OT TANIN AND ITS SPECIES. J99 



tfie quantity of tanin. It may alfo happen that the vegetable 



juices may contain many other fubftances capable of decom- 



pofing the muriate either direclly or indirectly, fo that this 



re-agent can never be depended upon with confidence. 



There are befides earthy falts in thefe juices as Vauquelin When earthy 



has fliewa, Some, as for example Sumac contain fulphate of v a e *g t "jf '"^ 



lime in abundance. If therefore we ufe the alkali to complete will Jet fall their 



the reparation of the tannate, the precipitate will alfo be k fe s when the 

 r \ r r alkali 1S a ddcd. 



charged with an earthy depofition. 



3. On reflecting upon the means of avoiding the errors caufed If the pure maxi- 

 by the muriatic acid, Mr. Prouft tried a procefs which he had ™%£££ eof da 

 fuccefsfully ufed to feparate the colouring principle from the agitated with 

 gelatinous mucilage which accompanies it in cochineal. It F|°[*" com "' 

 confifts in heating, or even limply agitating, the aftringent p i ace and> mu _ 

 juice with the oxide of tin prepared by nitric acid, and kept cilage, if prefent, 

 under water. The oxide in fact, becomes loaded with tanin 



in a few days. But if the juice of a plant which is not aftrin- but this proeefa 

 gent, or a diluted extrad, be treated in the fame manner, it ^eextf^T 

 will alfo become deprived of the whole of its extractive matter : matter, 

 and the gum and the fait will remain alone in the fluid. This 

 method cannot therefore lead us to our object. 



4. What appears moft furprifing to this chemift in the pre^ Singular event 

 fent experiment is the destruction of the gallic acid, or pro- l,"^^ e ^j, 

 bably its transition into a State in which it cannot perform the gallic acid is d*. 

 fundions of that acid. The fluid in fad when cleared of the ftr °y ed ' 

 tannated oxide by the filter has no longer either colour or taSte, 



and makes not the Slightest impreffion on the folutions of iron, 

 nor even upon turnfol. When examined by every trial the 

 fluid is found to be mere water. 



$. Sufpecting, neverthelefs, that this acid might be com- When the tanatc 

 bined with the oxide of tin, he patted the tannate of the laSt con J2Jj £y 

 defcribed procefs into potafh. The product was a coloured potato no gallic 

 fluid, in which he found no fign of the gallic acid : for it did™ d ^ fo ™ d of 

 not afTume by expofure to the air that green (hade which it tanin. 

 always indicates when faturated with an alkali. He pre- 

 cipitated the tanin by a diluted acid, a portion remaining in 

 folution, as happens in this cafe and he proceeded to examine 

 that which remained upon the filter ; but he foon perceived 

 that it had alfo advanced towards a Mate in which, a_s we (hall 

 fhew, he found infinitely lefs tanin than before, 



C Boiling 



