It seems evident however, that no very great change is wanting to convert the one of these into the other.
The question so much agitated, of the conversion of vegetables into coal, would appear to receive some illustration from the history of the compound which I have been describing, and since (as I shall by and by show) it has actually been confounded with bitumen, and has been adduced as an instance of the artificial production of coal by the action of fire, I shall make no apology for pursuing this subject. Indeed the general chemical resemblance between the mineral bitumens and this vegetable bitumen, if it may be so called, is so striking, that we may, at first sight, be easily led to suppose that the same agent has produced both, and excuse the mistakes which seem to have occurred on this subject. But a cursory view of the several substances which have been classed under the head of bitumens, may enable us to form a clearer notion of the limited extent of this analogy, at the same time that it will perhaps assist us in correcting some errors which have crept into our arrangements of them.
It is necessary to separate from the bitumens three or four mineral substances, which differ completely both in chemical and ordinary characters, but which are approximated to each other by some general resemblance. These are, amber, mellilite, and the subterraneous resins of Cologne, Bovey, and Highgate. The two first are more nearly associated by the property they have of yielding a peculiar acid; and of the three last, it may perhaps be fairly doubted, whether they are more entitled to be ranked among the mineral substances strictly so called, than the other vegetable matters which are found in alluvial soils.
The nature and relations of naphtha, petroleum, maltha, and asphaltum, will, I trust, appear sufficiently clear from what I have
