CHUGIN.j (J O In Nassau and Bavaria woody structure is very common, and it is from tbis circumstance that the term lignite is derived. The best varieties are black and pitchy in lustre, or even bright and scarcely to be distinguished from true coals. These kinds are most common in Eastern Europe. Lignites, as a rule, are generally found in strata of a newer geological age, but there are many instances of perfect coals being found in such strata. By the term "ash" is understood the mineral matter re maining unconsumed after the complete combustion of the carbonaceous portion of a coal. This represents part of the mineral matter present in the plants from which the coal was originally formed, with such further addition by infiltration and mechanical admixture as may have arisen during consolidation and subsequent changes. The com position of the ashes of different coals is subject to consi derable variation, as will be seen by the following list of analyses : TABLE II. Composition of the Ashes of Coals.
| a iS . 1 o 1 a o a> o a 22 Us 1 | ifl 1 & a o f! o 2< 1 99 3 feO ^ SU w
H True Coals. Dowlais, South Wales 39-64 39-20 11-84 1-81 2-58 3-01 98-03 Ebbw Vale, do. 53-00 35-01 3-94 2-20 4 -89 0-88 99-92 Konigsgrube. Silesia. 55-41 18-95 16-06 3-21 1-87 2-05 1-73 0-36 99-IH Ohio 44-60 41-10 7 40 3-61 1-28 1-82 69 0-29 IOU 69 Lignites. Helmstadt, Saxony... 17-27 11-57 5-57 23-67 2-58, 2-64 33-83 97-13 Edele"ney, Hungary.. S6-01 23-07 5-05 15-62 3 64 2-38, 12-35 98-12 The composition of the ash of true coal approximates to that of a fire-clay, allowance being made for lime, which may be present either as carbonate or sulphate, and for sulphuric acid. The latter is derived mainly from iron pyrites, which yields sulphate by combustion. An indi cation of the character of the ash of a coal is afforded by its colour, white ash coals being generally freer from sulphur than those containing iron pyrites, which yield a red ash. There are, however, several striking exceptions, as for instance in the anthracite from. Peru, given in Table I., which contains more than 10 per cent, of sulphur, and yields but a very small percentage of a white ash. In this coal, as Avell as in the lignite of Tasmania, known as white coal or Tasmanite, the sulphur occurs in organic com bination, but is so firmly held that it can only be very partially expelled, even by exposure to a very high and continued heating out of contact with the air. An anthracite occurring in connection with the old volcanic rocks of Arthur s Seat, Edinburgh, which contains a large amount of sulphur in proportion to the ash, has been found to behave in a similar manner. Under ordinary conditions, from |- to ^ of the whole amount of sulphur in a coal is volatilized during combustion, the remaining | to |- being found in the ash. The amount of water present in freshly raised coals varies very considerably. It is generally largest in lignites, which may sometimes contain 30 per cent, or even more, while in the coals of the coal measures it does not usually exceed from 5 to 10 per cent. The loss of weight by exposure to the atmosphere from drying may be from -g- to of the total amount of water contained. Coal is undoubtedly the result of the transformation of vegetable matter, mainly woody fibre, by the partial eli mination of oxygen and hydrogen giving rise to a substance richer in carbon than the original wood, the mineral matter being modified simultaneously by the almost entire removal of the alkalies and lime, and the addition of materials analagous iu composition to clay, as will be seen by comparing the analyses in Table IL The ^ L 47 following table, given by Percy, shows the relative pro portions of the different components of mineral fuels. TABLE III. Composition of Fuels (assuming Carbon - 100). Caibon. Hydro gen. Oxygen. Disposable Hydrogen. Wood 100 12-18 83-07 rso Peat 100 9 85 55 67 2 89 Lignite 100 8 37 42 42 3 07 fiiiek Coal, S. Staffordshire.. Hartley Steam Coal 100 100 6-12 5 91 21-23 18-32 3-47 3 62 South Wales Coals . . 100 475 5-23 4 09 Amevicaii Anthracite . 100 2 84 1 74 2 63 Mohr has computed that the transformation of wood into coal is attended with a loss of about 75 per cent, in weight ; and, having regard to the difference in density of the two substances, the volume of the coal can only be from -jY to i of the woody fibre from which it is derived. The nature of the change is essentially a slow oxidation under water or any covering sufficient to protect the dead wood from the direct action of atmospheric air, as -in the latter case the vegetable mould or humus would be pro duced. The products of such decomposition vary with the length of time and the nature of the plants acted on, and in the case of anthracite the change is so great that no portion of the original plant structure can be recognized, at the same time the density and conductivity for heat and electricity are increased. This, however, is a case of metamorphosis analogous to the transformation of sedi mentary into crystalline rocks, the extreme term of such metamorphosis being the production of graphite or plum bago. Daubre"e has shown that wood may be converted into anthracite by exposure to the actiun of superheated water at a temperature of 400 C. The plants concerned in the production of coal vary very Coal-pi > considerably in different geological periods. In the coal d " cui measures proper, acrogens, ferns, equise turns, and similar * allied forms are most abundant. It is stated by some observers that entire beds of coal are sometimes made up of the spores of ferns. This, however, appears to depend upon the inspection of microscopic sections, and may not be capable of rigorous quantitative demonstration. In the coals of newer date exogenous wood and leaves are more common than in those of the coal measures ; the former also contain resins, sometimes in considerable quantity. The number of species of land plants in the British sedimentary formations, which may be taken as a measure of the comparative prevalence of coal in the different series, is as follows : Devonian strata 9 species, Carboniferous do 320 Permian do 20 Triassic do 9 Lias and Oolitic do 160 Purbeckand Wealden do 38 Cretaceous do ^ Tertiary do 224 The most generally received opinion is that much if not all coal results from the transformation of plants upon the site of their growth. The principal evidence in favour of such a supposition is afforded by the common occur rence of a bed of clay, the so-called " wider-clay," con taining the roots of plants, representing the old soil, immediately below every coal seam a fact that was first pointed out by the late Sir W. E. Logan in South Wales. In Yorkshire the same thing is observed in the siliceous rock called ganister occurring in similar positions, show ing that the coal plants grew there upon sandy soils. The action of water in bringing down drift wood may have also contributed some material, but much less than
the local growth. This may probably have been concerned