WHAT IS COAL ?

West Coast Times, Issue 320, 2 October 1866, Page 1 (Supplement)

WHAT IS COAL ?

' (VEOJI VAGi? 3 GEOLOGY FOE GENEBAL

HEADERS.)

| What is co.il ?is a question more satis- | factority answered 'by a little roundabout explanation than by a direct reply. To i say that coal is altered and minerajised ; vegetable matter is true ; but the defini- ! tion is too curt to be" readily intelligible. I Everybody' knows something of peat and peat-mosses. Well, this peat is simp'y i coal in its first stage of development. Were i the peat-moss submerged and covered by deposits oi- mud, and clay, and sand, it would in the course of time undergo important chemical changes, by which part of its gaseous contents (oxygen, hydrogen, &c.) would be discharged, but the mass, reduced to a compact coaly substance known as lignite or brown coals. Such brown coals are abundant in many countries (Germany, Austria, New Zealand, &c.) and worked for economical purposes ; and were they subjected to still further changes, they would, in the course of ages, become converted into shining stony coals Jike those which are now raised so largely f^rom the coal-fields of Great Britain. The truth is, coal occurs in the earth's crust in' every shade of development, from the peat-mosses, and swamp-growths still in process of accumulation on the surface, , down through the tertiary brown-coals to the bituminous stone-coals of the secondary and primary periods, and from these again to the still older non-bituminous anthracites and graphites. All, in fact, had a similar origin. They were mere, vegetable masses that have undergone different degrees of mineralisation—the recent vegetable full of volatile matters, andjigru'te less so, thft bituminous coals giving off smoke and flame,, the anthracites barely smoking, and the graphite masses of pure debitumenised carbon. They are all coals, and belong to the same -family — those in the younger formation still retaining much of their vegetable structure and full of volatile matter, while those in the older formations have seemingly lost all traces of structure, and -have been all but deprived of their volatile constituents. But even when not obvious to the naked eye it can generally be rendered apparent by submitting thin ti ansparent slices to the microscope. By these means the vegetable origin of the irost compact and glistening coal ia often revealed as clearly as the tissiies in living plants, and thus the observer is enabled to determine not only the organic nature of the mass, but the botanical peculiarities of the order concerned in its formation. Like oil mixed rocks, however, coal presents itself in many varieties. We cannot conceive of vegetable matter (whether drifted or grown hi sittt) being associated with ' sedimentaiy strata without its being mixed more or less with the earthy impurities of these sediments. These impurities, according to their amount, v must necessarily confer on different coals different structures, different aspects, and different qualities. Besides, varieties will arise from the conditions of the vegetable mass itself, according as it may have been embedded while fresh or been long exposed to atmospheric decay, according as, it may have been suddenly 'covered up, or long exposed to maceration and comminution in water, and notably also, according to the nature of the plants composing the mass. These varieties, according to -their struc= ture, texture, and qualities, are known as caking-coal. which is soft and tender in the mass, like that of Newcastle, and swells and cakes together in 'burning ; splint or shale coal, which is slaty in texture, like most Scotch coals, and burns free and open ; cannel or parrot coal, which is compact and jet-like in texture, spirits, and crackles when thrown suddenly on the fire, but when ignited burns with a clear and candle-like flame, and from its composition is chiefly used in gas manufacture ;s; s and coarse, foliated, or cubic coal, which is more or less .soft, breaks up into large, square blocks, and contains m general a large percentage of impurities. Between these varieties, there is, of course, gradation — coals so pure as to leave only one or two per cent, of ash, others so mixed as to yield from ten to thirty per cent, and Tnany so impure as to be unfit for fuel, and to pass into shales, more or less bituminous.