Page:Proceedings of the Royal Society of London Vol 1.djvu/242

Rh still continued to yield the tanning substance, our author thinks it probable the process might be repeated until the whole of the bark became converted into the above substance.

From the foregoing experiments, and many others made by him, Mr. Hatchett thinks that the method of treating roasted vegetable substances here described is the most speedy and economical for obtaining the artiﬁcial tanning matter; and, as all refuse vegetables may be thus converted into that matter by simple and unexpensive means, he hopes the discovery may eventually be productive of some real public advantage.

In a former paper Mr. Hatchett observed, that he suspected the tannin found in some peat-moors was produced during the imperfect carbonization of the original vegetable substances: whether that is really the case, or whether it has been aﬂ'orded by heath or other vegetables growing upon and near the peat, is, he says, still uncer- tain, as he has never been able to detect any tanning substance in peat, although he has examined a considerable number of varieties of it. The great fadlity with which tannin is dissolved by water causes it to be speedily extracted and drained from the substances which at ﬁrst contained it: and that this facility of extraction ex- tends to the most solid vegetable bodies, is shown by an experiment made by our author on a piece of oak from the submerged forest at Sutton, on the coast of Lincolnshire, described in the Phil. Trans. for the year 1799. This oak, by decoction, afforded extractive matter, but no traces of tannin could be perceived; yet, by incineration, it even afforded potash.

Peat, however, although it does not contain tannin, is, by the im- perfect carbonization it has undergone, rendered capable of being converted, by treatment with nitric acid, into the artiﬁcial tanning substance, in the manner already mentioned with respect to roasted ligneous bodies.

In the following section of his paper, Mr. Hatchett compares the eﬁ'ect of the acetic, sulphuric, and rﬁtﬁc acids, upon resinous sub- stances. The ﬁrst of these he considers as the solvent of such sub- stances, as it dissolves them speedily, without producing any apparent subsequent change in their natural properties; so that, by proper precipitants, they may be separated from that acid in an unaltered state. Sulphuric acid immediately dissolves resinous substances; but the moment the solution is complete, progressive alterations ap- pear to take place in the dissolved substance, coal being the ultimate product.

The effects of nitric acid seem to be the reverse of those of the sulphuric; for by nitric acid the resins are converted into a brittle porous substance, then into a soluble product intermediate between extractive matter and resin, which product is converted into the ﬁrst variety of the tanning substance; beyond which our author has not been able to eﬂ'ect any change. A table of the quantity of coal remaining after the treatment of various resinous substances with sulphuric acid is now given; respecting which we shall