FIG. 5. - Boghead from New South Wales, X500.
From what precedes it seems to result, then, that anthracite is in a much less appreciable state of preservation than cannel coal, and that it is only rarely, and according to locality, that we can discover vegetable organs in it. Soft coal comes nearer to amorphous carbon. Boghead appears to be of an entirely different character (Fig. 5, magnified X300). It is easily reduced to a thin transparent plate, and shows itself to be formed of a multitude of very small lenses, differing in size and shape, and much more transparent than the bands that separate them. In the interior of these lenses we distinguish very fine lines radiating from the center and afterward branching several times. The ramifications are lost in the periphery amid fine granulations that resemble spores. We might say that we here had to do with numerous mycelia moulded in a slightly colored resin. Preparations made from New South Wales and Autun boghead presented the same aspect.
If boghead was derived from the carbonization of parts that were soluble, or that became so through maceration, and were made insoluble at a given moment by carbonization, we can understand the very peculiar aspect that this combustible presents when it is seen under the microscope.
The following figures were made in order to show the details of anatomical structure that are still visible in coal, and to permit of estimating the shrinkage that the organic substance has undergone in becoming converted into coal.
It is not rare in coal mines to find fragments of wood, of which a portion has been preserved by carbonates of iron and lime, and another portion converted into coal. This being the case, it was considered of interest to ascertain whether the carbonized portion had preserved a structure that was still recognizable, and, in such an event, to compare this structure with that of the portion of the specimen that was preserved in all its details by mineralization.
FIG. 6. - Arthropitus gallica, St. Etienne; transverse section, X200.
Fig. 6 shows a transverse section of a specimen of Arthropitus Gallica found under such conditions. The region marked c is carbonized; the organic elements of the wood-cells, tracheae, etc., have undergone but little change in shape. Moreover, no change at all exists in the internal parts of another specimen (Fig. 8), where we easily distinguish by their form and dimensions the ligneous cells, aa, and the elements, bb, of the wood itself.
FIG. 8. - Arthropitus gallica, St. Etienne; transverse section through the carbonized part.
In the region, b, of Fig. 6, the ligneous elements have undergone an evident change of form, and the walls have been broken. This region, already filled by petrifying salts, but not completely hardened, has not been able to resist, as the region, a, an external pressure, and has become more or less misshapened. As for the not yet mineralized external portion, c, it has completely given way under the pressure, the walls of the different organic elements have come into contact, the calcareous or other salts have been expressed, and this region exhibits the aspect of ordinary coal, while at the same time preserving a little more hardness on account of the small quantity of mineral salts that has remained in them despite the compression.
From the standpoint of carbonization there seems to us but little difference between the organic elements that occupy the region, a, and those that occupy b. If the former had not been filled with hardened petrifying matter, they would have been compressed and flattened like those of region c, and would have given a compact and brilliant coal, having very likely before petrifaction reached the same degree of carbonization as the latter. The layer of coal in contact with the carbonized or silicified part of the specimens is due, then, to a compression of the organic elements already chemically carbonized, but in which the mineral matter was not yet hardened and was able to escape.
FIG. 7. - Arthropitus gallica, St. Etienne; tangential longitudinal section.
If this be so, we ought to find the remains of organic structure in this region c. In fact, on referring to Fig. 7, which represents a tangential, longitudinal section of the same specimen, we perceive at ab a ligneous duct and some unchanged tracheae situated in the carbonized region, and then at c the same elements, though flattened, in which, however, we still clearly distinguish the bands of the tracheae; at d is found a trachea whose contents were already solidified, and which has not been flattened; then, near the surface, in the region, e, the pressure having been greater, it is no longer possible to recognize traces of organization in a tangential section. In a large number of cases, the fact that the coal does not seem to be organized must be due to the too great compression that the carbonized cells and vessels have undergone when yet soft and elastic, at the time this slow but continuous pressure was being exerted.
It also became of interest to find out whether, through the very fact of carbonization, the dimensions of the organic elements had perceptibly varied - a sort of research that presents certain difficulties. At present we have no living plant that is comparable, even remotely, with those that grew during the coal epoch. Moreover, the organic elements have absolutely nothing constant in their dimensions.