To what modification has the vegetation of the ancient world been subjected to attain that carbonised state, which constitutes coal? The submerged plants would, at first, be a light, spongy mass, in all respects resembling the peat-moss of our moors and marshes. While under water, and afterwards, when covered with sediment, these vegetable masses underwent a partial decomposition--a moist, putrefactive fermentation, accompanied by the production of much carburetted hydrogen and carbonic acid gas. In this way, the hydrogen escaping in the form of carburetted hydrogen, and the oxygen in the form of carbonic acid gas, the carbon became more concentrated, and coal was ultimately formed. This emission of carburetted hydrogen gas would, probably, continue after the peat-beds were buried beneath the strata which were deposited and accumulated upon them. The mere weight and pressure of the superincumbent mass, continued at an increasing ratio during a long series of ages, have given to the coal its density and compact state. The heat emanating from the interior of the globe would, also, exercise a great influence upon the final result. It is to these two causes--that is to say, to pressure and to the central heat--that we may attribute the differences which exist in the mineral characters of various kinds of coal. The inferior beds are _drier_ and more compact than the upper ones; or less bituminous, because their mineralisation has been completed under the influence of a higher temperature, and at the same time under a greater pressure. An experiment, attempted for the first time in 1833, at Sain-Bel, afterwards repeated by M. Cagniard de la Tour, and completed at Saint-Etienne by M. Baroulier in 1858, fully demonstrates the process by which coal was formed. These gentlemen succeeded in producing a very compact coal artificially, by subjecting wood and other vegetable substances to the double influence of heat and pressure combined. The apparatus employed for this experiment by M. Baroulier, at Saint-Etienne, allowed the exposure of the strongly compressed vegetable matter enveloped in moist clay, to the influence of a long-continued temperature of from 200° to 300° Centigrade. This apparatus, without being absolutely closed, offered obstacles to the escape of gases or vapours in such a manner that the decomposition of the organic matters took place in the medium saturated with moisture, and under a pressure which prevented the escape of the elements of which it was composed. By placing in these conditions the sawdust of various kinds of wood, products were obtained which resembled in many respects, sometimes brilliant shining coal, and at others a dull coal. These differences, moreover, varied with the conditions of the experiment and the nature of the wood employed; thus explaining the striped appearance of coal when composed alternately of shining and dull veins.