Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/56

Rh occurrence, since, although the softening takes place at a low temperature, still it marks the point at which destructive distillation commences, and hydrocarbons both of a solid and gaseous character are formed. That nothing analagous to bitumen exists in coals is proved by the fact that the ordinary solvents for bituminous substances, such as bisul phide of carbon and benzole, have no effect upon them, as would be the case if they contained bitumen soluble in these re-agents. The term is, however, a convenient one, and one whose use is almost a necessity, from its having an almost universal currency among coal miners. The propor tion of carbon in bituminous coals may vary from 80 to 90 per cent. the amount being highest as they approach the character of anthracite, and least in those which are nearest to lignites. The amount of hydrogen is from 4i to 6 per cent., while the oxygen may vary within much wider limits, or from about 3 to 14 per cent. These variations in com position are attended with corresponding differences in quali ties, which are distinguished by special names. Thus the semi-anthracitic coals of South Wales are known as dry&quot; or &quot;steam coals,&quot; being especially valuable for use in marine steam-boilers, as they burn more readily than anthracite and with a larger amount of flame, while giving out a great amount of heat, and practically without producing smoke. Coals richer in hydrogen, on the other hand, are more useful for burning in open fires smiths forges and furnaces where a long flame is required. The excess of hydrogen in a coal, above the amount necessary to combine with its oxygen to form water, is known as &quot; disposable &quot; hydrogen, and is a measure of the TABLE I. Elementary Composition of Coal (the figures denote the amounts per cent}. Composition calculated exclusive of Water, Sulphur, and Ash. Localities. Specific Gravity. Carbon. Hydrogen. Oxygen. Nitrogen. Sulphur. Ash. Water. Carbon. Hydrogen. 0. and N. A ntUracite. 1. South Wales 1-392 1-462 90-39 90-45 82-70 75-49 86-80 78-65 78-57 79-90 80-07 63.10 82-67 79-34 63-29 66-31 50-72 3-28 2-43 1-41 4-73 4-25 4-65 5-29 4-85 5-53 8-91 9-14 10-41 4-98 5-63 5-34 2-98 2-45 o- 6- 3- 13- 12-88 12-75 8-08 7- 8&quot; 4- 26- 22-86 33-18 0-83 85 78 06 36 1-84 0-64 2-12 25 19 33 24 0-57 2-80 0-91 10-35 1-21 0-83 0-55 0-39 0-20 1-50 0-96 5-32 2-36 0-90 1-61 4-67 3-75 10-67 4-40 2-49 1-03 1-66 2-70 19-78 8-49 2-36 7-86 2-00 0-94 1-12 0-66 1-13 0-91 93-54 94-89 97-34 86-78 92-24 80-67 79-70 81-45 85-48 79-61 82-67 83-80 66-97 69-53 55-11 3-39 2-54 1-66 5-43 4-51 4-76 5-37 4-92 5-90 11-24 9-14 10-99 5-27 5-90 5-80 3-82 2-57 1-00 7-79 3-25 14-5 14-9 13-63 8-62 9-15 8-19 5-21 27-76 24-57 39 09 2 Pennsylvania 3. Peru Bituminous Steam mid Coking Coal. 4. Eisca, South Wales 5. Aberdare, Do 6. Hartley, Northumberl d 7. Dudley, Staffordshire ... 8. Strauitzen, Styria 1-278 Cannel or Gas Coal. 9. Wigan, Lancashire 10. Boghead, Scotland 11. Albertite, Nova Scotia.. 12. Tasmanite, Van Die- ) man s Land. ( 1-276 1-18 1-100 Lignite and Broivn Coal. 13. Cologne 14. Bovey, Devonshire 15. Trifail, Styria fitness of the coal for use in gas-making. This excess is greatest in what we know as cannel coal, the Lancashire kennel or candle coal, so named from the bright light it gives out when burning. This, although of very small value as fuel, commands a specially high price for gas- making. Canuel is more compact and duller than ordi nary coal, and can be wrought in the lathe and polished. These properties are most highly developed in the substance known as jet, which is a variety of cannel found in the lower oolitic strata of Yorkshire, and is almost entirely used for ornamental purposes, the whole quantity produced near Whitby, together with a further supply from Spain, being manufactured into articles of jewellery at that town. _ When coal is heated to redness out of contact with the air, the more volatile constituents, water, hydrogen, oxygen, and nitrogen are expelled, a portion of the carbon bein^ also volatilized in the form of hydrocarbons and carbonic oxide, the greater part, however, remaining behind, to gether with all the mineral matter or ash, in the form of coke, or, as it is also called, &quot;fixed carbon.&quot; The proportion of this residue is greatest in the more anthracitic or drier coals, but a more valuable product is yielded by those richer in hydrogen. Very important distinctions those of caking or non-caking are founded on the behaviour of coals when subjected to the process of coking. The former class undergo an incipient fusion or softening when heated, so that the fragments coalesce and yield a compact coke, while the latter (also called free-burning) preserve their form, producing a coke which is only serviceable when made from large pieces of coal, the smaller pieces being incoherent and of no value. The reason of this difference is not clearly made out, as non-caking coals are often of very similar ultimate chemical composition as those in which the caking property is very highly developed. As matter of experience, it is found that caking coals lose that property when exposed to the action of the air for a lengthened period, or by heating to about 300 C., and that the dust or slack of non-caking coal may, in some instances, be converted into a coherent cake by exposing it suddenly to a very high temperature. Lignite or brown coal includes all varieties which are Liguit intermediate in properties between wood and coals of the older formations. A coal of this kind is generally to be distinguished by its brown colour, either in mass or in the blacker varieties in the streak. The proportion of carbon is comparatively low, usually not exceeding 70 per cent., while the oxygen and hygroscopic water are much higher than in true coals. The property of caking or yielding a coherent cake is usually absent, and the ash is often verj&amp;gt; high. The specific gravity is low when not brought up by an excessive amount of earthy matter. Sometimes it is almost pasty, and crumbles to powder when dried, so as to be susceptible of use as a pigment, forming the colour known as Cologne earth, which resembles umber or