66 EFFECTS OF EVAPORATION. 



bladder ; and it maintains its level, even after all 

 the water has disappeared. 



The height of the mercury in the narrow tube is 

 an exact measure of the pressure acting on the sur- 

 face of the bladder. The pressure in the inside of 

 the tube is less than the existing pressure of the 

 atmosphere outside by the height of that column 

 of mercury. 



This difference of level between the mercury in 

 the vessel and that in the tube is the limit of the 

 pressure, under which air passes into the water 

 through the pores of the bladder ; or under which 

 the molecules of water in the pores are displaced 

 by the molecules of air. 



If we fill the tube entirely with water, and 

 place the narrow end in mercury, while the wide 

 end, closed with bladder, is exposed to the air, the 

 mercury rises in the narrow limb, and at last 

 reaches a point, identical with that to which it fell 

 in the preceding experiment. For each specimen 

 of bladder, according to its thickness, the level to 

 which the mercury reaches is of course different. 



When the diameter of the wide part of the tube, 

 which is closed with bladder, is 12 millimetres, and 

 that of the narrow tube 1 millimetre, the mercury 

 rises, with ox-bladder, according to the temperature 

 and the hygrometric condition of the air, to from 22 

 to 65 millimetres in one hour. 



The cause of the rise of the mercury in this 

 experiment hardly requires a special explanation. 



