70 WATER PASSES MORE EASILY THAN 



the mercury is caused by water being forced through 

 the bladder into the tube, in virtue of the existence 

 of an external pressure greater than the pressure on 

 the inside of the tube. 



Water To displace the aqueous particles in the pores of 



through the bladder by other aqueous particles, requires 



moist blad- 

 der more obviously a much smaller pressure than is necessary 



easily than 



air does. to displace them by particles of air. In the one 

 case, where both surfaces of the bladder are in con- 

 tact with the liquid, the attractive force (that of the 

 bladder for the water and of the water for the blad- 

 der) is equal on both sides ; but not so in the other 

 case, where one side of the bladder is in contact 

 with air. If the bladder had the same absorbent 

 power for the particles of air as for those of water, 

 the particles of air and water would pass through 

 the bladder under the same pressure ; the experi- 

 ment shews, that the absorbent power and perme- 

 ability of the bladder for air is far less than for 

 water. Hence it comes to pass, that when, with a 

 given portion of bladder, in the apparatus, Fig. 11, 

 mercury is raised by evaporation to a height of 12 

 inches, less than 12 inches of mercury are required, 

 in the apparatus, Fig. 1, to cause water to pass 

 through the bladder. 



