SCIENTIFIC. THE ORIGIN OF COAL.

Otago Witness, Issue 1899, 13 April 1888, Page 36

SCIENTIFIC. THE ORIGIN OF COAL.

(Written for the Witness by F. G. S,)

We have recently read in the columns of this journal a few facts relating to the products of coal, and have seen that these substances are not only of varying qualities, but of characteristics so utterly dissimilar that we can hardly credit them with a common origin. It now follows, as a natural sequence, that we should inquire — firstly, what is this substance which is capable of yielding so bountifully for the use of man 1 and, secondly, whence was it originally derived ?

The first question would appear to be easily answered :

"WHAT IS COAL?" Surely there is not an intelligent person living who does not think he could give a satisfactory answer, and yet the exact definition, legal and scientific together, has actually never been satisfactorily settled, and in the year 1853 a very important trial took place in Edinburgh, the object being to determine whether a certain mineral was or was not entitled to the name of coal. An immense amount of evidence was called on each side ; chemists, geologists, and commercial men contradicted each other as freely and positively as witnesses always do, and still the matter was not definitely settled. Dr Percy, the eminent metallurgist, has suggested the following definition : — "Coal is a solid, stratified, mineral combustible substance, varying in colour from dark brown to black, opaque except in extremely thin slices, brittle, not fusible without decomposition, not sensibly soluble in ether, benzol, or turpentine, not containing sufficient earthy matter to render it incapable of being applied with advantage as a source of heat either in ordinary fireplaces or in furnaces." • Now, if we take a piece of ,coal and analyse it, we find that besides a varying proportion of incombustible matter,, .known as ash, we have in 20 parts about 3 of oxygen and nitrogen, 1 of hydrogen, and 16 of carbon. Out of these four elements, therefore, are obtained all the varied derivatives of coal. If we look at the kingdom of nature, we shall see that there are some common substances containing the same elements in very nearly the same proportions as the above. Taking the inferior varieties of coal — lignite, for instance — we find that there is less carbon and much more oxygen and nitrogen, and if we examine into the chemical constituents of some timbers we find a most striking resemblance, and that the woods, so to speak, take another step downhill by having still less carbon, and more oxygen, nitrogen, and hydrogen.

If we tabulate the results, the gradations become still more apparent (omitting ash in each case) : — Carbon. Hydrogen. Oxygen and Nitrogen. Average bituminous : coal 80*7 . 4'B 145 . . Lignite ... ... 67*9 , s*B 26-3, , Peat .„ ... 61*0 s*B ' , 332 .. \jTood- 50-2 ""6-2 ' . ', ; .43-6 ; Nobody can resist the evidence ' afiidfcdecl&y

this table that wood, by becoming' gradually carbonised, may become peat ; that peat by the same process may change to lignite, and lignite to bituminous coal ; and if additional proof were required, we have particulars from America of an anthracite coal mine/ in which some mine timber was sealed up for upwards of 30 years,' during the whole of which time the mine was on fire, smouldering gradually. When the mine was opened up, in working through one "of the piles of refuse which filled the workings from floor to roof,' the miners found a pit prop which exhibited very peculiar characteristics. The lower portion was practically unaltered ; halfway up the wood was charred externally, and for some distance above this it consisted of soft, crumbling charcoal. The top, which showed evidence of great compression, had changed to a substance resembling mineral coal, and the resemblance was even more complete in an overlying wedge, which had a cross fracture like that of anthracite ,coal, a similar hardness, a specific gravity of T3B, and a jet-black colour. The analysis proved, however, that it was not anthracite. We have here the conversion in a very short period of timber into a substance resembling coal, the conditions being very heavy pressure from the superincumbent mass of rocky and a smouldering heat, with exclusion of air.

But it is not only to the destructive analysis of the chemist that we have to look for an insight into mineral substances, for < the researches of modern microscopical science have revealed the facfc that in very thin slices many substances which yield, when in their opaque condition, no clue to their origin, DISCLOSE EVEKY DETAIL OF THEIE STRUO-

TURK, and unfold their history to us in a very marvellous manner. Let us therefore take a small fragment of coal, saw it into a thin slice, fasten this on to a slip of glass, grind it down slowly and carefully until it becomes transparent, cover it with a little piece of very thin glass, and put it under a powerful microscope, we shall find in many cases that we have before us a mass of minute rounded bodies known as spores, which were shed by a tree called Lepidodendron, or scaly tree ; so called because its trunk was covered with lozenge-shaped markings be?ring a certain , resemblance to the scales of a fish. It is not •in all cases that the spores are present,, neither are all substances containing them coals, but they are frequently to be found and are believed by 3ome, but not by ali, .geologists to form a large portion of many of our known seams. ' Let us examine a peat bed where the process 9f growth and decay is now going on. | On the surface we find a confused and matted mass of mos3, and roots, and various plants ; lower down the peat becomes more compact, until' at a considerable depth we come upon a dense black mass, which is indistinguishable from some lignites. -On examining

SOME OF THE LIGNITES OF OTASO, we find large trunks of trees, masses of leave 3 and twigs, matted together, and containing lumps of fossil resins known as ambrite, the whole forming a very excellent fuel for local purposes. This resin, which is no older in appearance than the kauri gum of the North Island, is at present either thrown on one side as waste, or filled up with the lignite, but the day will no doubt come when it will be carefully picked out and sold for the manufacture of varnish. The fossil gums of the world are gradually becoming less and less plentiful, and as the better known and consequently longer- worked deposits' become exhausted attention will be no doubt called to the inferior qualities, such as we have in large quantities in Otago. That this resin is the fossil gum of some coniferous tree there can be no doubt, and the ambrite of to-day is merely the exudation from the trees of an ancient forest which grew on the slopes of a f ormei Otago many thousands of years before the first human being « set foot on these shores. The trees which we cut out of the iignites, well preserved as they are, and showing every detail of , their woody structure, lived and died in an age which, though geologically comparatively moderr, is in the limits of time as known to man so remote that the oldest record of the Egyptians and Assyrians become in comparison but the chronicles of yesterday.

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