HEAT 



135 



liquid at all times. The boiling point of a liquid is the 

 temperature at which the vapor pressure of the liquid 



becomes greater 

 than the atmos- 

 pheric pressure on 

 the liquid. Since 

 the atmospheric 

 pressure becomes 

 less and less as we 

 ascend, the boiling 

 point for any cer- 

 tain liquid will be 

 lowered as we as- 

 Water boils 



FIG. 117. Boiling Water at a Temperature 

 below 100 Centigrade by Reducing the Air Pres- Cend. 



sure with the Aid of an Air Pump. 



at 



temperature on the tops of our high mountains than at 

 sea level. 



Experiment 38. Boil some water in a flask and then remove it 

 from the fire and place it under the receiver of an air pump (Figure 

 117). Exhaust some 

 of the air and note 

 the result. How do 

 you explain the fact 

 that the water again 

 ,begins to boil vio- 

 lently when it is evi- 

 dently much colder 

 than when it was boil- 

 ing over the fire? 



Experiment 39. - 

 Fill a Florence flask 

 half full of water and 

 boil the water vigor- 

 ously for a minute. FlG- us. Boiling Water at a Temperature below 

 Now close the flask 100 Centigrade. 



