Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/20

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Dr. Mac Culloch on certain products

resin and the wood pitch which I have been describing. To this admixture, and not to that of adventitious charcoal produced in combustion, is the black colour of common pitch owing.

The analogy between this wood pitch and the bitumens is equally striking, and the preceding history of these compounds will throw light on the several varieties of the bituminous substances.

Assuming the tar as the medium form, it is seen that when exposed to heat it gives over oil, and that pitch remains. Thus, petroleum yields naphtha and asphaltum; and thus too, asphaltum exhibits all the gradations which I have described in the pitch, in properties varying in a similar manner, according to its particular state. In the process of distillation the principal difference will be found to consist in the relative quantities of acetic acid and ammonia, which they severally yield; the former chiefly characterizing the wood tar, and the latter the petroleum. From the same chemical cause which produces this effect arises also the difference in the nature of the inflammable gasses which are produced from these different substances.

The sensible qualities of the bitumens (their taste and smell) are in all states utterly and entirely different from those of the vegetable tar. Petroleum is also much less soluble in alcohol, and further differs from the vegetable tar in being perfectly soluble in naphtha. In their solubilities in oil of turpentine they resemble, each other, as well as in their habitudes with acetic acid and the alkaline lixivia, although the vegetable tar will be found the more readily soluble of the two. I need not repeat the circumstances in which the essential oil of wood differs from naphtha. It is a sufficiently characteristic one, that it forms no union with this latter.

It has been already shown that the difference between the pitch and asphaltum is considerable, when the former is in its first state, particularly with regard to its solubility in alcohol.