138 



GENERAL SCIENCE 



FIG. 105. The earth's tiny share of the 

 sun's heat. 



titles of heat are given off because of its great pressure. The sun 

 is about 1,300,000 times the size of the earth. 



Radium and other 

 elements are also sup- 

 posed to assist in giving 

 off heat. 



Oxygen in Other 

 Things. The amount 

 of oxygen in the rocks 

 a few feet below the sur- 

 face of the earth equals 

 the amount of oxygen 

 in the air above them. 

 Oxygen is found in 

 water, food, sand, clay, gravel, wood and in the majority of sub- 

 stances which we use every day. It is not found free, as we find 

 it in the air, but united with the substances. 



Prevention of Oxidation. Many methods of preventing oxygen 

 from uniting with substances have been tried. Painting wood, 

 iron, etc., is the usual method. Sometimes the outside of the sub- 

 stance is oxidized, or covered with a lacquer or shellac. 



The leathery skin formed on linseed oil left exposed to the 

 air is due to the oxygen in the air. We often use paints and oils 

 which oxidize and form a hard substance over the material we 

 wish to keep from the oxygen. Paint containing linseed oil, fish 

 oil, or china oil is said to contain drying oils. When these oils do 

 not oxidize rapidly enough, a drier is added. Certain metals, as 

 nickel, oxidize very slowly, and are often used to cover other metals 

 which oxidize rapidly. 



Rapid oxidation or burning is often prevented by adding water, 

 thus shutting off the supply of oxygen. The water lowers the tem- 

 perature, changes to steam, or covers the material with a thin film 

 of water and water vapor an action which prevents oxygen from 

 uniting with the burning material. 



Water cannot be used to extinguish the flame of burning oil 

 because the oil is lighter than water and will float on it, thus coming 

 in contact with the air and continuing to burn, 



