334 Common Science 



carbon dioxid). This combining (burning or oxidizing) 

 gives us our body heat and the energy to move. The 

 free oxygen is carried to the different parts of our bodies 

 by the red blood corpuscles that float in the liquid part 

 of the blood. The liquid part of the blood also carries 

 the food to the different parts of the body, and the food 

 contains the carbon and hydrogen that is to be burned. 

 Then in a muscle, for instance, the oxygen that has 

 been carried by the corpuscles combines with the carbon 

 to form carbon dioxid, and with the hydrogen to form 

 water. The corpuscles carry part of the carbon dioxid 

 back to the lungs, and the water is carried with other 

 wastes and the rest of the carbon dioxid in the liquid 

 part of the blood. In the lungs the carbon dioxid is ex- 

 changed for the free oxygen we have just inhaled, and we 

 exhale the carbon dioxid. A good deal of water is also 

 breathed out, as you can tell from the way the mist 

 gathers on a window pane when you blow on it. 



If there were only animals (including people) in the 

 world, all the free oxygen in the air would in time be 

 combined by the animals with hydrogen to make water 

 and with carbon to make carbon dioxid (CO*). As 

 animals cannot breathe water and cannot get any good 

 from carbon dioxid, they would all smother. 



But the plants, as we have already said, use carbon 

 dioxid (C0 2 ) and water (H 2 O) to make food. They 

 do not need so much oxygen, and so they set some of 

 it free. The countless plants in the world set the 

 oxygen free as rapidly as the countless animals com- 

 bine it with hydrogen to make water and with carbon 

 to make carbon dioxid. Since the water and carbon 



