64 BOTANT. 



dries into a translucent mass. It gives the firmness and con- 

 sistence to apple, currant, and other fruifc jellies. Unripe 

 fruits contain a substance insoluble in water, alcohol, and 

 ether, which, during the process of ripening, or under the 

 action of heat, acids, and ferments, is converted into pectin. 



91. — In addition to oxalic acid (C^ H, 0,), which is found 

 generally combined with calcium, there are other vegetable 

 acids, some of which are even more common ; they occur 

 either free, or united with organic or inorganic bases. 



(a) Malic Acid (C, H^ OJ is abundant in many sour fruits 

 — e.g., apples, cherries, strawberries, currants, etc.; it is 

 likewise abundant in rhubarb, where it accompanies oxalic 

 and phosphoric acids. 



(5) Tartaric Acid (C^ H^ OJ occurs in the grape, tama- 

 rind, berries of the mountain ash (unripe), and other plants. 



(c) Citric Acid (0, Hj 0,) is found in abundance in the 

 lime, lemon, and other fruits of the Aurantiacese. It also 

 occurs in other sour fruits associated with rrialic acid, as in 

 gooseberries, raspberries, strawberries, cherries, etc. 



{d) Tannic Acid (C^, H^^ Oj,) occurs in the bark and 

 leaves of oak, elm, willow, and many other trees, in the 

 wood and bark of sumach and whortleberry, and the roots of 

 some Kosacess and Polygonaceee, and gives to them their as- 

 tringency. 



Nearly related to tannic acid is quinic acid, which occurs 

 in the bark of Cinchona (Peruvian Bark) in combination 

 with organic bases, of which quinia is the most important. 



There.are many other substances which occur in plants as 

 the products of cells — e.g., the vegetable alkaloids, many 

 coloring-matters, etc. As, however, this whole matter be- 

 longs rather to Organic Chemistry, it will not be carried 

 further in this place. 



