of Astringent Substances. 337 



acetate, which "throws them down of a bluish-purple colour, 

 which on standing changes to a leaden .gray. When boiled 

 with alkalies they immediately assume a fine red colour, but 

 they differ from the tannin of the larch in not being reddened 

 by sulphuric acid. I think it unnecessary to go into more 

 minute details respecting them, as I have been unable to de- 

 rive from them any determinate or crystalline compounds. I 

 shall leave this subject, therefore, for the present with one or 

 two general observations. 



The great difficulty of examining the different species of 

 tannin with a view to classifying them, is chiefly owing to their 

 amorphous nature, to the great similarity of their properties, 

 and to the circumstance, that except in the case of nut-galls 

 and shumac, the products of their decomposition are of a very 

 indeterminate character. We think however that there are 

 good grounds for believing that both nut-galls and shumac con- 

 tain the same species of tannin, for the effects of reagents upon 

 it are exactly the same, and the products of its decomposi- 

 tion, when boiled with either sulphuric or muriatic acid, when 

 destructively distilled, or when left to spontaneous decomposi- 

 tion, are in every instance identical, from whichever of these 

 sources it has been derived. It is remarkable also that in so 

 many instances, in eight cases out of ten which I have ex- 

 amined, the species of tannin which give bluish-black preci- 

 pitates with protosulphate of iron are accompanied with larger 

 or smaller quantities of gallic acid. In the present state of 

 our knowledge it is impossible to say whether the gallic acid 

 has originally existed in these substances, or has resulted from 

 the decomposition of the tannin they contain. In the case of 

 galls and shumac the latter opinion is probable enough, as we 

 are easily able to effect this change by artificial means, and it 

 also, as is well known, occurs spontaneously. In the case of 

 the other species of tannin, however, we are still unacquainted 

 with any instance of a similar transformation. It is to be 

 hoped that subsequent researches may yet throw light on this 

 very obscure subject. It is also rather singular that in the 

 case of some of those species of tannin which give green pre- 

 cipitates with salts of iron, a somewhat similar circumstance 

 occurs. Thus the tannin of catechu is accompanied by a 

 crystalline acid body, catechine, which also gives green preci- 

 pitates with salts of iron. I have likewise observed that in the 

 case of infusions of birch bark, alder bark, &c, when the whole 

 of the tannin they contain had been removed by gelatine, the 

 clear liquid when filtered still contained a substance which 

 precipitated salts of iron olive-green, just as the tannin had 

 done, and which threw down salts of lead as copious dark yel- 



Phil, Mag. S. 3. Vol. 23. No. 153. Nov. 1843. Z 



