Observations on (lie Processes of Tann'tmr. 59 



viums, saturated in clill'civnl lUoivcs with the astrlnrjeiit 

 prinei-pies of the bark. 'I'he hxi\iniiis first eniplovecrare 

 nsuallv weak; but lor the coinpKiion ui' the process they 

 arc made as strong as possible. 



In the process of tanning, the skin gains new chemical 

 properties; it increases inwtight^ and becomes insoluble in 

 boihng waicr. 



The infusions of oak bark, when chemically examined, 

 are found to contain two princijial substances ; one is pre- 

 cipitable by solution of gelatine, made from glue or isin- 

 glass; and gives a dense black with solution of common 

 sulphate of iron. The other is not thrown down by solu- 

 tion of gelatine ; but it precipitates the salts of iron of a 

 brownish black, and the salts of tin of a fawn colour. 



The substance precipitable by solution of gelatine is the 

 tanning principle,- or the tannin of Seguin. It is essential 

 to the conversion of skin into leather, and in the process of 

 tanning it enters into chemical union with the matter of 

 skin, so as to form with it an insoluble compound. The 

 other substance, the substance not precipitable by gelatine, 

 is the colouring or extractive matter ; it is capable of enter- 

 ing into union with skin, and it gives to it a brown col(>ur j 

 but it docs not render it insoluble in boiling water. 



It has been usually supposed that the infusion of oak bark 

 contains a peculiar acid, called gallic acid; but some late 

 experiments render this opinion doubtful : and this prin- 

 ciple, if it exists in oak bark, is in intimate combination 

 with the extractive or colouring matter. 



In the connnon process of tanning, the skin, which is 

 chiefly composed of gelatine, slow'ly combines in its or- 

 ganized form with the tannin and extractive matter of the 

 infusions of Imrk ; the greater proportion of its increase of 

 weigiu is however owing to tannin, and it is from this sub- 

 stance the leather derives its characteristic properties ; but 

 its colour, and the degree of its llexibilitv, appear to be in- 

 fluenced by the cjuantily of colouring matter that it contains. 



When skin, in large quantity, is suffered to exert its full 

 action upon a small portion of infusion of bark, containing 

 tannin and extractive matter, tlie fluid is found colourless. 

 It u'ivcs no precipitate to solution of glue, and produces very 

 little efleet upon the salts of iron, or of tin. 



The taiming prineijile of oak bark is more soluble in 

 water than the extractive matter: and the relative propor- 

 tion of tannin to extractive matter is much greater in strong 

 infusions of oak bark than in weak ones; and when strong 



infusions 



