Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/25

Rh, or the substances with which it has accidentally been mixed. The power of yielding naphtha on distillation, is rather to be resorted to as the distinction between the one end and the other of the series; and it would be surely equally correct to call coal a compound of charcoal and naphtha, as a compound of charcoal and asphaltum.

Its several varieties will moreover be found to vary from each other by containing greater or less proportions of carbon, compared with their other ingredients; just as in asphaltum the relative proportions of the hydrogen, azote, and oxygen, to the carbon, are found to differ from those which constitute petroleum or naphtha.

The last link of the chain of coal (chemically considered) is anthracite, which contains only carbon, if we reckon the earths mixed with it as adventitious matter. So the last result of the distillation of asphaltum is charcoal, and the intermediate steps through which asphaltum passes in its progress to charcoal, resemble precisely the corresponding changes which occur in the distillation of coal till coak is formed, and confirm by their chemical analogy the view here held forth of the chemical composition of coal, and the gradation to be traced in nature from fat coal to anthracite. If asphaltum be subjected to distillation it gives petroleum. By degrees its solubility in naphta diminishes, in consequence of its carbonaceous ingredient becoming more disproportioned to its hydrogen. At a particular period of this distillation it will be found to resemble fat coal; by and by, it resembles blind coal, and gives no stain to naphtha; ultimately, pure charcoal remains. All these bituminous compounds may therefore properly be said to belong to one genus or family, of which the principal chemical ingredients are carbon and hydrogen, and it is chiefly to the difference in the relative proportions of those two substances that we are to look