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I02 SCIENCE PRIMERS. [§ vi. 



How then does the carbonic acid at the bottom of 

 the lungs get out? How do the capillaries in the 

 air-cells get their fresh oxygen ? 



The stationary air mingles with the tidal air at every 

 breath. If you want to ventilate a room, you are not 

 obliged to take a pair of bellows and drive out every 

 bit of the old air in the room, and supply its place with 

 new air : it will be enough if you open a window or a 

 door and let in a draught of pure air across one corner, 

 say, of the room. That current of pure air flowing 

 across the comer will mingle with all the rest of the air 

 until the whole air in the room becomes pure; and 

 the mingling will take place very quickly. So it is 

 in the lungs. The tidal air comes in with each inspi- 

 ration as pure air from without ; but before it comes 

 out at the next expiration it gives up some of its 

 oxygen to the stationary air, a. ^ robs the stationary 

 air of some of its carbonic acid. For each breath of 

 tidal air the stationary air is so much the better, 

 having lost some of its carbonic acid and gained some 

 fresh oxygen. ' The tidal air rapidly purifies the 

 stationary air, and the stationary air purifies the blood. 



Thus it comes to pass that the tidal air, which at 

 each pull of the diaphragm and push of the sternum 

 goes into the chest as pure air with twenty-one parts 

 oxygen to seventy-nine parts nitrogen in every hundred 

 parts, comes out, when the diaphragm goes up and 

 the sternum falls back, as impure air with only sixteen 

 parts oxygen, but with five parts carbonic acid to 

 seventy-nine of nitrogen. That ioFt oxygen is carried 

 through the stationary air to the blood in the capil- 

 laries, and the gained carbonic acid came through the 

 Stationary air from the blood in the capillaries. So 



