!^20 Miscellaneous Intelligence, 



36. Oil the Preparation of Tannin. — According to M. Berze- 

 lius, tannin is not in the very impure state, in an infusion of galls, 

 generally supposed. He prepares it pure in two ways, either by 

 the action of sulphuric acid, or of carbonate of potash. First me- 

 thod. A hot infusion of gall-nuts is to be filtered through a cloth, 

 a very small quantity of weak sulphuric acid added, and the whole 

 well mixed ; the coagulum formed is to be separated, and the liquid 

 filtered. Sulphuric acid, diluted with half its weight of water, is to 

 be added in small quantities with agitation ; the precipitate, after 

 an hour* s rest, acquires a half-fluid glutinous state ; then the fluid 

 is to be decanted, and carefully mixed with concentrated sulphuric 

 acid, as long as any precipitate is formed. It is a compound of 

 sulphuric acid and tannin, yellowish-white, and insoluble in a weak 

 acid. Being put on a filter, it is to be washed with diluted sul- 

 phuric acid, pressed between bibulous paper, dissolved in pure 

 water, and carbonate of lead in fine powder added to the fluid, 

 until the free sulphuric acid is neutralized ; ebullition for a short 

 time also removes the acid combined with the tannin ; perfect satura- 

 tion is indicated by the deep yellow colour taken by the solution. 

 The filtered fluid is to be evaporated carefully to dryness in an air- 

 pump if possible ; the brown extract obtained pulverized, and 

 digested in ether, at a temperature of 86° Fahr. The ethereal solu- 

 tion evaporated yields a pale yellow transparent substance, which is 

 pure tannin. It suffers no change in the air. 



Second method. A concentrated solution of carbonate of potash 

 is to be added to a filtered infusion of galls, only as long as a white 

 precipitate is formed. The precipitate is to be washed on a filter 

 with ice-cold water, and then dissolved in weak acetic acid. By 

 filtration a brown matter is separated ; the clear fluid is to be pre- 

 cipitated by acetate of lead ; the compound of tannin and oxide of 

 lead washed, and then decomposed by sulphuretted hydrogen. The 

 filtered liquid is then colourless, and being evaporated under the 

 air-pump receiver, gives transparent yellowish hard scales, which, 

 treated with ether as before, yield pure tannin. 



Pure tannin is colourless, but sometimes becomes coloured by 

 alteration in the air ; it is not deliquescent, is easily pulverized, 

 and dissolves readily in water. By distillation it yields no ammo- 

 nia, but a yellow oil and a liquid, which, on cooling, deposit 

 crystals different from those of gallic acid ; they have a hot taste, 

 and colour or precipitate salts of iron of a yellowish or grayish 

 green. 



The combinations of tannin with acids, when exactly saturated, 

 have no sourness, but a pure astringent taste. AVhen pure, they 

 are usually very soluble in water, and precipitated only by a great 

 excess of acid. With salifiable bases, tannin forms very curious 

 compounds. The neutral compound, v/ith potash or ammonia, is 

 little soluble in cold water, more so in hot, separating from the 

 latter, as the temperature diminishes, in the form of a white powr 



