366 CHEMISTRY. 



acid called gallic acid is obtained, and then by the decompo- 

 sition of this latter several other acids can be produced. 



510. How Obtained. The mode of obtaining tannin is as 



follows: Into a globular funnel, 5, Fig. 115, 

 which can be closed at the top by a stopper, 

 is introduced a quantity of powdered nut-galls 

 after the tube of the funnel, c, has been stopped 

 with a little cotton. The funnel is then placed 

 in the bottle, a, and is filled up with the ether 

 of the shops, which is about one tenth water. 

 The apparatus being allowed to stand several 

 days, there appear two layers of liquid. The 

 lower one, which is as thick as sirup, is a con- 

 centrated solution of the tannin in water, with 

 very little ether in it, while the upper is ether containing 

 a mere trace of tannic and gallic acids. The theory of 

 this process is that the tannin has such a greedy affinity 

 for water that, as the liquid passes through the powder, 

 the tannin in it seizes the water, withdrawing it from the 

 ether. The tannin is obtained from this sirup-like solution 

 by evaporation. 



511. Tanning. In the common process of tanning the 

 tannic acid in the bark is the effective agent. It is a chem- 

 ical union of this acid with the gelatin of the skin that 

 converts the skin into leather. This combination prevents 

 the decay or putrefaction which would otherwise take place 

 in the skin, just as the chemical union of corrosive subli- 

 mate with the albumen of the wood in kyanizing prevents 

 the decay of the wood. A black color is given to the leath- 

 er by washing it with a solution of iron, the tannin of the 

 leather imiting w^ith the iron to form a tannate of iron. 

 The reason that drops of tea upon a knife-blade become of 

 a dark color is that the tannic acid in the tea forms a tan- 

 nate with the iron. 



