64 HUMAN PHYSIOLOGY CHAP. 



mixed throughout the entire world over land and sea. 

 Although such a uniform distribution is never reached, 

 yet this means prevents the air from becoming entirely 

 unfit for breathing in certain places. If it were not for 

 the principle of diffusion of gases, people could not live 

 in large cities. 



2. Winds. All the time there are currents of air 

 moving from place to place which not only aid in the 

 mixing of gases but move the entire body of air and 

 tend to distribute the temperature equally. 



3. Rain. Gases, dust, and other impurities are taken 

 up by the raindrops and washed out of the air. Snow 

 in the winter has a similar purifying effect. The mois- 

 ture and oxygen of the air change many impurities of 

 the atmosphere by oxidizing them to harmless products. 



4. Vegetation. Plants take carbon dioxid from the 

 air, and water from the ground as their food. The 

 green cells of the leaves change these two substances 

 into starch by the aid of the heat which the sun fur- 

 nishes. When the plant puts carbon dioxid and water 

 together to form starch, it has quite a large amount of 

 oxygen which it does not need, and this it gives back 

 to the air. The plant inhales carbon dioxid and exhales 

 oxygen. In animals the very opposite occurs. The 

 plant purifies the air for us, and we purify the air for 

 the plant. 



59. Ventilation. Everything we eat, drink, and wear 

 should come in contact with life-giving, germ-destroying 

 oxygen and sunshine. How pale and sickly a plant 

 looks that is kept in a dark, close room ! The rooms in 



