68 EXPERIMENTAL GENERAL SCIENCE 



liquefied by pressure alone. A good illustration of the heat 

 that develops when a gas is compressed may be found when air 

 is pumped into a bicycle or automobile tire. After a few 

 strokes of the pump the tube connecting the pump and tire 

 becomes noticeably warmer. When gas under pressure is 

 allowed to expand, however, the molecules at once move 

 further apart and, taking up some of the heat in the process, 

 become cooler. As might be inferred, liquids exposed to gases 

 under pressure absorb much more than they otherwise would. 

 The carbonated water used at soda fountains consists of carbon 

 dioxide forced into the water under pressure. 



64. The Pressure Cooker. In the pres- 

 sure cooker, which consists of a vessel with 

 a close-fitting cover that allows no steam 

 to escapa advantage is taken of the fact 

 that pressure retards a change of state and 

 causes an increase in the temperature of a 

 substance. The contents of such a cooker 

 may be heated to temperatures of 250F. or 

 more and thus be quickly cooked. A valve 

 FIG. 23. Pressure in the cover prevents the pressure from be- 

 coming high enough to cause an explosion. 

 A similar contrivance used in the scientific 

 laboratory is called an autoclave. Housewives make use of 

 this principle when they cover the kettle in which food is 

 cooking. In canning, the use of the pressure cooker enables 

 one to preserve many substances that are otherwise very diffi- 

 cult to keep. The great heat to which they are thus subjected 

 kills the germs of decay that are often unharmed by ordinary 

 processes of canning. 



65. Refrigeration. All our mechanical systems of refrig- 

 eration are based on the relations of pressure and heat to the 

 change of state. The cooling desired is accomplished by com- 

 pressing a gas until it becomes liquid and then removing the 



