BURNING 137 



burn or make things burn. This gas is necessary to prevent the 

 presence of too large a percentage of oxygen in the air. 



Oxidation. The oxygen disappeared from the air in the bottle 

 experiment because this gas unites with any material which is 

 burning. Whenever oxygen unites with a substance, the substance 

 is said to oxidize. If the oxidation takes place very rapidly, heat 

 and light are given off. Rapid oxidation is called burning or com- 

 bustion. When the food in our body oxidizes, giving us the neces- 

 sary heat to maintain life, the process is called wet burning. Some- 

 times oxygen unites so slowly with substances as to give off little 

 heat at any one time. This process is called slow oxidation. 



Iron rust is caused by oxygen uniting with the iron. When- 

 ever oxygen unites with a substance, the resulting compound is 

 called an oxide. Iron oxide, copper oxide, etc., simply means that 

 oxygen has united with the substance. 



Meats turn brown when exposed to the air. An apple which has 

 been cut open and exposed to the air will turn dark brown, probably 

 because the oxygen unites with the iron in the apple, forming oxides. 



It is believed that nearly all the colors in nature are due to different forms 

 of iron. Different compounds of iron give the colors of blue, green, red, and 

 brown. 



Spontaneous Combustion. Heaps of cotton waste soaked in oil 

 slowly oxidize, giving off heat until the heat produced raises the ma- 

 terial to the kindling point, and fire results. Such fires are said to 

 be caused by spontaneous combustion. 



The Production of Light and Heat without Oxygen. Burning, 

 then, means a rapid combining of oxygen with other substances, 

 usually with the production of heat and light. Unless oxygen is 

 present, burning can not take place. Substances may glow, as in an 

 electric light bulb. The filament becomes very hot, but does not 

 oxidize or burn. 



Scientists have come to the conclusion that there is no oxidation 

 in the sun. The sun is really not burning up but glowing, and heat 

 is given off by its constant shrinking. This shrinkage is supposed 

 to be about 300 feet each year. So great is the weight of the sun, 

 1,980,000,000,000,000,000,000,000,000,000 pounds, that great quan- 



