ATMOSPHERIC PRESSURE 39 



possible for them to get a thorough knowledge of many of 

 our common phenomena. We now know that the atmos- 

 phere contains gases and living forms of great activity, 

 and that these have very important effects upon the sub- 

 stances and the living things of the earth's surface. Thus, 

 the respiration, or breathing, of animals and plants, and 

 the decay of all substances of animal and vegetable origin, 

 are due to the air. So are the rusting, or tarnishing, of 

 metals, and the phenomenon of fire, or burning. 



39. Weight of Air. The atmosphere is drawn toward 

 the earth's center as all other bodies are; hence air has 

 weight (cf. 4). One liter of air ordinarily weighs about 

 1.2 grams. It takes a little over 4 liters (about a gallon) 

 to weigh as much as a nickel five-cent piece; that is, 5 

 grams. 



We can get an idea of the weight of air in another way: 

 A room 20 x 20 x 10 feet holds 4,000 cubic feet of air. One cubic 

 foot of air weighs about 134 ounces; hence the air of the room weighs 

 4,OOOX1M, or 5,000 ounces. Dividing this by 16 we get 312.5, the 

 weight in pounds. 



40. Atmospheric Pressure; the Barometer. Because 

 air has weight, it exerts pressure on all bodies in it, and 

 on the earth, which supports it. That the atmosphere 

 has pressure was first proved by Torricelli (pronounced 

 Tor-ri-tchell-y), a pupil of Galileo, in 1643. Finding that 

 no pump could lift water higher than 32 feet above the 

 water of a well, he reasoned that this was probably the 

 greatest height to which the atmospheric pressure could 

 push a water column. If this were so, then a mercury 



