82. 



GENERAL SCIENCE 



storm? From what directions does the wind usually blow after a 

 storm? 



3. Compare the velocity of the wind just before, during and 

 after a storm. 



4. Compare the temperature before, after and during a storm. 



5. Just before a storm, is the sky more or less cloudy? 



6. How does the sky look after a storm? 



BOILING POINT 



Atmospheric Pressure Affects the Rate of Evaporation. Water 

 evaporates more rapidly in high places, where the atmospheric 

 pressure is low, than in valleys. 



Effect of Air Pressure on the Boiling of Water. The pressure 

 of the atmosphere causes a difference in the temperature at which 

 water boils. At sea level if we fill a flask 

 half full of water and apply heat, the water 

 will not begin to boil until it has reached a 

 temperature of about 212 F.; but if we hap- 

 pen to live in Denver, Colo., the water will 

 boil at about 203 F. because Denver is over 

 5000 feet above sea level, and the air pressure 

 is not so great. An ascent of about 596 feet 

 produces a difference of 1 F. 



Water while boiling is violently agitated 

 by steam bubbles which are formed at the 

 bottom of the flask, nearest the flame. The 

 steam bubbles rise and burst, sending off steam 

 FIG. 63. The water w ith a pressure greater than that of the air. 

 the flask will boil In Qther word the ure of gteam at sea 



more vigorously if , , . , 



ice is used in place eve must be over 14.7 pounds per square inch, 

 of cold water and an( * in Denver 11.4 per square inch, 

 sponge. Remove the flame from under the flask, 



quickly close it with a rubber stopper and 



invert the flask on a ring stand. While the water was boiling, 

 the steam forced out all the air, leaving in the flask nothing but 

 steam and water. The pressure of the steam keeps the water 



