performed by the action of heat, or of other causes volatilizing the hydrogen, or by the contact of oxygen converting it into water, cannot now be determined by any facts that we are acquainted with. Experiments on the induration of the essential oils may throw some light on this question. It will here perhaps be remarked, that there is a difference in the substances as they are produced artificially by the distillation of coal, and as they are found in nature. Thus, for example, the artificial petroleum of coal differs from that of nature, in being much more soluble in alcohol. Yet, this circumstance may arise from the insensible gradation of difference which I have above remarked in the similar compounds, and thus in the series of gradation, specimens absolutely corresponding, whether artificial or natural, may exhibit the same chemical characters.
Thus, as I have shown that there is a sort of gradation from naphtha to asphaltum, through a series of undefinable petrolea, so this analogy may be extended to the next general variety of the bitumens, coal.
The several varieties of coal are supposed to consist of charcoal and asphaltum, or of charcoal and bitumen, combined in as many different proportions. Charcoal is undoubtedly found mixed with coal, but it does not appear correct to consider pit coal as either a mixture or combination of any bitumen with charcoal. The action of naphtha on its varieties, often none and always sparing, shows that bitumen does net exist in it in a mixed state. It will be more consonant to the analogies of the other bituminous substances, to consider coal in its several varieties as a bitumen, varying in its composition, from the fattest specimens of Newcastle to the driest of Kilkenny, and owing its compactness, as well as the other modifications which it exhibits, to the peculiar circumstances under which it has been formed, the changes it may subsequently have undergone,