175. In the petroleum lake of the Island of Trinidad, described _Phil Trans._ 1789, the petroleum evidently exudes from the rock, and is collected in a variety of springs in the bottom, after which it hardens, and acquires the consistency of pitch. The manner, therefore, in which petroleum exists in the strata, is very consistent with the idea of its having been introduced in the form of a hot vapour. Even amber appears to have some relation to coal. It is found in the unconsolidated earth in Prussia and Pomerania; but I am not sure whether this earth is _travelled_ or not. In the same earth where the amber is found, there is often a mixture of coaly matter, which burns in the fire; it is apparently fibrous, and has been considered as a kind of fossil-wood.[84] [Footnote 84: Buffon, Hist. Nat. des Mineraux, tom. ii. p. 5.] These circumstances make out a connection between the purer bitumens and ordinary coal; but do not, it must be acknowledged, establish any thing with respect to the more immediate relation, supposed in this theory to exist between them and blind coal. It is probable, indeed, that, to discover any facts of that kind, the natural history of both substances must be more carefully examined; the natural history of blind coal, in particular, has hitherto been but little attended to. 176. A fact is mentioned by Mr Kirwan, which must not be regarded as less valuable for being adverse to this theory. It is, that neither petroleum, nor any fossil bitumen, is found in the vicinity of the Kilkenny coal, as might be expected, if that coal was deprived of its bituminous part by subterraneous distillation.[85] This, however, admits of explanation. Though a general connection, on the above hypothesis, might be expected between bitumens and infusible coal, we cannot look for it in every instance. The heat which drove off the bitumen from one part of a stratum of coal, may only have forced it to a colder part of the same stratum; and thus, in separating it from one portion of carbonic matter, may have united it to another. Blind coal may therefore be found where no bitumen has been actually extricated. In like manner, bitumen may have been separated, where the coal was not reduced to the state of coke, as a part of the bitumen only may have been driven off, and enough left to prevent the coal from becoming absolutely infusible. [Footnote 85: Geol. Essays, p. 473.] It should be considered too, if the bitumen was really separated, and forced, in the state of vapour, into some argillaceous or limestone stratum, that this stratum may have been wasted and worn away long ago, so that the bitumen it contained may have entirely disappeared. It does not therefore necessarily follow, that, wherever we find blind coal, there also we should discover some of the purer bitumens. NOTE X. § 37.