OXYGEN 35 



oxid, SO 2 . Other elements, as phosphorus and iron also 

 unite with oxygen, forming oxids. Different elements 

 unite with oxygen at different temperatures. Phosphorus 

 and sulfur combine with oxygen at a comparatively low 

 temperature, while carbon and iron require a higher tem- 

 perature. The sulfur and the splinter of wood burned 

 more brilliantly in the oxygen than in the air because air 

 is diluted with other gases and elements and is not pure 

 oxygen. Oxygen is more active at a high than at a low 

 temperature. 



The oxidation of some of the elements and compounds 

 results in the production of light and heat ; this is com- 

 monly called combustion. Oxygen forms stable com- 

 pounds with many of the elements. It has such affinity 

 for some elements, as aluminum and carbon, that it is 

 separated from them with difficulty. With other ele- 

 ments it forms less stable compounds. When a substance 

 contains oxygen, it does not necessarily follow that it is 

 combustible, because it may be the product of combus- 

 tion as carbon dioxid or sulfur dioxid. When an ele- 

 ment, as oxygen, enters into chemical combination, it 

 loses its identity or individuality as an element. The 

 oxygen that is present in the minerals forming the crust 

 of the earth, and in plant and animal tissues, is not free 

 but combined with other elements. 



38. Importance. Oxygen takes an important part in 

 life affairs, and is necessary to the existence of plant and 

 animal bodies. The combustion of wood, coal, and other 

 fuel is due to the oxygen of the air. The production of 

 heat in the body is due to the oxidation of food, and many 



