OILS AND FATS. 365 



is found in grapes, citric acid in lemons and some other 

 fruits, malic acid in apples, lactic acid in sour milk, etc. 



These organic acids form salts by replacement of hydro- 

 gen with a base just like the mineral acids. In fact the 

 acids mentioned as found in fruits do not exist as such, 

 but combined with potassium, sodium, or possibly calcium. 

 Thus in the case of tartaric acid it is combined with potas- 

 sium in the plant. Acid potassium tartrate, or so-called 

 cream of tartar, is gradually deposited in wine-casks from 

 the wine, and this is one cause of the improvement of wine 

 by age. Rochelle salt is a double salt a tartrate of potas- 

 sium and sodium. So tartar emetic is a tartrate of potas- 

 sium and antimony. Then there is a double tartrate of 

 potassium and iron, which is a valuable medicine. 



Many of the organic acids char on heating, owing to the 

 imperfect combustion of the carbon. If they are heated 

 more strongly complete decomposition ensues, just as in 

 the case of wood, sugar, etc. This distinguishes them in 

 their reactions from the mineral acids. 



509. Tannic Acid. This body is not a true acid, and 

 strictly belongs to another group of bodies called gluco- 

 sides, but it is of so much importance in many ways in the 

 arts that it should not be passed by. It exists extensively 

 in the bark of many trees, as the oak, horse-chestnut, hem- 

 lock, birch, etc., and is also found in some roots, and in the 

 leaves of roses and pomegranates. It exists most abun- 

 dantly in the gall-nut of the oak. Here we have a valuable 

 vegetable product as the result of disease, for the gall-nut is 

 a morbid growth which comes from the wound of an insect 

 made in the oak for the purpose of depositing its eggs.* 

 Tannin, as this body is commonly called, is a very astringent 

 substance. By the decomposition of tannic acid another 



* See Hooker's "Natural History," page 270, for further particulars. 



