802 THE BOILING POINT. 



turned rapidly in the direction of the arrow so that 

 the whole water falls from the bulb into the empty 

 tube, and strikes against the glass. 



It has been previously stated that at the temperature 

 of the boiling point the tension of the vapour is equal 

 to the pressure of the atmosphere upon the liquid. It 

 would, however, be more correct to say, conversely, that 

 the lolling point of a liquid is that temperature at which the 

 tension of its vapour is equal to the pressure it supports. 

 When the vapour has a lower tension, it is unable to 

 displace the atmosphere, and can only diffuse itself 

 through the air, but as soon as its tension becomes 

 equal to the pressure of the atmosphere it is capable of 

 lifting the air before it and of expanding freely. 



Under diminished atmospheric pressure the tension 

 of the vapour need not be so 

 high as to support a column of 

 mercury of 760 mm , and the tem- 

 perature is therefore less than 

 100 when the liquid boils. 

 Hence ebullition may be pro- 

 duced at very low temperatures 

 if the pressure be artificially 

 diminished. To prove this a 



FIG. 394 (I real size). L 



flask half filled with water at 



40 or 50 may be placed under the receiver of the air- 

 pump. When the air is exhausted the water begins to 

 boil briskly. Or, without using an air-pump, water 

 may be boiled in a retort until the air is expelled ; 

 the neck is then closed, and the retort immersed in a 

 vessel with cold water. The rapid condensation of the 

 steam in the retort produces a partial vacuum in it, and 



