Tannin. 75 



and we know that these extracts are at least triple combinations 

 of acid, the colouring substance, and of vegeto-animal matter, 

 because tiie precipitates which form the tannin matter in those 

 solutions arc constantly coloured and sometimes very brilliant. But 

 if we reflect that gallic acid always accompanies tannin, and that 

 the colours of the ])recipitates furnished by the tanning matter, 

 and the metallic solutions, are the same with those manifested 

 bv the addition of the gallic acid, and the same metallic solu- 

 tions ; may we not conclude with M. Thenard, that the colour- 

 ing of these precipitates is owing to the gallic acid, from which 

 we can never entirely separate tannin? or is it the property 

 which tannin has of combining with animal matter and pre- 

 serving it from putridity ? A multiplicity of combinations of 

 vegetable matter also possess this property ; and without men- 

 tioning the astringent matter formed by the action of mineral 

 acids on charcoal and several vegetable substances, or the ex- 

 periments of M. Chevreul on hemniine (which acquired this 

 property) and who disbelieves the existence of tannin ? I shall 

 mention some facts to prove that gallic acid can combine with 

 several vegetable substances, and thus acquire the properties of 

 tannin. If we put a solution of pure gelatine in gallic acid, no 

 precipitate is formed : this acid does not produce any tnrbidness 

 in t!ie gummy solutions, but they cannot be resolved without im- 

 mediately becoming turbid in white flakes which are soon pre- 

 cipitated. Among the pharmaceutical extracts there is a great 

 number which do not contain the astringent principle, and 

 wliich form no precipitate in the solution of gelatine; but by 

 the addition of gallic acid they acquire this property. The same 

 phrenomenon does not take place with the other vegetable acids, 

 which on the contrary seem to oppose themselves to the preci- 

 pitation of gelatine. 



We know that pure gallic acid forms no precipitate in a so- 

 lution of suljjhate of iron at the maximum, but it there becomes 

 a beautiful deep-blue colour. Infusion of nut-galls produce^;, tm 

 tlie contrary, a precipitate which is attributed to tannin; but gallic 

 acid of itself acquires the jjropertv of partly precipitating the 

 iron from this solution v.'hen it is combined with extractive 

 matter : most of the vegetable infusions unite with gallic acid 

 and gelatine, the same as the extractive substances, for which 

 wc can assign no reason. The phenomenon is very perceptible 

 with the cold infusion of satfron : the properties of these pre- 

 cipitates catniot be absolutely identical ; they must difler ac- 

 cording to the nature of the substances wiiich enter into each 

 combination: that formed by gum arabic, gelatine and gallic 

 acid is the only one which I have hitherto b£en able to examine: 

 it differs from the others by its extreme atlhercncc to water, with 



which 



