OXIDATION 225 



only a process of combustion, and the word fire means 

 all that combustion means, the chemical union of dif- 

 ferent elements, together with the heat, flame, light, 

 etc., that may occur in the process. A little thought 

 will make the matter clear. The chemical action in com- 

 mon fires is between the elements H or C and O ; the 

 heat is given off as from any chemical union ( 206) ; 

 flames show that a gas (usually H) is burning ; light is 

 given off from glowing solid particles (commonly of 

 C) ; smoke is a mass of solid particles that were not 

 entirely burned; and ashes are made up of mineral 

 matter that could not burn. 



264. Oxidation. We have learned that oxygen com- 

 bines directly with many elements ( 222), and that it 

 does this rapidly if they be heated to a high enough 

 degree ( 260). Now it also happens that several ele- 

 ments will combine with oxygen even at the ordinary 

 temperature of the air, but they do this very slowly. 

 The process is called oxidation; the compound formed 

 is called an oxide. 



Experiment 145. File a piece of iron till bright ; dip it in 

 water, remove it, and without even shaking off the drops of water, 

 set it aside. In two or three days examine it, and tell what has 

 happened. 



Experiment 146. Scrape a piece of lead till its surface is 

 bright and clean, then set it aside. In a few days examine the 

 lead, note its surface, and explain the change. 



Most of the metals will combine directly with O ; 

 gold does not, and silver forms a sulphide rather than 

 an oxide in air. The presence of water usually assists 



