HOW THE ATMOSPHERE IS PURIFIED 57 



The answer is that the impurities are constantly being 

 removed. The wind scatters foul air, and mixes it with 

 fresh air from the country, the mountains, and the sea. 

 The rain washes out impure gases, dust, smoke, and 

 bacteria, or germs. Sunlight destroys many bacteria that 

 produce disease. Other bacteria bring about the oxida- 

 tion of dead organic matter, and so destroy it. Plants 

 use up the carbon dioxide cast off by animals, and give 

 back oxygen in its place (cf. 310). 



59. Summary. The atmosphere is the gaseous ocean that forms 

 the outer layer of the earth. Air is the substance that makes up the 

 atmosphere. 



Air is matter, for it takes up space, and has weight, inertia, etc. 



The pressure of the atmosphere is due to the weight of the air. It 

 is measured by the barometer, and is equal to about 15 Ibs. for each 

 square inch of surface at sea level. The pressure becomes less as we 

 go up a mountain, and greater as we go down into a mine. 



The lift pump removes from a pipe the air that is over a liquid, 

 so that atmospheric pressure will raise the liquid. Its limit is about 

 34 feet for water, and 30 inches for mercury. 



Compressed air, like other compressed gases, can do work when it 

 expands. 



Gases are " collected " by displacement of air or water. 



Priestley discovered oxygen in 1774 by heating mercury oxide. 



Lavoisier proved that air consists of two substances, oxygen and 

 nitrogen. 



Oxygen makes up about one fifth, by volume, of air. 



Burning in air is union with oxygen, or oxidation. Oxidation may 

 be slow, as in rusting and decay, or rapid, as in burning (combustion). 



A flame is a burning gas. 



Oxygen is prepared by heating mercury oxide or a mixture of 

 potassium chlorate and manganese dioxide, or by putting hydrogen 

 peroxide with potassium permanganate. 



Oxygen is colorless, tasteless, odorless, and slightly soluble in water. 



