Part II.

Materia Medica.

281

other process ; but it still requires and deserves a more mi-nute examination. In my experiments, a very weak infusionof nut-galls was precipitated by sulphuiic acid, lime-water,sub-carbonate of potass, acetate of lead, sulphate of copper,nitrate of silver, sulphate of iron, tartrate of antimony, nitrateof mercury, infusion of officinal cinchona, and solution of ge-latine; it was not precipitated by nitrous acid, ammonia, sul-phate of zinc, muriate of mercury, infusion of quassia, or in-fusion of saffron. To what principles these precipitates areowing remains still to be ascertained. Vauquelin justly ob-serves, that the infusions of nut-galls and of cinchona agree inprecipitating both gelatine and tartrate of antimony, but thatthey precipitate each other: Another fact, equally curious, oc-curred in my experiments : a mutually saturated mixture ofthe infusions of nut-galls and cinchona still precipitates gela-tine ; but these infusions, separately saturated by gelatine, donot act on each other. Hence it appears, that the action ofthese infusions on each other depends on principles containedin each, compatible with the presence of tannin, but re-actingon each other, and that gelatine precipitates these principlesalong with the tannin. Sir II. Davy has concluded that tanninand gelatine unite in fixed proportions, viz. 16 of tannin with54. gelatine : were this correct, it would very much facilitatethe analysis of astringents, but unfortunately my experimentsdo not confirm it. A twelve hours' infusion of 500 grains ofnut-galls in twelve ounces of water, precipitated successivelywith equal quantities of solution of gelatine, containing eachtwenty-four grains, gave precipitates weighing 98, 64, 4ti, and36 grains : hence, if we suppose the whole gelatine used to becontained in each precipitate, these consisted of 2t grains ofgelatine, and 74, 4-0, 24, and 12 grains of tannin; so that,from the weight of the precipitate alone, we cannot estimatethe tannin. Dr Bostock has drawn the same conclusionsfrom a set of experiments which he made, without any know-ledge of mine. It has been generally asserted, that the preci-pitate of tannin and gelatine is insoluble in water, either coldor hot; but I find that in boiling water it not only becomes softand viscid, but a certain portion is dissolved, which separatesagain when the solution cools. 1 may also remark, that if theprecipitate be dried without any heat, it has a yellowish-whiteappearance, opaque, and without lustre; but if exposed to avery moderate increase of temperature before it be dry, itseems to undergo a kind of fusion, and acquires transparency,a dark brown-red colour, and a resinous lustre ; with a high-er temperature, even when elmost dry, it will become so fluid