In a certain number of specimens the diminution in volume of the tracheæ is less than that that we have observed in the same organs of corresponding genera. The quantity of oxygen and hydrogen that they contain is greater, and seems to bring them near the lignites.
We cannot attribute these differences to the nature of the plants converted into coal, since we have just seen that they are the same in the one case as in the other. Neither does time count for anything here, since, according to accepted ideas, the burial having been longer, the carbonization ought to have been more perfect, while the contrary is the case.
If we admit (1) that vegetable remains alter more and more through maceration in ordinary water and in certain mineral waters; (2) that, beginning with their burial in sufficiently thick strata of clay and sand, their chemical composition scarcely varies any further; and (3) that these are important changes only as regards their physical properties, due to loss of water and compression, we succeed quite easily in learning what has occurred.
In fact, when, as a consequence of the aforesaid alteration, the vegetable matter had taken the chemical composition that we find in the less advanced coal of the pebbles, it was in the first place covered with sand and protected against further destruction, and it gradually acquired the physical properties that we now find in it. At the period that channels were formed, the coal was torn from the beds in fragments, and these latter were rolled about for a time, sometimes being broken, and then covered anew, and this too at the same time as were the plants less advanced in composition that we meet with at the same level. These latter, being like them protected against ulterior alteration, we now find less advanced in carbonization (notwithstanding their more ancient origin) than the other vegetable fragments that were converted into coal after them, but that were more thoroughly altered at the time of burial.
There are yet a few other important deductions to be made from the foregoing facts: (1) the same coal basin may, at the same level, contain fragments of coal of very different ages; (2) its contour may have been much modified owing to the ravines made by the water which transported the ancient parts into the lowest regions of the basin; and (3) finally, since the most recent sandstones and schists of the same basin may contain coal which is more ancient, but which is formed from the same species of plants that we find at this more recent level, we must admit that the conversion of the vegetable tissues into coal was relatively rapid, and far from requiring an enormous length of time, as we are generally led to believe.
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
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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.