215 



influenced by the nature of the acid constituent of the iron-salt, by 

 the greater or less acidity, and by the concentration of the liquids, 

 and all this to such an extent that it is possible to produce all the 

 gradations of blue or of green, and sometimes even from blue to 

 green with the same substance. Besides, as stated already, there 

 are a few Tannic acids which, under the same conditions, produce 

 neither of the two colours, but only a dirty-brown deposit. Even 

 the property of precipitating glue does not seem to be general, as 

 evidenced by a few of the iron-greening acids. 



Another distinguishing feature of Tannins is their behaviour, 

 when submitted to dry distillation. Those which })roduce green 

 iron compounds, yield, under these conditions, Pyrocatechin (Pyro- 

 catechuic acid), while the Tannic acids of the blue reaction with 

 iron-salts yield sometimes Pyrogallic, sometimes Pyrocatechuic 

 acid. 



Tannic acids are widely diffused in plants, especially prerennial 

 ones. They occur in all parts of their organism, but predominantly 

 in roots, barks, and young woods, less frequently in the teguments 

 of fruits and seeds, seldom in the leaves. If not present in too 

 small quantity, they are easily detected by the taste and by the 

 reaction of their solutions with glue and iron-salts. Some plants 

 contain two different Tannic acids, i.e., one of them producing a 

 green iron compound, the other a bhie one undei- equal conditions ; 

 or the two acids may ajjpear identical in regard to ii'on-salts, but 

 yet differ in other respects. For instance, the Tannic acids of nut- 

 galls, and of oak-bark, produce sevei-ally l)lue iron-precipitates, but 

 the former substance, by dry distillation, yields Pyrogallic acid, 

 while the latter does not. 



To determine qiiantitatively the amount of a Tannic acid, of 

 Avhatever kind : prejiare an aqueous solution or extract of one to 

 two grammes of the substance in question, add to the ffltered liquid 

 solution of acetate of baryta, as long as a precipitate ensues, filter 

 and precipitate the filtrate with acetate of lead, collect the deposit 

 on a filter, wash dry at 120°, and note down the weiglit of the dry 

 precipitate of Tannate of lead, incinerate the latter in a porcelain 

 crucible at a red heat, moisten with nitric acid, and heat again, 

 ascertain the weight of the remaining lead-oxyd, and deduct this 

 from the weight of the dried Tannate of lead — the rest represents 

 tlie weight of the Tannic acid. 



'faimiiift'enic Aci(l=CATECHuic Acid, 



Taimopinic Acid=C.2s H15 O13. Known to exist in the leaves 

 of Pinus sylvestris, and during spring-time, to replace the oxypino- 

 tannic acid, oxydises readily in warm, moist air, yields a red 

 product on heating with diluted acids. 



