If, then, lignites have not become soft coal, and if the latter has not become anthracite, it is not that time was wanting, but climatic conditions and environment. Most analyses of specimens of coal have been made up to the present with fragments so selected as to give a mean composition of the mass; it is rare that trouble has been taken to select bits of wood, bark, etc., of the same plant, determined in advance by means of thin and transparent sections in order to assure the chemist of the sole origin and of the absolute purity of the coal submitted to analysis. This void has been partially fitted, and we give in the following table the results published by Mr. Carnot of analyses made of different portions of plants previously determined by us:
Carbon Hydrogen Oxygen Nitrogen 1. Calamodendron (5 specimens) 82.95 4.78 11.89 0.48 2. Cordaites (4 specimens) 82.94 4.88 11.84 0.44 3. Lepidodendron (3 specimens) 83.28 4.88 11.45 0.39 4. Psaronius (4 specimens) 81.64 4.80 13.11 0.44 \----v----/ 5. Ptychopteris (1 specimen) 80.62 4.85 14.53 6. Megaphyton (1 specimen) 83.37 4.40 12.23
As seen from this table, the elementary composition of the various specimens is nearly the same, notwithstanding that the selection was made from among plants that are widely separated in the botanical scale, or from among very different parts of plants. In fact, with Numbers 1 and 2 the analysis was made solely of the wood, and with No. 3 only of the prosenchymatous and suberose parts of the bark. Here we remark a slight increase in carbon, as should be the case. With No. 4 the analysis was of the roots and the parenchymatous tissue that descends along the stem, and with No. 6 of the bark and small roots. One will remark here again a slight increase in the proportion of carbon, as was to be foreseen. The elementary composition found nearly corresponds with that of the coal taken from the large Commentry deposit.
Carbon. Hydrogen. Oxygen and Nitrogen. Regnault 82.92 5.39 11.78 Mr Carnot 83.21 5.57 11.22
Although the chemical composition is nearly the same, the manner in which the different species or fragments of vegetables behave under distillation is quite different.
In fact, according to Mr. Carnot, the plants already cited furnish the following results on distillation:
Volatile Fixed Coke. matters. residue. Calamodendron 35.5 64.7 Well agglomerated. Cordaites 42.1 57.8 Quite porous. Lepidodendron 34.7 55.3 Well agglomerated. Psaronius 29.4 60.5 Slightly porous. Ptychopteris 39.4 60.5 Megaphyton 35.5 64.5 Well agglomerated. Coal of the Great Bed 40.5 59.5 Slightly porous.
These differences in the proportions of volatile substances, of fixed residua, and of density in the coke obtained seem to be in harmony with the primitive organic nature of the carbonized tissues. We know, in fact, that the wood of the Calamodendrons is composed of alternately radiating bands formed of ligneous and thick walled prosenchymatous tissue, while the wood of Cordaites, which is less dense, recalls that of certain coniferae of the present day (Araucariae).
We have remarked above that the portions of Lepidodendron analyzed belonged to that part of the bark that was considerably thickened and lignefied. So too the portion of the Megaphyton that was submitted to distillation was the external part of the hard bark, formed of hypodermic fibers and traversed by small roots. The Psaronius, on the contrary, was represented by a mixture of roots and of parenchymatous tissue in which they descend along the trunk.
It results from these remarks that we may admit that those parts of the vegetable that are ordinarily hard, compact, and profoundly lignefied furnish a compact coke and relatively less volatile matter, while the tissues that are usually not much lignefied, or are parenchymatous, give a bubbly, porous coke and a larger quantity of gas. The influence of the varied mode of grouping of the elements in the primitive tissues is again found, then, even after carbonization, and is shown by the notable differences in the quantities and physical properties of the products of distillation.
The elementary chemical composition, which is perceptibly the same in the specimens isolated in the sandstones and in those taken from the great deposit, demonstrates that the difference in composition of the environment serving as gangue did not have a great influence upon the definitive state of the coal, a conclusion that we had already reached upon examining the structure and properties of the coal pebbles.
We may get an idea of the nearly similar composition of the coal produced by very different plants or parts thereof, in remarking that as the cells, fibers, and vessels are formed of cellulose, and some of them isomeric, the difference in composition is especially connected with the contents of the cells, canals, etc., such as protoplasm, oils, resins, gums, sugars, and various acids, various incrustations, etc. After the prolonged action of water that was more or less mineralized and of multiple organisms, matters that were soluble, or that were rendered so by maceration, were removed, and the organic skeletons of the different plants were brought to a nearly similar centesimal composition representing the carbonized derivatives of the cellulose and its isomers. The vegetable debris thus transformed, but still resistant and elastic, were the ones that were petrified in the mineral waters or covered with sand and clay. Under the influence of gradual pressure, and of a desiccation brought about by it, and by a rising of the ground, the walls of the organic elements came into contact, and the physical properties that we now see gradually made their appearance.
The waters derived from a prolonged steeping of vegetables, and charged with all the soluble principles extracted therefrom, have, after their sojourn in a proper medium, deposited the carbonized residua that have themselves become soluble, and have there formed masses of combustibles of a different composition from that resulting from the skeletons of plants, such as cannel coal, pitch coal, boghead, etc.