BOILING POINT 



83 



from boiling. Place a piece of ice on the bottom of the flask. As 

 the ice melts, and cold water runs down over the flask, steam con- 

 denses, and reduces the pressure on the surface of the water. 

 Since water boils more easily at a low pressure than at high pres- 

 sure, the water begins to boil immediately. Why does the water 

 stop boiling every time the ice is removed? 



FIG. 64. How hot would boiling water be on Pike's Peak? In Denver, Colo.? 

 In Spica, Kan.? In Kanfield, Colo.? In New York?^the barometer read- 

 ing 30 inches at sea level. How is it possible for an aeronaut to tell how 

 high he is above the ground with a barometer? Why could Mark Twain 

 tell how high up in the Alps he was by " boiling his thermometer"? 



Boiling point varies about .88 of a degree Fahrenheit for a 

 variation in the barometer of half an inch. Water boils in a prac- 

 tical vacuum, or when the mercury has fallen to .24 of an inch 

 (0.1176 pound pressure), at about 40 F. which is the lowest tem- 

 perature at which water will boil. 



At the top of Mt. Blanc in Switzerland, which is three miles high, 

 water boils at 185 F. A traveler could not boil eggs at the top of 

 this mountain, as the white of an egg will not harden unless the 

 temperature is far in excess of 185 F. 



