inflammability, I have proved to be a different substance, and that the igneous theory of the origin of coal will receive no support from them, as far at least as relates to the conversion of vegetable matter into bitumen. I need take no notice of the modifications derived from a mixture of animal matter in these experiments, as it is not my desire to enter into a discussion of the general question, but to state such chemical facts as arose in the experiments I undertook. And since it is certain that vegetables alone are competent to the production of bitumen, and that the geological history of coal does not justify a supposition that animals have been concerned in its production, it is perhaps unnecessary to investigate that question further.
To satisfy myself whether any essential chemical difference would result from the experiments performed by simple heat, and those performed by heat under pressure, I repeated these trials, by heating wood in close gun barrels, introducing occasionally lime, clay, or other matters to absorb the acid generated, and give the greater chance for the disoxygenation and bituminization of the wood. But the produce only differed from that of the experiments in open vessels, by the circumstance which is mentioned in Sir James Hall's, paper; namely, the mixture of a porous charcoal, or a half destroyed vegetable structure. In all cases the bituminous looking matter was vegetable tar, not bitumen.
Thus far then perhaps we are justified in concluding that the action of water, and not that of fire, has converted the vegetable matters into bitumen. It is another question to determine how that bituminous matter in its several forms of peat or lignite, has been converted into coal, into a substance differing mechanically, rather than chemically from it, if, without misleading, I may use the contrast of these terms.