Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/36

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Dr. Mac Culloch on certain products

As it was no part of my design to examine the vegetable elements, I did not pursue any experiments with this substance distilled in earthen vessels so as to ascertain whether in this case also it would contain iron, but I did enough to satisfy myself that the pitch was essentially the same in which ever way produced.

It is already known that a substance resembling plumbago is formed in water, it having been discovered by Fabroni in the country round Naples. It is equally known to be formed in the iron foundries, and the advocates for the igneous origin of coal have also contended for that of plumbago, and have supposed it to have been produced by the contact of melted greenstone with beds of coal. But even if we admit this cause of its formation, something else seems necessary for the production of the substance, and some other mode of applying the heat required before it can be produced. Nor indeed does the explanation sufficiently correspond with the general geological position of plumbago.

In numerous trials to combine iron with charcoal so as to form this substance, I have uniformly failed of success, except where as in the case above related, the charcoal or carbon has been in a state of previous combination, or was actually held in solution. In many trials on this principle, the results have been tolerably successful. If therefore we are to adopt an igneous theory of the formation of plumbago, it will be as easy to suppose that the action of subterraneous fire on mixtures of bitumen and iron has produced the compound of charcoal and iron, on the principles I have described, and this supposition will be more consonant to the chemical facts. But we are too little acquainted with the geological relations of plumbago to lay much stress at present upon this or any other hypothesis. It is evident that plumbago may be a produce of art, and could it be produced in as solid and compact a state as Nature affords it, the