226 



THE REALITIES OF MODERN SCIENCE 



II 



FIG. 26. 



dropper into the mercury immediately under the 

 column. Evaporation then takes place from this 

 drop, which should be some highly volatile substance, 

 like ether, so that the effect will be marked at ordinary 

 temperatures. The evaporation continues until the 

 number of molecules in the space above the liquid is 

 such that at each instant just as many 

 are condensing into the liquid form as 

 there are evaporating from the surface. 

 A statistical equilibrium is thus estab- 

 lished between the liquid and its vapor. 

 The pressure exerted by the vapor 

 molecules plus that of the weight of 

 the column of mercury (the weight of 

 the drop of liquid is negligible) must 

 balance the external atmosphere. The 

 mercury in' the column therefore falls. The amount 

 that this column is reduced, as shown by a second 

 barometer column, II, is the measure of the vapor 

 pressure in centimeters of mercury. 



If the barometer tube is pushed down into the mer- 

 cury the liquid condenses. If the tube is moved slowly 

 the temperature will remain constant, for the tube and 

 its contents will lose energy to their surroundings as 

 fast as it is released by the condensation. The reduc- 

 tion of the volume, occupied by the liquid and its vapor, 

 will be found to occur at constant pressure, as will be 

 evidenced by the constancy of the height of the mercury 

 volume. Similarly, if the volume is increased by 

 raising the tube it will be found that this also occasions 

 no change in the pressure as long as any liquid is in 

 contact with the vapor. After all the liquid is evap- 



