Page:Proceedings of the Royal Society of London Vol 1.djvu/212

Rh reduced 3. portion of isinglass to the state of coal, and digested it in nitIic acid, which at ﬁrst did not appear to act upon it, but at length slowly dissolved almost the whole of it. The solution resembled those which have been described, but was of a deeper brown colour: and, when evaporated to dryness, left a residuum, which, upon being examined by the re-agents employed in the former experiments, was found to produce similar effects.

It appears evident, therefore, that'tan may be formed from animal as well as from vegetable and mineral coal; and it also appears, from what has been stated, that it is composed of carbon, combined with a certain proportion of oxygen. It seems, however, necessary that the carbon should be uncombined with any other substance. In support of this opinion, Mr. Hatchett mentions the following expe- riments'i—

l. A piece of Bovey coal, which appeared like half-charred wood, upon being treated with nitric acid, formed a solution of a deep yel- low colour: this solution, when evaporated, left a residuum, which, dissolved in distilled water, and examined by various re-agents. par- ticularly by gelatine, did not show any signs of its containing tan; the predominant substance appearing to be oxalic acid.

2. Another piece of Bovey coal, which was more perfectly car- bonized, afforded abrown solution, which, unlike the former, yielded a considerable quantity of tan.

3. A portion of the ﬁrst-mentioned sort of Bovey coal, by being exposed to a red heat in a close vcssel, and then treated as before, was thus converted, almost entirely, into tan.

4. A coal from Sussex, very like the second sort of Bovey coal, also afforded tan.

5. A piece of Surturbrand, from Iceland, yielded a similar result.

6. Deal sawdust, treated in the same manner as the former sub- stances, afforded oxalic acid, but not any tan.

'7. Another portion of the same sawdust was reduced into char- coal, which, treated as before, was thereby converted into tan.

8. Teak wood, which Mr. Hatchett had previously ascertained not to contain either gallic acid or tan, was reduced into charcoal, which was as readily converted into tan as the substances already mentioned.

Mr. Hatchett then adverts to a series of experiments he is making on the slow carbonization of vegetable substances in the humid way, a few of which, he says, he is compelled to notice, on account of their being intimately connected with the present subject. In these experiments he has observed, that concentrated sulphuric acid dissolves resinous substances, forming a yellowish brown transparent solution, which, by digestion, becomes intensely black. Concentrated sulphuric acid readily dissolves the common turpentine of the shops. If a portion of this solution be immediately poured into cold water, the turpentine is precipitated, in the state of common yellow resin. But if another portion, of the same solution be, after the lapse of an hour or more, poured. into cold Water, the resin thus formed is not yellow,