342 PHYSIOLOGY AT THE FAEM. 



salts of gallic acid do not cause a precipitate in solutions of 

 gelatine. 



A solution of gallotannic acid occasions a violet black pre- 

 cipitate in solutions of the persalts of iron. Hence the basis of 

 ordinary writing-ink is gallotannate of iron. To be distinguished 

 from the effect of gallotannic acid on the persalts of iron, is 

 that of gallic acid on a mixture of the protosalts and persalts 

 of the same metal. When gallic acid is added to a mixture of 

 the protosalts and persalts of iron, a deep bluish-black solution 

 is formed ; if the solutions be free from acid, and particularly 

 if a solution of bicarbonate of lime be added, the reaction is 

 found to be one of extreme delicacy. 



Most astringent vegetable substances, such as the bark and 

 leaves of the oak, the elm, the willow, the horse-chestnut, the 

 pine, the pear, the plum, the wood and bark of the sumach and 

 whortle-berry, the roots of tormentilla and bistort, precipitate 

 the persalts of iron of a bluish-black colour, and if a free acid 

 be present the solution assumes a dark-green colour. Tea and 

 Paraguay tea give the same colour, but coffee has no such effect. 

 There are astringent vegetable bodies which precipitate the 

 persalts of iron of a dark-green instead of a blue colour, of 

 which catechu and kino are good examples. Again, some as- 

 tringent plants, of which so powerful an astringent as rhatany 

 (Krameria triandra) is an instance, precipitate persalts of iron 

 of a grey colour. The variety of tannin exhibited in catechu and 

 kino is termed mimotannic acid, as derived chiefly from plants 

 belonging to the section Mimosce of the leguminous family. 



Lactic Acid. Lactic acid is the acid of sour milk, yet it is 

 formed by various vegetable substances. For example, when 

 oatmeal diffused in a large quantity of water becomes sour, it 

 is found that lactic acid has been produced. It is commonly 

 procured from cane-sugar. When eight parts of cane-sugar 

 are dissolved in fifty parts of water, and this solution, after the 



