202 OXALIC ACID. 



of powdered animal charcoal, the color may be re- 

 moved. By merely exposing the crystals of tartar 

 to the sun and air, they become bleached to a consi- 

 derable extent ; for this purpose they are spread out 

 upon large sheets of canvas, and left some time thus 

 exposed, after which, they are dissolved and recrys- 

 tallized. 



501. Tartaric a'cid is a white crystalline solid, like 

 citric acid, easily soluble in water, and uniting to bases 

 to form salts called tartrates. It is chiefly used in 

 dyeing, and calico-printing; it is also used in medi- 

 cine, and in the manufacture of saline and effervescing 

 drinks. 



502. Malic acid occurs associated with citric acid 

 in apples, pears, and in the stems of common rhubarb. 

 It forms salts called malates, but neither the acid nor 

 any of its salts are of much importance; they are 

 not used for any practical purpose. 



503. Oxalic acid exists in considerable quantity 

 in rhubarb, sorrel, and a great many other plants ; 

 it is, however, generally made artificially from sugar. 

 When sugar, starch, lignin, &c., are heated with strong 

 nitric acid, violent action takes place, and the organic 

 substances are decomposed, the whole of their hydro- 

 gen being abstracted, and the carbon and oxygen 

 left in those proportions which form oxalic acid (180). 



504. The commonest salts of oxalic acid in plants 

 are oxalate of lime and super-oxalate of potash. The 

 former is a white insoluble salt, readily obtained by 

 adding oxalic acid to a solution of lime. In conse- 



