64 Experiments and Observations on the 



guin, have shown that many astvingcnt soUitions undcrjj'? 

 a change by cxpotiure to the atmosphere ; an insoluble mat- 

 ter being pre 'iritated from them. A precipitation is like- 

 wise occaaioned in them by the action of heat; and these 

 circumstances reader it extremely di.'Bcult to ascertain, witli 

 any degree of precision , the <iinntiiies of their constituent 

 parts, as they exist in the primitive combination. 



After trying several experiments on ditlerent methods of 

 ascertaining the quantity of tannin iii astringent infusions, 

 I was induced to employ the common process of precipita- 

 tion bv gelatine, as being the most accurate. 



This process, however, requires many precautions. The 

 tanning principle in difll-rent vegetables, as will be seen 

 hereafter, demands for its saturation different proportions 

 of gelatine ; and the quantity of the precipitate obtained 

 by filtration is not always exactly proportional to the quan- 

 tities of tannin and gelatine in solutions, but is influenced 

 by the degree of their concentration., llius I found that 

 10 grains of dry isinglass, dissolved in two ounces of di- 

 stilled water, gave, with solution of galls in excess, a pre- 

 cipitate weighing, w ben dry, 1 7 grains ; whilst the same 

 quantity, dissolved in six ounces of water, produced, all 

 other circumstances being similar, not quite 15 grains. 

 With more diluted solutions, the loss was still greater; 

 and analogous effects took place, Avhen equal portions of 

 the same solution of isinglass were acted on by equal por- 

 tions of the same infusion of galls diluted in different de- 

 grees with water ; the least quantity of precipitate being 

 always produced by the least concentrated liquor. In all 

 cases, when the weak solutions were used, it was observed 

 that the re,>idual fluid, though passed two or three time? 

 thronoh the filter, still remained more or less turbid and 

 opaque ; so that it is most likely that the deficiency arose 

 from the continued suspension of some of the minutely di- 

 vided solid matter in the licjuid mass. 



The solutions of gelatine, for the purposes of analysis, 

 should be employed only when quite fresh, and in as high 

 a state of saturation as is compatible with their perfect flu- 

 idity. 1 have observed that, in cases when they approach 

 towards the state of jeliy, their power of acting upon tannin 

 is materially altered, and they produce only a very slight 

 precipitation. As the degree of fluidity of solutions of ge- 

 latine is influenced by their temperature, I have found it 

 expedient, in all comparative experiments, to bring them, 

 and the astringent infusions on which they are designed to 

 act, as nearly as possible to a common degree of heat. My 



standard 



