424 TANNIC ACIDS. 



white powder, easity soluble in hot water and in alco 

 hol. By boiling with dilute acids, it yields sugar and 

 a yellowish resin glycyrrhetin, C 18 H 26 4 (?). 



24. Digitalin. 



In Digitalis purpurea. Small colorless crystals ; spar 

 ingly soluble in water, easily soluble in alcohol, of an 

 intensely bitter taste. Exceedingly poisonous. Very 

 difficult to obtain in a pure state, and hence but little 

 known as yet. Is resolved by sulphuric acid into sugar 

 and amorphous digitalretin. 



25. Tannic Adds. 



By the name tannic acids is understood a class of 

 weak acids, which are widely distributed in the vege 

 table kingdom, and which bear a close relation to each 

 other, as regards their properties, as well as their com 

 position ; the composition is, however, not yet deter 

 mined with certainty for all of them. Most of the 

 tannic acids have been shown to be glucosides. In 

 general they are characterized by a sharp astringent 

 taste ; by the property of giving bluish-black or green 

 compounds with iron salts; of precipitating solutions 

 of gelatin ; and by the ability to tan animal hides ; i. e. 

 to convert them into leather. Their important uses in 

 dyeing, in the preparation of ink, and dressing of 

 leather, depend upon these properties. They also con 

 stitute the active principles of a number of plants em 

 ployed in medicine. 



Gallotannic acid (Tannin), C 27 H 22 17 . Occurs particu 

 larly in gall-nuts, the excrescences found on the young 

 branches of Quercus infectoria, caused by the punctures 

 of the gall-wasp ; these contain about half their weight 

 of tannic acid ; in still larger quantity in Chinese gall- 

 iiuts, formed in a similar manner ; also in the various 

 species of sumach (the branches of Rhus coriaria) ; and 

 probably in still other plants. 



Eight parts powdered gall-nuts (most profitably 



