THE VOLATILE PART OF PLANTS. 81 



with potash or nitric acid. Commercial oxalic acid Is thus made from 

 sawdust. 



Gum (Arabic), sugar and starch yield tartaric acid by the action of 

 nitric acid. 



Definition of Acids, Bases, and Salts. — In the popular 

 seuse, au acid is any body having a sour taste. It is, in 

 fact, true that all sour substances are acids, but all acids 

 are not sour, some being tasteless, others bitter, and some 

 sweet. A better characteristic of an acid is its capability 

 of forming salts by its interaction with bases. The strong- 

 est acids, i. e., those bodies whose acid characters are most 

 highly developed, if soluble, so as to have any effect on 

 the nerves of taste, are sour, viz., sulphuric acid, phos- 

 phoric acid, nitric acid, etc. 



Bases are the opposite of acids. The strongest bases, 

 when soluble, are bitter and biting to the taste, and cor- 

 rode the skin. Potash, soda, lime, and ammonia are ex- 

 amples. Magnesia, oxide of iron, and many other com- 

 pounds of metals with oxygen, are insoluble bases, and 

 hence destitute of taste. Potash, soda, and ammonia 

 are termed alkalies ; lime and magnesia, alkali-earths. 



Salts are compounds that result from the mutual ac- 

 tion of acids and bases. Thus, in Exp. 30, the salt, cal- 

 cium phosphate, was produced by bringing together 

 phosphoric acid, and the base, lime. In Exp. 37, cal- 

 cium oxalate was made in a similar manner. Common 

 salt — in chemical language, sodium chloride — is formed 

 when caustic soda is mixed with hydrochloric acid, water 

 being, in this case, produced at the same time. 



NaOH + HCl = NaCl + HjO 



SoiMuTn hydroxide. Hydrochloric acid.. Sodium chloride. Water. 



In genera,!, salts having a metalUc base are formed by 

 substituting the metal for the hydrogen of the acid ; or if 

 an organic acid, for the hydrogen that is united to oxy- 

 gen, i.e., of carboxyl, COOH. 



Ammonia, NHg, and many organic bases unite directly 

 to acids in forming salts. 

 6 



