424 TANNIC ACIDS. 



white powder, easily 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 Acids. 



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- 

 nuts, 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 



