Page:Proceedings of the Royal Society of London Vol 1.djvu/135

Rh very delicate precautions: previous to the experiments the infusions are materially affected by exposure to the atmosphere; the tanning principle in different vegetables demands for its saturation different proportions of gelatine; and the quantity of the precipitate obtained by ﬁltration'is not always proportionate to the quantity of tannin and gelatine in the solution, but is materially inﬂuenced by the de- gree of their concentration. Hence it follows that the solutions of gelatine, for the purposes of analysis, should be employed in as high a state of saturation as is compatible with their perfect ﬂuidity. They should be used only when quite fresh; and as their relative effects Were found to be inﬂuenced by their temperature, it was found ex- pedient to bring them, and the infusions on which they were designed to act. as nearly as possible, to a common degree of heat: great care must also be taken to prevent any excess of gelatine. Duly attending to all these precautions, the general result is, that in any given-case, when the quantity of gelatine in the solution employed upon an as- tringent infusion is compared with the quantity of the precipitate ob- tained, the difference between them may be considered as the pro- portion of tannin contained in the infusion.

The tannin being thus separated, it remained to ascertain the proportion of the two other ingredients in the infusion, viz. the gallic acid, and the extractive matter. The ﬁrst step here was slow evapo- ration, by which the latter substance is in part rendered insoluble, so as to subside at the bottom of the vessel. A proportionate quan- tity of alcohol being next poured upon the ﬂuid thus reduced to a thick consistency, both the gallic acid and the soluble extractive matter, if there be any remaining in the infusion, will be dissolved. The great difﬁculty now was to separate the gallic acid and the ex- tractive matter. Ether and alumine were tried without the desired effect, the afﬁnities of these two agents with those substances not being sufﬁciently distinct to produce the separation. Some judge- ment, however, may be formed of their relative proportions, by means of the salts of alumine and the oxygenated salts of iron. Muriate of alumine precipitates much of the extractive matter from solutions. without acting materially upon gallic acid; and after this precipi- tation, some idea may be formed concerning the quantity of the gallic acid, by the colour it gives with the oxygenated sulphate of iron.

2. Concerning the Infusions of Galls.—The strongest solution of gall- nuts was obtained by repeatedly pouring distilled water upon the best Aleppo galls, broken into small pieces; it was of the speciﬁc gravity 1'068. 400 grains of this solution produced by evaporation 53 grains of solid matter, which, as well as could be estimated by the methods of analysis described in the preceding section, consisted of aboutaﬁths of gallic acid, united to a minute portion of extractive matter. 100 grains, moreover, of this solid matter left, after inci- neration, nearly 4% grains of ashes, which were a mixture of lime with carbonate of lime, and a small portion of ﬁxed alkali.

Here follows a long series of experiments on the infusion of gall-nuts, in which it was exposed to, or combined with, all manner of