for the differences which characterize the several bitumens, from naphtha placed at one extreme, to anthracite placed at the other. The chasm in this series from asphalt um to fat coal, is in fact, rather apparent than real, being more properly a mechanical or accidental, than a chemical or essential one. I cannot here avoid taking notice of the very loose experiments of Mr. Kirwan on the analysis of coal, (which consisted in projecting portions of coal on melted nitre) as his deductions are at war with this view of the subject, although not more so than with all chemical reasoning. They were founded on an assumption, that coal was carbon impregnated sometimes with maltha and sometimes with asphaltum—a distinction quite unnecessary if the supposition were true. If we conceive coal to be compounded in this way, it would be more obvious to consider it as formed of carbon and petroleum, since by a regulated heat it can be separated into those two substances. The theory of the experiment is equally assumed and the conclusions equally groundless, when it is inferred that of this compound, (coal) the carbon alone possesses, the power of decomposing the nitre, and that the proportions of these supposed ingredients may thus be determined. The varying temperature of the nitre, would necessarily produce considerable variations and uncertainty in its action, and in the consequent accuracy of the results; but it is plain, that the effect of this contrivance was to separate by a sort of distillation the petroleum which fire elicits from coal, and that the method could neither be so accurate as that of ordinary distillation, from the greater irregularities to which it was subject and the difficulty of conducting it, and that it proves nothing with regard to the composition or nature of coal. In the examination of maltha, and asphaltum, the defect of this method is still more apparent. If heat and flame be applied to these bitumens, with, access of air, they are either consumed without leaving any carbon,
Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/26
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Dr. Mac Culloch on certain products