158 CHfMISTRY APPLIED TO AGRICDLT0RE. 



bol. Its predominant characteristic is that of affording 

 an insoluble precipitate when added to a solution of gela- 

 tine. It combines with a solution of iron, and forms a 

 black precipitate. It enters into the composition of writ- 

 ing ink, and of the greater part of the black dyes for 

 eloth. 



Tannin cannot be procured perfectly pure without a 

 great deal of difficulty, and the operations require a de- 

 gree of nicety which can only be acquired by a close ac- 

 quaintance with chymical manipulation. For the greater 

 part of the purposes to which it is applied it is not requi- 

 site that it should be freed from all foreign substances. 

 The great affinity which tannin has for gelatine causes 

 it to combine with that principle whenever presented to it^ 

 till the substances containing the gelatine are completely 

 exhausted of it : the various proportions of tannin con- 

 tained in the different kinds of bark used in the manufac- 

 ture of leather are determined from this circumstance. 



The most important purpose to which tannin is applied 

 is that of converting skins into leather, and for this pur- 

 pose the tannin contained in the bark of the oak is gener- 

 ally preferred. In this process layers of ground bark are 

 placed alternately with layers of skins in a pit, the layers 

 of bark being slightly moistened in order that the tannin 

 mfy act readily. As the tannin combines with the gela- 

 tine of the skin, the latter changes its color to a reddish 

 brown, and its opacity and consistency are at the same time 

 increased, till by the progress of the operation the change 

 is carried on through the whole substance of the skin, and 

 it is thus brought to the firmness of leather. This new 

 eombination, which consists entirely of a union of tan- 

 nin and gelatine, is compact and resists putrefaction ; it 

 can be cut with a knife by quick strokes, and employed 

 for numerous purposes. 



The best leather is that which, by being allowed to re- 

 main in the pit a long time, is formed gradually : in this 

 case the slowness with which the combination takes place 

 renders it more close and complete, than when the tan- 

 nin is dissolved in water and the skins plunged into it. 

 By this last process the thickest skin may be tanned in a 

 few days, but the quality of the leather will be very 

 inferior. 



An astonishing improvement has been made in the art 

 of tanning since M. Seguin discovered that it consisted 



