64 



LIVING IN AN OCEAN OF AIR 



of them hit against the tube (see diagram 2), thus increas- 

 ing pressure against it. Thus air pressure causes the tube 

 to bulge out and we say "the tire is up." The tire stays 

 up as long as it holds these gas molecules, and it goes down 

 when the number of the molecules decreases so that there 

 are too few to strike enough blows to maintain the pressure. 

 A Household Use of the Vacuum. Recall the demon- 

 stration in which steam drove the air from a flask. When 

 the steam condensed, the pressure in the flask was less 

 than that of the atmosphere. In canning fruit and 

 vegetables, the heat used produces three important re- 

 sults. It cooks the food, it kills the bacteria that might 

 cause it to spoil, and it produces steam that drives all the 

 air out. If the cover is put on before the water present 

 cools, after the cooling and condensing of the steam a 



vacuum is formed. Out- 

 side atmospheric pressure, 

 being so much greater 

 than the pressure inside, 

 presses the cover on so 

 tightly that no germs 

 (bacteria) can get in to 

 harm the food. 



How to Empty a Liquid 

 from a Vessel with a 

 Small Opening. Many 

 people make no use of 

 their science outside the 

 classroom because it is 

 difficult for them to apply 

 scientific facts and prin- 

 ciples to new situations. 

 Did you ever try to suck water out of a bottle that is full 

 of water having a glass tube passing into it through a 

 tightly fitting stopper ? Try it if you think you can do it. 



.cork 



Try sucking water from each of these bot- 

 tles. Which gives the better result ? 



