OXALIC ACID TANNIC ACID. 341 



Oxalic acid is also found free in several species of Rumex, 

 as in the common sorrel (Rumex acetosa), the sheep's sorrel 

 (Rumex acetosella), the French sorrel (Rumex scutatus) ; and 

 all of these, particularly the cultivated common sorrel, have 

 for ages been employed in dietetics, and no harm seems to have 

 resulted from their use. The wood sorrel (Oxalis acetosella') 

 also contains oxalic acid, and is eaten in salads ; it is used 

 sometimes also as an antiscorbutic. The very poisonous 

 effects of the pure oxalic acid have driven its preparations 

 very much out of use in medicine. 



Tannic Acid. The term tannine was originally employed 

 to denote the several varieties of astringent principle which 

 were employed in the tanning of hides. As most of these 

 possess an acid reaction, the name tannic acid has come to be 

 applied to them. The tannic acid of the British Pharmacopoeia 

 is C^H^O 34 , the same to which the name gallotannic acid 

 is still sometimes applied. It is obtained from gall-nuts. 

 These nuts, excrescences formed on the Quercus infectoria, 

 contain two-thirds of their weight of tannic acid, and only 

 two per cent of gallic acid. Gallotannic acid is freely soluble 

 in water ; also soluble in dilute alcohol, and sparingly in sul- 

 phuric ether. Its diluted aqueous solution absorbs oxygen 

 from the air, and is converted into gallic acid. All the forms 

 of tannic acid, indeed, when moist freely absorb oxygen. A 

 particular ferment in gall-nuts favours the conversion of its 

 gallotannic acid into gallic acid. Hence when powdered gall- 

 nuts, moistened freely with water, are allowed to stand in a 

 warm place exposed to the air, they absorb oxygen, and be- 

 come mouldy, while abundance of gallic acid in crystals is 

 formed. 



The most remarkable property of tannic acid is its power of 

 precipitating animal gelatine from its solution an effect on 

 which the tanning of leather depends. On the contrary, the 



