The dispensatory of the United States of America / By George B. Wood and Franklin Bache.
- George Bacon Wood
- Date:
- 1836
Licence: Public Domain Mark
Credit: The dispensatory of the United States of America / By George B. Wood and Franklin Bache. Source: Wellcome Collection.
1131/1204 page 1105
![medicinally only in the composition of ointments and plasters, and little even for this pur- pose. Its properties are analogous to those of the turpentines. It is sometimes used as incense. ‘ TANNIN. Tannic acid. Since the publication of the experiments of M. Pelouze, this substance has been universally admitted to rank with the acids, and is consequently deno- minated, in recent chemical works, tannic acid. Its extensive diffusion in vegetables, the wide circle of its affinities, and the consequent influence which it exercises in numerous pharmaceutical operations, both extemporaneous and officinal, are sufficient grounds, independently of its effects as a remedial agent, for giving it a place in this work, though it does not rank among the medicines or preparations directed either in the United States or British Pharmacopeias, The term tannin, like the term sugar, resin, &c. has been considered as applicable not so much to one distinct proximate principle, invariable in its character and composition, as to a set of substances, which, though differing somewhat in their nature, are very closely allied to each other, and possess many properties in common. It has been ob- served, that tannin, as procured from certain vegetables, affords, with a solution of a per- salt of iron, a black precipitate, and colours the liquid deep blue; while, as furnished b others, it gives a grayish-green precipitate with the same solution, and colours the liquid green. Hence has arisen a distinction of this substance into two varieties; 1. tannin which is rendered black or blue, and 2. tannin which is coloured green, by the persalts of iron. The first, which is the most abundant, is that found in the different species of oak, and consequently in galls; the second exists in Peruvian bark, catechu, and various other vegetable products. It has not yet been determined whether these are really distinct bodies, or whether their difference is not ascribable to the presence of foreign substances in one or both. As they do not materially differ in other respects, and as the tannin pro- cured from galls is that which has attracted most zttention, we shall confine our observa- tions to this variety. Numerous processes for obtaining tannin in a state of purity have been proposed ; but most of them are complex, and, in consequence of the readiness with which this principle undergoes alteration, are insufficient for the end in view. ‘The process of M. Pelouze has superseded all others, being preferable both for its facility and the purity of the resulting ‘preparation. An elongated narrow glass vessel, open at both ends, is taken. The lower orifice is loosely closed by a piece of linen or cotton; and above this is placed some very fine powder-of galls, which is lightly pressed together, and added till it fills half the capa- city of the vessel. Sulphuric ether is then poured upon the powder till it fills the vessel, the upper aperture of which is afterwards closed to prevent the evaporation of the ether. The spp tus is placed in the mouth of a bottle, in which the ether is received as it per- colates through the galls. The water always present in the ether of the shops dissolves the tannin, to the exclusion of all the other principles of the galls, and forms a saturated solution, which separates from the ether, and remains asa distinct layer at the bottom of the ‘bottle. When the powder has been exhausted by the addition of successive portions of the solvent, the apparatus is taken apart, the ether is separated from the aqueous solution of tannin beneath it, and the latter, after being washed with some fresh ether, is evaporated by a very gentle heat, or in a vacuum with sulphuric acid. From 35 to 40 per cent. of tannin is thus obtained from galls. . Pure tannin is solid, uncrystallizable, white or very slightly yellowish, inodorous, strong] ‘astringent to the taste without bitterness, very soluble in water, much less soluble in-aleohol and ether, especially when anhydrous, and insoluble in the fixed and volatile oils. It may be ‘kept unchanged in the solid state. Exposed to heat, it partly melts, swells up, blackens, takes fire, and burns with a brilliant flame. The aqueous solution, when exposed to the air, gradually becomes turbid, and deposites a crystalline matter, consisting chiefly of gallic acid. During the change, oxygen is absorbed, and an equal volume of carbonic acid disengaged. Tannin has an affinity for acids, and when in solution affords precipitates with the sul- “phuric, nitric, muriatic, phosphoric, and arsenic acids, but not with the oxalic, tartaric, ‘lactic, acetic, or citric. The precipitates are compounds of tannin with the respective acids, and are soluble in pure water, but insoluble in water with an excess of acid. Hence, in ‘order to ensure precipitation, it is necessary to add the acid in excess to the solution of ‘tannin. This principle also unites with most of the salifiable bases. With potassa it forms a compound but slightly soluble, and is therefore precipitated by this alkali or its carbo- nates from a solution which is not too dilute, though a certain excess of alkali will cuuse the precipitate to be redissolved. Its combination with soda is much more soluble; and this alkali affords no precipitate unless with a very concentrated solution of tannin. With ‘ammonia its relations are similar to those with potassa. Baryta, strontia, lime, and mag- ‘nesia, added in the state of hydrates, form with it compounds of little solubility.. The same is the case with most of the metallic oxides, when presented, in the state of salts, to . 94 . 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