AIR THROUGH MOIST MEMBRANES. 



71 



When the tube (Fig. 13) is filled 

 with water, closed with bladder at 

 both ends, and exposed to evapo- 

 ration, the bladders in a short time 

 become concave, that is, they are 

 pressed inwards. As the evaporation 

 of the water through the moist sur- 

 faces of the bladder proceeds, there 

 is formed in the upper part of the tube a vacuum, 

 which is filled with aqueous vapour, and which con- 

 tinues to increase. The place of the water which 

 evaporates is, as in the experiments previously de- 

 scribed, gradually occupied by air, which enters the 

 tube through the bladder. 



It is evident, that when air enters the tube (Fig. 

 13), the pressure on the surface of the bladder is 

 equal to the absorbent force of that bladder for the 

 water. In the apparatus, Fig. 11, with the same 

 bladder, the mercury might have been raised, in 

 consequence of the evaporation, to a height of 4, 6, 

 12, or more inches, according to the thickness of 

 the membrane. 



When the longer limb of the bent 

 tube, after it has been filled with 

 water, and closed at both ends with 

 bladder, is placed in a vessel contain- 

 ing brine, and exposed to evaporate in 

 the air, as in Fig. 14, it is plain, that 

 when the atmospheric pressure, in- 

 creasing in consequence of the evapo- 

 ration of the water on both "the sur- 



Experi- 

 ments with 

 a tube 

 closed at 

 both ends 

 with blad- 

 der: 



with one 

 end in 

 brine, the 

 tube being 

 filled with 

 water : 



