THE AIR 149 



A book lying on the desk exerts a force equivalent to 

 its own weight. Water in a vessel exerts a force against 

 the sides and bottom of the ves- 

 sel. Air is matter and so it 

 also exerts a force on the sides 

 of vessels containing it. You 

 have experienced this with bi- 

 cycle tires and footballs. Air As THE AIR is EXHAUSTED THE 

 also exerts a force on the SHEET RUBBER is FORCED IN 



earth's surface. This force is due to the weight of the 

 entire atmosphere. 



If a rubber membrane is stretched across a large opening 

 in a glass vessel and the air is then exhausted from it, the 

 membrane will be forced down into the vessel by the out- 

 side air pressure. Again, put a small quantity of water into 

 a tin can that can be tightly closed by a cork, and heat 

 it to boiling point. The air will be expelled by the steam. 

 While the water is still boiling, cork the can tightly, 

 remove it from the fire, and pour cold water over it. 

 The steam inside will be condensed and the air outside 

 will crush the can. When we drink lemonade through 

 a straw, we exhaust the air from the tube and the pres- 

 sure of the air on the liquid forces it up through the 

 straw. 



The height to which air will force water in a tube can 

 be determined from the following experiment. Take a 

 glass tube 34 inches long and closed at one end. Fill 

 the tube with mercury, close the end with your finger, 

 and invert it. Now place the bottom into a dish of 

 mercury and remove the finger. Notice the mercury 

 in the tube; it will fall a few inches, although no air 

 can get in at the top, thus making a perfect vacuum in 

 the top of the tube, The weight of the mercury in the 



