Page:Proceedings of the Royal Society of London Vol 1.djvu/138

Rh found in fruits, in which it is modiﬁed by sugar and acids; that he has found it in great abundance in the juice of sloes, and that a friend of his had discovered its presence even in port wine. It also appears that it may exist in a state of combination in different substances, in which its presence cannot be made evident by the common means of solutions of gelatine; and that in these cases, in order to detect its existence, it may be necessary to have recourse to the action of di~ luted acids.

General Observations.—After a few strictures concerning a con- jecture of Mr. Proust, that there are different species of the tanning principle possessed of different properties, and different powers of acting upon re-agents, from which our author thinks himself autho- rized to dissent, he draws the general conclusion,—that in all the dif- ferent astringent infusions the tanning principle is found possessed of the same general properties and powers of combination. In all instances it is capable of entering into union with the acids, alkalies, and earths; and of forming insoluble compounds with gelatine and with skin. That in the processes of tanning, if the astringent in- fusion contain extractive and colouring matter, these as well as tannin enter into chemical combination with the skin; but that in no case is there any reason to believe that gallic acid is absorbed in this pro- cess. That hence the different qualities of leather made with the same kind of skin, seem to depend very much upon the different quantities of extractive and colouring matter it contains; the leather prepared by means of infusions of galls being generally found harder, and more liable to crack than that obtained from the infusions of bark.

When skins are slowly tanned in weak solutions of the barks, or of extract of catechu, it combines with a considerable proportion of extractive matter, whereby it is rendered perfectly insoluble in water, and yet soft and very strong. The inference, perhaps the most es- sential, deduced from this inquiry is, that of all the astringent sub- stances as yet examined, the extracts of catechu are those that con- tain the largest proportion of tannin, half a pound of this extract being found to produce the same effect in tanning as from four to ﬁve pounds of common oak bark.

How material this must be in a country where oak timber is not an object of trivial importance, need not be here insisted upon.—-The paper closes with a table, in which oak bark being taken as the standard of comparison as to its quantity of tannin, the different astringent substances are arranged in the order of their powers.

In a former paper Mr. Knight related some experiments on trees, from which be inferred that their sap, having been absorbed by the