229. The decoction of galls precipitates the oxide of iron of a deep black colour, and forms the well-known substance ink. The solutions of mercury, mixed with an infusion of galls, assumes a brick colour, which soon becomes of a reddish grey. Copper is easily precipi tated: the precipitate is at first green, but becomes of an ash grey, and acquires a reddish coppery tinge in dry ing. Zinc yields a dirty green precipitate. Cobalt forms a light blue precipitate, which quickly becomes of an ash grey.
230. If the astringent property was owing to some in dividual principle, which was always the same in the different vegetables in which it occurs, the precipitates obtained by their means from a solution of iron, would always form the same compound, and exhibit the same general appearances; but this is not the case. The pre cipitate produced by galls is of a blackish blue ; that by logwood has a different shade of blue; that by oak is of a fawn-colour; that by quinquina of a blackish green, &c. These precipitates are also formed with different attendant circumstances, and when fixed on stuffs, are differently affected by alum and tartar. So that the sub stances denominated astringents form, with iron, differ ent kinds of precipitates, and, consequently, do not de rive their properties from an individual principle found in different vegetables. This diversity of properties has induced Dr Bancroft to rank the substances denominated astringents as colouring matters.---“ I have invariably applied the name of colouring matter," says he, g‘ to those parts of vegetable dyeing drugs which are found to produce colour with an earthy or a metallic basis ; and I have certainly never been able to discover any good reason for doing otherwise, in regard to those ve getable matters which afford ink, or a black (lye with iron ; matters which, indeed, are extremely various in their other properties, and even in the sorts of black which they produce; though chemists have, as I think improperly, confounded most of them under the general denomination of astringents ; a term which may be un objectionable, as signifying acerbity in vegetables, but not as indicating, or being invariably connected with, any such property of matter as they have been supposed universally to possess, that of producing a black colour with iron." 231. Although chemists have considered the astrin gent principle as always the same, experience had taught us that all astringent substances were not equally proper for producing a beautiful and durable black. Of twenty
one species of astringents compared with galls, oak saw dust, and yellow myrobolans, were the only substances which produced a fine black, but inferior in beauty and durability to that obtained by means of the common gall. NI. Beunie, who mode the comparatiye trial of their properties in this respect, found that the oak sow dust Wa% preferable to the bark, w hich is employ eel by the dyers of thread, and he remarks it is cheaper.
232. Lavoister, Vandermonde, Fourcroy, and Ber thollet, were desired to try experiments on the different astringents, for the purpose of making a report on the subject to the academy. The substances upon which they instituted the experiments of comparison, were galls, oak-bark, raspings of heart of oak, of the exter nAl part of oak, of logwood, and sumach. To deter mine the proportion of astringent principle contained in these substances. they took two ounces of each sepa w nich they boiled half an hour in three pounds of water ; after the first water they added a second, which underwent a similar ebullition, and continued these operations until the substances appeared exhaust ed; they then mixed together the decoctions belonging to each. They employed a transparent solution of sul phate of iron, in which the proportions of water and sulphate were exactly known. They first estimated the quantity of the astringent principle by the quantity of sulphate which each liquor could decompose, and after wards by the weight of the black precipitate which was formed. To saturate the decoction of two ounces of galls, three drachms and sixty one grains of sulphate of iron were required, and the precipitate, when collect ed and dried, weighed seven drachms twenty four grains. The decoction of oak bark is of a deep yellow; a very small portion of sulphate of iron gives it a dirty reddish colour, and a larger one changes it into a deep brown. The quantity of sulphate required to saturate the decoc don of two ounces of this bark was eighteen grains, and the precipitate, when collected and dried, weighed twenty-two grains. The inner bark of the oak afforded cry nearly the same result. The decoction of the rasp ings of the heart of oak required for its saturation one drachm twenty four grains, and the precipitate obtained was of the same weight. The decoction of the external wood of the oak produced very little precipitate. The decoction of sumach acquired a reddish violet colour w hen a small quantity of the sulphate of iron was added.