24 CHEMICAL COMPOSITION OF LIVING MATTER 



into vinegar or lemon juice it will turn red again. The 

 reason for this is that vinegar and lemon juice contain 

 certain combinations of nonmetallic elements called 

 acids. The acid present in vinegar is a combination of 

 hydrogen, oxygen, and carbon, called acetic acid. Some 

 other powerful acids are sulphuric acid (hydrogen, sul- 

 phur, and oxygen), nitric acid (hydrogen, oxygen, and 

 nitrogen), and hydrochloric acid (hydrogen and chlorine). 

 All acids have a sour taste, and contain no metals in their 

 composition. 



If we add drop by drop some of a base to an acid, 

 we shall evidently come to a point when the tendency 

 of the acid to turn litmus red is just counterbalanced by 

 the tendency of the base to turn it blue, "\^'hen this point 

 is reached the base is said to have neutralized the acid, 

 and the process is called neutralization. Finally, if we 

 drive off the water of such a neutralized mixture by evap- 

 oration, we have left a product which turns litmus neither 

 blue nor red and is called a salt. Analysis of such a prod- 

 uct shows it to be composed partly of the metal which 

 was in the base and partly of the nonmetals which were 

 in the acid. You can make common table salt yourself 

 by mixing hydrochloric acid and the base caustic soda 

 (oxide of sodium) in this way. (See Ex. XII.) Salts, or 

 compounds of metals and nonmetals, form an important 

 part of the composition of our bodies, bones being a salt 

 of calcium, phosphorus, and oxygen. Since most of the 

 minerals in nature, limestone, quartz, etc., are metallic 

 salts, such salts in the living matter are spoken of as 

 mineral salts to distinguish them from compounds of 

 purely nonmetallic elements. 



