COAL
IN its most general sense the term coal includes all varieties of carbonaceous minerals used as fuel, but it is now usual in England to restrict it to the particular varie ties of such minerals occurring in the older Carboniferous formations. On the continent of Europe it is customary to consider coal as divisible into two great classes, depend ing upon differences of colour, namely, brown coal, corre spending to the term " lignite " used in England and France, and black or stone coal, which is equivalent to coal as understood in England. Stone coal is also a local English term, but with a signification restricted to the substance known by mineralogists as anthracite. In old English writings the terms pit-coal and sea-coal are com monly used. These have reference to the mode in which the mineral is obtained, and the manner in which it is transported to market.
The root kol is common to all the Teutonic nations, while in French and other Romance languages derivatives of the Latin carbo are used, e.g., charbon de terre. In France and Belgium, however, a peculiar word, kouille, is generally used to signify mineral coal. This word is supposed to be derived from the Walloon hole, correspond ing to the mediaeval Latin kulla;. Littre" suggests that it may be related to the Gothic haurja, coal. Anthracite is from the Greek a.v6pa, and the term lithanthrax, stone coal, still survives, with the same meaning in the Italian litantrace.
It must be borne in mind that the signification now attached to the word coal is different from that which for merly obtained when wood was the only fuel in general use. Coal then meant the carbonaceous residue obtained in the destructive distillation of wood, or what is known as charcoal, and the name collier was applied indifferently to both coal-miners and charcoal-burners.
The spelling "cole" was generally used up to the middle of the 17th century, when it was gradually superseded by the modern form, "coal." . The plural, coals, seems to have been used from a very early period to signify the broken fragments of the mineral as prepared for use.
Coal is an amorphous substance of variable composition, and therefore cannot be as strictly defined as a crystallized or definite mineral can. It varies in colour from a light brown in the newest lignites to a pure black, often with a bluish or yellowish tint in the more compact anthracite of the older formations. It is opaque, except in exceed ingly thin slices, such as made for microscopic investigation, which are imperfectly transparent, and of a dark brown colour by transmitted light. The streak is black in an thracite, but more or less brown in the softer varieties. The maximum hardness is from 2 5 to 3 in anthracite and hard bituminous coals, but considerably less in lignites, which are nearly as soft as rotten wood. A greater hardness is due to the presence of earthy impurities. The densest anthracite is often of a semi-metallic lustre, resembling somewhat that of graphite. Bright, glance, or pitch coal is another brilliant variety, brittle, and breaking into regu lar fragments of a black colour and pitchy lustre. Lignite and cannel are usually dull and earthy, and of an irregular fracture, the latter being much tougher than the black coal. Some lignites are, however, quite as brilliant as anthracite; cannel and jet may be turned in the lathe, and are suscep tible of taking a brilliant polish. The specific gravity is highest in anthracite and lowest in lignite, bituminous coals giving intermediate values (see Table I.) As a rule the density increases with the amount of carbon, but in some instances a very high specific gravity is due to inter mixed earthy matters, which may be separated by me chanical treatment.
Coal is perfectly amorphous, the nearest approach to any thing like crystalline structure being a compound fibrous grouping resembling that of gypsum or arragonite, which occurs in some of the steam coals of S. Wales, and is locally known as " cone in cone," but no definite form or arrangement can be made out of the fibres. The impres sions of leaves,_ woody fibre, and other vegetable remains are to be considered as pseudomorphs in coaly matter of the original plant structures, and do not actually represent the structure of the coal itself. There is generally a ten dency in coals towards cleaving into cubical or prismatic blocks, but sometimes the cohesion between the particles is so feeble that the mass breaks up into dust when struck. These peculiarities of structure may vary very considerably within small areas ; and the position of the divisional planes or cleats with reference to the mass, and the pro portion of small coal or slack to the larger fragments when the coal is broken up by cutting-tools, are points of great importance in the working of coal on a large scale.
The divisional planes often contain small films of other minerals, the commonest being calcite, gypsum, and iron pyrites, but in some cases zeolitic minerals and galena have been observed. Salt, in the form of brine, is some times present in coal. Some years ago a weak brine occur ring in this way was utilized at a bathing establishment at Ashby-de-la-Zouche. Hydrocarbons, such as petroleum, bitumen, paraffin, &c., are also found occasionally in coal, but more generally in the associated sandstones and lime stones of the Carboniferous formation. Gases, consisting principally of light carburetted hydrogen or marsh gas, are often present in considerable quantity in coal, in a dissolved or occluded state, and the evolution of these upon exposure to the air, especially when a sudden diminution of atmospheric pressure takes place, constitutes one of the most formidable dangers that the coal miner has to encounter.
The classification of the different kinds of coal may be considered from various points of view, such as their chemical composition, their behaviour when subjected to heat or when burnt, and their geological position and origin. They all contain carbon, hydrogen, oxygen, and nitrogen, forming the carbonaceous or combustible portion, and some quantity of mineral matter, which remains after combustion as a residue or "ash." As the amount of ash varies very considerably in different coals, and stands in no relation to the proportion of the other constituents, it is necessary in forming a chemical classification to compute the results of analysis after deduction of the ash and hygroscopic water. Examples of analyses treated in this manner are furnished in the last column of Table I., from which it will be seen that the nearest approach to pure carbon is fur nished by anthracite, which contains above 90 per cent. Anthracite. This class of coal burns with a very small amount of flame, producing intense local heat and no smoke. It is especially used for drying hops and malt, and in air or blast fur naces where a high temperature is required, but is not suited for reverberatory furnaces. The American anthracite is largely used in iron smelting, as is also that of South Wales, but to a less extent, the latter having the disad vantageous property of decrepitating when first heated.
The most important class of coals is that generally known as bituminous, from their property of softening or undergoing an apparent fusion when heated to a temperature far below that at which actual combustion takes place. This term is founded on a misapprehension of the nature of the
