115 



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 nitration 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 

 about ^-Jths of gallic acid, united to a minute portion of extractive 

 matter. 100 grains, moreover, of this solid matter left, after inci- 

 neration, nearly 4f- 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 

 i 2 



