the characterisilc Properties nfTanmn. 75 



that the tannin is also diminished, and (I may add,) is ul- 

 timately destroyed by the process ot" roasting; that when 

 galis have not been so far roasted as to destroy the whole of 

 the tannin, then [he remainder of this seems to be destroy- 

 ed by the addition of nitric acid, whilst at the same time a 

 small portion of the artificial tanning substance is produced ; 

 and that this last is aKvavs pk-ntifallv afforded by roasted 

 galls when digested with nitric acid, similar to other vege- 

 table bodies when thus treated. 



These remarks are also partly- confirmed by tlie following 

 experiments upon oak bark. 



1. 200 grains of oak bark, reduced into very small frag- 

 ments, were infused in about four ovnices of water, after 

 which the infusion was examined by dissolved isingtass, and 

 yielded a considerable precipitate. 



2. 200 grains of the same sort of bark were slightly roasted, 

 and afterwards di<icsttd in water ; a much darker coloured 

 liquor was obtained than in the former case ; but alibongh 

 it afibrded precipitates by the addition of niuriate of liin, 

 acetite of lead, and sulphate of iron, vci iiot the saiallest 

 efl'ect was produced by solution of isinglass. 



3. The residuum, with tlie remaining pirt of the above- 

 mentioned liquor, was then digested with a small pt*rtion of 

 nitric acid; this was completely eviiporatcd, and a brovtrn 

 solution was formed by water, which abundantly precipi- 

 tated gela'inc. 



4. One ounce of oak bark, reduced into very small frag- 

 ments, was repeatedly digested m difierent portions of water 

 ontil the whole of lis tannin was extracted. The residuum 

 or exhausted bark (as it. is called by the tauners) was dned> 

 «nd was afterwards moijerately roasted. It was then moist- 

 ened with diluted nitric acid, which was evaporated in a 

 heat not much exceedmg 300" until the baik was become 

 |KrfectIy dry. This was digested in water, and speedily 

 ibrmed a ycliowish-biown liquor, which abundantly preci- 

 pitated gelatine. 



5. The baik, which after being exhausted of its na- 

 tural tannin had thus afibrded the ailillri;d tanning sub- 

 stance. 



