MATTER FORMED OF SMALL PARTICLES 



109 



73. The Measurement of the Pressure of Aqueous Vapour 

 in contact with Excess of Water at different Temperatures. 



When a small quantity of water is introduced into the vacuum 

 of a barometer, it will be found that the column of mercury is 

 depressed through a distance which does not change on the 

 introduction of more water, provided the temperature of the 

 whole be unaltered. But if the temperature be caused to rise, it 

 will be found that there is an increase of the depression of the 

 mercury column, caused by the pressure of the water vapour ; 

 and if a temperature of 100 C. be reached, the pressure of the 

 contained vapour would be equal to that of the atmosphere, i.e. 

 the column of mercury inside be depressed to the level of the 

 mercury outside. It will also be noticed that, if the tube 

 itself be raised or lowered, the height of the column of mercury 

 is unaltered. That is, the pressure of the vapour 

 in presence of water remains the same, whatever 

 the change in the space throughout which the 

 pressure has to be exerted. The quantity of liquid 

 may be noticed at the same time to diminish or 

 increase as the space is increased or diminished. 

 In each experiment there must be excess of 

 water. 



Any vapour which behaves in this manner is 

 called a saturated vapour. These results are inde- 

 pendent of the presence of air, as may be shown by 

 allowing a small quantity to enter the tube. 



In conducting these experiments firm stands 

 for the tubes are essential, and a tube with a 

 vacuum free from water is necessary for compari- 

 son. In order to show that the pressure is inde- 

 pendent of the space through which it is exerted, 

 provided only there be excess of water, the cistern 

 shown (fig. 39) is used. In order to show that 

 the pressure of the vapour at 100 is equal to that 

 of the atmosphere, complicated apparatus is re- 

 quired. What is strictly true is that water boils 

 at that temperature at which its saturated vapour has a 

 pressure equal to that which its surface bears. By diminishing 



Fig 39. 



