few of the main points. Coal gas is made, as you are all aware, by heating coal or cannel, which is the special form of coal most valued for the purpose, on account of the high quality of gas it produces in cylindrical fireclay retorts.

The by-products obtained in the manufacture of coal gas, the tar and the ammonia water, are nowadays scarcely less important than the coal gas itself. The ammonia water furnishes large quantities of salts to be used, among other applications, as food for plants. We thus restore to-day to our vegetation the nitrogen which existed in plants of primeval times. The tar, black and noisome though it be, is a marvelous product, by the reason of scores of beautiful substances which are concealed within it.

Coal tar when distilled yields three main products: naphtha, dead oil, and pitch or asphalt. The naphtha on redistillation yields benzine, from which are prepared some of our most beautiful dyes; the dead oil, as the less volatile portion is termed, furnishes carbolic acid, used as a disinfectant and antiseptic, together with anthracene and naphthaline; all three substances the starting points of new series of coloring matters.

This discovery of these coloring matters marks an era in the history of chemical science; it exercised an extraordinary influence on the development of organic chemistry. Theoretical and applied chemistry were knit together in closer union than ever, and dye followed dye in quick succession; after mauve came magenta, and in close attendance followed a brilliant train of reds, yellows, oranges, greens, blues, and violets; in fact, all the simple and beautiful colors of the rainbow.