The liquor of the first lixiviation exhi bits a very different character. It is more coloured and astringent ; it sot only ex hibits the properties of the gallic acid, by the alterations it causes in the blue colours of vegetables, and the black pre cipitate it forms with the sulphate of iron; but it likewise possesses the remarkable quality of forming, with animal gelatine, or glue, a yellowish abundant precipitate, insoluble in water, not putrescible, which becomes hard and brittle by drying ; and if a piece of skin properly prepared be immersed in this fluid, it becomes gradu ally more compact, and is converted into leather.
There exist, therefore, in the same fluid, two very different substances : the one, which precipitates a black matter from iron, is the gallic acid or principle ; the other, which precipitates animal ge latine or glue, is called the tanning prin ciple, on account of its efficacy in the preparation of leather.
To leave no doubt on this important point, it was proved, by a number of ex periments easy to be repeated. 1. That the liquor of the last lixiviation, though coloured, and of an astringent taste, af fords no precipitate with glue ; a fact, which seems to show that the gallit acid contained in the bark is less soluble than the tanning principle. In fact, as has al ready been remarked, when water is suc cessively poured on the tan, an infusion is at last obtained, which no longer precipi tates glue, though it precipitates sulphate of iron very well. 2. The liquor of the first lixiviation, after having been saturat ed with glue or animal gelatine, and forming an abundant precipitate with that substance, is entirely deprived of the tanning principle. It no longer differs from the liquor of the last filtrations, and contains merely a portion of the gallic acid. Hence the addition of sulphate of iron affords a new precipitate with this liquor. 3. As the tanning principle has a strong attraction to the animal gelatine, with which it always forms an insoluble precipitate, this property affords a very convenient re-agent to ascertain its pre sence immediately in any fluid, and to de termine with precision its quantity. Ac cordingly, the infusion of tan poured in tol milk, whey, serum, broth, &c. forms, with these liquors, a precipitate more or less abundant, according to the quantity of gelatine they contain.
This peculiar property of the tanning principle affords an application, which rilay become of great importance in the art of treating diseases, to determine the nature of urine, and to ascertain some-of its changes. In the healthy subject, all
whose functions are duly exercised, the urine does not contain gelatine, nor af ford a precipitate with the infusion of tan: on the contrary, in all the gastric affec tions, the urine is more or less charged with gelatine ; and forms, with the infu sion or tan, a precipitate more or less abundant. The same observation is ap plicable to acute and chronical diseases, in which the assimilating or digestive forces are troubled, deranged, or per verted. 4. The gallic acid, or, if other terms be preferred, the principle which precipitates the sulphate of iron, is often found alone, or at least without being ac companied by the tanning principle. Thus, quinquina, crude or torrefied cof fee, the roots of the strawberry.plant, scrotidaria, milfoil, arnica, the flowers of Roman camomile, and all the multitude of plants vaguely comprised under the title of astringents, contain the gxdlic acid only. All these form with the sulphate of iron a precipitate more or less colour ed and abundant ; but none of them pro duce the slightest change in the solution of animal glue. On the contrary, the tanning principle has never been found alone, but always united or combined with the gallic principle. It was long supposed to exist exclusively in the oak, the nut-gall, and sumac, the only sub stances used at the tan-works ; but it is found more or less abundantly in the siliquastrum, the rose-tree, the larix, se veral species of pines, the acacias, the lotus, the squill, the roots of bistort, of rhubarb, of parella, and several other plants.. We have also found this princi ple in the products of distillation of dif ferent vegetable substances, where it was in some measure formed during the operation.
From these different considerations, founded on experiment, the following general principles may be deduced : 1. Every substance of which the infusion is capable of precipitating animal jelly, pos sesses the tanning property. 2. Every substance which possesses the tanning property, likewise precipitates the sul phate of iron black. 3. Every substance which precipitates the sulphate of iron, but not the solution of glue, does not pos sess the tanning property.