possessing iJte Proper lies oj' Tannin. 15? 



©hserved, that diluted nitric acid acts upon charcoal more 

 effectually in the formation of tlie tanning substance than 

 when it is employed in a concentrated state; and it appears 

 therefore very probable that hydrogen may have been afforded 

 by a portion of water decomposed during the process. For, 

 admitting that the new compound (formed by the action of 

 nitric acid upon charcoal) may possess a certain degree of 

 affinitv for hydrogen, this, being exerted simultaneously with 

 the affinity for oxygen possessed by nitrous gas, may (espe- 

 cially when the last is in a nascent state) effect a decompo- 

 sition of a portion of water, the hydrogen of which would 

 therefore enter into the composition of thft tanning sub- 

 stance, whilst the oxygen would supply the place of part of 

 that which had been taken from the nitric acid. 



Many of the properties of the tanning substance prepared 

 from coal by nitric acid are very remarkable, particularlv 

 those which have been noticed in § III. experiments F and 

 G ; for surely it is not a little singular, that this substance 

 when burned should emit an odour so very similar to ani- 

 mal matter, notwithstanding that the tanning substance had 

 been prepared from pure vegetable charcoal. And again, in 

 experiment G, the portion which had not been precipitated 

 by solution of isinglass was, when dried, found to possess 

 a strong vegetable odour very analogous to oak bark, al- 

 though charcoal is inodorous, and isinglass very nearly so. 



But, after all, the most extraordinary properties of this sub- 

 stance are certainly those which so nearly approach it to the 

 vegetable principle called tannin; for it perfectly resembles 

 this principle by its solubility in water and in alcohol, by 

 its action upon gelatine and upon skin, by the effects which 

 it produces upon metallic solutions, upon those of the earths, 

 and of the alkalis. 



The sulphuric and muriatic acids also affect the solutions 

 of it as they do those of tannin ; and the only marked dif- 

 ference which as yet has been found in the characters of the 

 artificial substance and of tannin is, that the former is pro- 

 duced, whilst the varieties of the latter are more or less dc* 

 stroycd by nitric acid. This, for the present at least, must 

 draw a line of separation between them; but we must not 



forget. 



