composed of a limited number of species appropriate by their nature to the growth and composition of the coal, while a different kind of vegetation inhabited dry or high ground? All the specimens of fossil plants found in beds of sandstone, even where no trace of coal is found, represent the same species, or at least the same genera, as those found in connection with coal beds.
Fig. 13.—1. Carpolithes. 2. Trigonocarpus. 3. Cardiocarpus.
Hard fruits, resembling nuts and named trigonocarpum, carpolithes, &c., are locally abundant in sandstone, but sometimes the shale of the coal has them in plenty. Nothing is known as to the relation of these fruits, which have never been found attached to any kind of vegetable remains. They may have been floated by water from a distance, and may therefore, as has been supposed, represent species of plants not found in the true coal basins; but this is very doubtful. The composition of the coal flora, like the formation of the coal over immense surfaces, indicates a great degree of humidity in the atmosphere. The vapors covering the land, extending over the whole emerged surface, developed everywhere the same kind of vegetation, diversified only in its exuberance according to the composition, especially the solidity of the ground. Even the vegetation of the hills, if there were any in proximity to the coal basins, should have been exposed to the same influence. In the southern islands, where fern trees are most common, these plants everywhere follow or ascend the slopes of the mountains in exact concordance with the line of the fogs; therefore a general and permanent degree of humidity should have influenced the same uniform character for the vegetation of the whole land. This seems still better evidenced by the identity of the essential vegetable types in the carboniferous formation of the whole world. On this subject, however, as on other questions concerning the distribution of the old floras, there is still a great deal of uncertainty, especially caused by insufficiency of materials for observation.
COAL PRODUCTS. The readiness shown by the elements of coal to enter into new combinations where it is exposed to an increase of temperature, and the great variety of the combinations obtained under different degrees of heat, or by the admission or exclusion of air, indicate the close relation of coal to the elements of the vegetable kingdom. It consists of carbon, hydrogen, oxygen, and nitrogen, which make up the great bulk of vegetable matters, and these show the same disposition as in the plants themselves to separate from existing combinations and enter into new. The number of the new products thus formed is almost unlimited. They differ from one another, and from the original substance from which they are generated, as do those obtained in the processes of vegetable fermentation. When heat is applied without access of air, the vapor of water set free acts upon the existing combinations of the elements. These are broken up, and hydrogen and oxygen are evolved under the most favorable circumstances, in their nascent state, to form new compounds with the carbon present, the characters of which vary greatly with the temperature. The process is called dry distillation. By keeping the retorts in which it is conducted at a cherry-red heat, the gases used for illumination are most copiously evolved, the tar itself being decomposed and converted into gaseous matters. (See Gas.) But if the object is to obtain the coal oils, paraffine, benzole, and other hydrocarbons of this
| TABLE REPRESENTING THE PRODUCTS DERIVED FROM THE VOLATILE MATTERS OBTAINED IN THE DISTILLATION OF COAL. | |||||
| |||||
| |||||
| |||||
| |||||
| |||||
Liquids condensed and collected in tar cistern. Gases and vapors Coal tar gives, on redistillation with water or steam Distilled tar; which affords, on further distillation Pitch; distilled in ovens, affords Dead, or pitch oil, consisting of Crude coal tar naphtha, which consists of Ammonical liquor, containing Water, hydrosulphate, carbonate, muriate, acetate, hydrocyanate, sulphite, and gallate of ammonia. Gases and vapors separated in line purifier Gases and vapors separated sometimes by additional chemical agents Gases and vapors conducted to gas-holder Olefiant gas, Vapors of hydrocarbons, Light carburetted hydrogen, Hydrogen, Carbonic oxide. Nitrogen, Vapor of bisulphuret of carbon, Ammonia.
