252 PHYSIOLOGY. 



about 300 square inches ; by adding some carbonic acid to 

 the jar, in a short time 26 cubic inches of oxygen were 

 evolved. What then must be the amount given off by an 

 entire tree, especially in cities, where carbonic acid abounds ; 

 hence how important it is that cities should be thickly plant- 

 ed with trees, not only for the sake of ornament, but much 

 more for that of utility. 



21. The quantity of oxygen consumed by a man in a 

 minute, is about 30 cubic inches. He breathes 20 times in 

 a minute ; and every time he breathes, takes into his lungs 15 

 cubic inches of atmospheric air, which contains three cubic 

 inches of oxygen, so that one half of that which is inspired 

 disappears in every act of respiration. This will amount to 

 about 2,000 cubic inches in an hour, ancU45,000 cubic inches 

 in 24 hours.y Thus one man will consume, in 24 hours, all 

 the oxygen contained in a space of 312 square feet ; or in 12 

 hours, in 156 square feet. And still our churches, and school- 

 rooms, and dwelling-houses, are constructed with no view to 

 a renovation of the air we breathe ! 



22. Dr. Southwood Smith has lately performed a series 

 of very interesting experiments, from which he deduces the 

 following general results : " 1. The volume of air ordinarily 

 present in the lungs is about 12 pints. 2. The volume of 

 air received by the lungs at an ordinary inspiration is one 

 pint. 3. The volume of air expelled from the lungs at an 

 ordinary expiration, is a little less than one pint. 4. Of the 

 volume of air received by the lungs at one inspiration, only 

 one-fourth part is decomposed at one action of the heart, and 

 this is so decomposed in the five-sixth parts of one second 

 of time. 5. The blood circulates through the system, and 

 returns to the heart in 160 seconds of time, which is exactly 

 the time in which the whole volume of air in the lungs is de- 

 composed. These circuits are performed every eight minutes ; 

 540 circuits are performed every 24 hours. 6. The whole 

 volume of air decomposed in 24 hours is 221,882 cubic inches, 

 exactly 540 times the volume of the contents of the lungs. 



