84 



GENERAL SCIENCE 



FIG. 65. Denver Cooker. 



" Singing " of a Teakettle. When water is first heated, tiny 



bubbles of air, which was dissolved 

 in the water, pass off. Steam bub- 

 bles next appear on the sides and 

 bottom of the flask. They rise a short 

 distance and condense, since the water 

 is not as hot as the bubbles of steam. 

 This often causes a noise which is 

 known as the " singing of a teakettle." 



Pressure Cooker. The pressure 

 cooker is designed to cook at 20 

 pounds of steam pressure in addition 

 to the atmospheric pressure. Water 

 will boil at a higher temperature 

 under such conditions; food, therefore, 

 cooks much more quickly. 



The following table will show how 

 the boiling-point i ises \\ ith the pressure : 



Degrees. 



At atmospheric pressure, 10,000 feet above sea level. . 194 

 At atmospheric pressure, 5000 feet above sea level. . . . 203 



At atmospheric pressure, at the sea level 212 



At 5 pounds steam pressure . . .- 227 



At 10 pounds steam pressure . . . 239 



At 15 pounds steam pressure 250 



At 20 pounds steam pressure 259 



At 25 pounds steam pressure 267 



Boiling Point of Different Substances. Not all substances boil 

 at the same temperature. The following table gives a few of the 

 common substances which have different boiling pcints. 



Fahrenheit. 



Water 212 



Alcohol 172 



Ether 98 



Gasoline 1 12-140 



Mercury 674 



Sulphur 831 



Kerosene.. . 365-392 



