The pitch coal, which has a brownish-black colour, and is generally found massive in plates, the bovey coal, called brown coal, and bituminous wood, with the anthracite coal, and some others of lesser note, form the remaining varieties of coal.
When coal is employed in fire-works, it is to be pulverized, and sifted in the usual way. For some purposes it is preferred to charcoal, in consequence of the bitumen it contains, which appears to contribute to the rapidity of the combustion. It is to be observed, that, as the base of coal is carbon, its action is the same as charcoal, and therefore, by producing the same effects, or nearly so, as charcoal itself, the phenomena it presents are analogous. As 12.709 parts of carbon, according to Kirwan, are required to decompose 100 parts of nitrate of potassa, we may readily ascertain the quantity of real carbon in any specimen of coal. According to Kirwan, 50 grains of Kilkenny coal will decompose 480 grains of nitrate of potassa, from which it is inferred, that ten grains would have decomposed 96 of nitrate of potassa, precisely the same quantity of charcoal, which would have produced the same effect. Therefore, Kilkenny coal is composed almost entirely of carbon. Cannel coal, when treated in the same manner with nitrate of potassa, left a residuum of 3.12 in the hundred parts of earthy ashes; and 66.5 of it were required to decompose 480 grains of nitrate of potassa, but 50 of charcoal would have been sufficient. From this experiment, it appears, that 66.5 grains of cannel coal contain 50 grains of charcoal, and 2.08 of earth; the remaining 14.42 grains must be bitumen. In a similar manner, by knowing the quantity of coal required to decompose a given quantity of nitrate of potassa, when melted in a crucible, the quantity of carbon in any variety of this substance may be ascertained.
With respect to the earthy and metallic ingredients of coal, we may ascertain them by burning the coal, with free access of air. What remains unburnt must be considered an impurity. Its weight may be ascertained, and its nature by analysis. As the object, however, is generally to determine the relative proportion of combustible matter, or carbon, which different species of coal are capable of yielding, that point may be determined in the manner already stated.
That coal originates from vegetables, whatever opinion may be formed to the contrary, we may fairly infer from a variety of vegetable remains, and impressions of animals that are both found in the strata of coal, and in earthy strata above and below them. Of its submarine origin, there can also be no doubt; or why do we find in it shells, the impression of fish, and other productions of the ocean? That coals grow like vegetables, an opinion with the uninformed, is contrary to fact, and the nature of things.
We may notice, in this place, another substance which sometimes is found partially carbonized; we mean turf.
Turf or peat, obtained from morasses, consists of a multitude or congeries of vegetable fibres, partly in a decomposed state, and is frequently so inflammable as to inflame by a spark. Very extensive morasses are found in some countries from which the inhabitants are supplied with fuel. Some improvements in the manner of preparing turf for use, have been made; that of charring it in kilns is one. By this process it kindles sooner, burns with less air, and forms a moderate and uniform fire, without much smoke, though it is not so lasting as that produced by turf. The method of reducing turf to coal is still practised in some parts of Bohemia, Silesia, and Upper Saxony, which was first proposed in 1669, by John Joachim Becher, who also recommended, at that time, a process for depriving coals of their sulphur, by burning them in an oven, and the use of the oil procured from them. What are our modern patents on this subject? What are lord Dundonald's coke ovens and coal tar? Are they original? Boyle (Usefulness of Natural Philosophy,) speaks of Becher's invention. Anderson, (History of Commerce,) however, observes, that something of the kind was attempted before Becher's time; for in the year 1627, John Hacket and Octavius Strada obtained a patent for their invention of rendering coals as "useful as wood for fuel in houses, without hurting any thing by their smoke."
With respect to turf, it appears that Hans Charles von Carlowitz, to save wood, introduced the use of it in Saxony, in the smelting houses, in 1708.
Turf has been known for a long time. It was used from the earliest periods, in the greater part of Lower Saxony, and throughout the Netherlands; as is fully proved by Pliny's account of the Chauci, who inhabited that part of Germany. Pliny (Hist. Nat. lib. xvi, c. i.) observes, that they pressed together with their hands, a kind of mossy earth which they dried by the wind rather than by the sun, and which they used, not only for cooking their victuals, but also for warming their bodies. We also read that a morass in Thessaly, having become dry, took fire, and the same thing ensued in some part of Russia, where a morass burned several days and did much damage. Very dry turf is nearly as inflammable as spunk, and when prepared with nitre, has been used for the same purpose. See [Pyrotechnical sponge.]
Ure (Chemical Dictionary) observes, that "turf has been charred lately in France, it is said by a peculiar process, &c." The truth is, that the charring of turf is by no means a recent invention, as we stated above. Sonnini (Journal, &c.) says, that it is superior to wood. It kindles slower than charcoal of wood, but emits more flame and burns longer. In a gold-smith's furnace, it fused eleven ounces of gold in eight minutes, while wood charcoal required sixteen.