ORGANIC ACIDS. 207 



For the properties, methods of preparation, and constitution 

 of these acids the reader is referred to any modern text-book 

 on organic chemistry. The free acids themselves often occur 

 in plants, particularly in fruits, e.g., malic acid, occurring in 

 apples, mountain-ash berries, gooseberries, red currants, black- 

 berries, and sour or morella cherries. 



In other cases, acid potassium or calcium salts are the cause 

 of the acidity, e.g., potassium hydrogen tartrate in grapes, 

 acid potassium malate in sweet cherries. In many fruits 

 several organic acids or their salts occur in association, e.g., 

 gooseberries, currants, and cherries contain both malic and 

 citric acids, while mountain-ash berries and tamarinds contain 

 malic, citric, and tartaric acids. 



Oxalates of potassium and, particularly, of calcium are 

 extremely widely distributed in the vegetable kingdom, the 

 latter occurring often in the solid, crystalline state in the cells 

 of plants. The crystals of the calcium salt have the composi- 

 tion CaC 2 4 .3H 2 O and are soluble in saccharine solutions. Acid 

 potassium oxalates, KHC 2 4 and KHC 2 O 4 + H 2 C 2 4 , occur in 

 solution in the sap of certain plants, e.g., sorrel, rhubarb, &c. 



Tannic acid or tannin is very abundantly distributed in the 

 vegetable kingdom. It varies in composition and properties 

 according to its origin. 



Tannin was formerly looked upon as a glucoside, but the 

 tannic acid, its chief constituent, is now looked upon as an 

 anhydro-acid." Gallo-tannic acid, for example, is said to have 

 the constitution C e H 2 (OH) 3 .CO.O.C 6 H 2 (OH) 2 .COOH. 



It is hydrolysed by dilute acids or by ferments, with the 

 production of gallic acid, C 6 H 2 (OH) 3 .COOH. 



The natural tannins, however, are usually associated or 

 combined with glucosides, and on hydrolysis yield glucose in 

 addition to gallic acid. 



Tannin is present in almost all plants, occurring in the bark, 

 leaves, root, or fruit. Its origin and function in the plant are 

 much debated questions. By some investigators it is thought 

 to be formed by chlorophyll from carbon dioxide and water, 

 and in many ways to resemble starch in its relationship to the 



* Schiff Ann. 170, 43. 



