EVAPORATION THROUGH MEMBRANES. 



The bladder is penetrated with water, covered 



tion 



on one side with water, and on the other in con- 

 tact with a space (the air) not saturated with 

 aqueous vapour. The water contained in the- pores 

 of the side of the bladder turned towards the air 

 evaporates ; the space which it had occupied in the 

 pores is filled with successive portions of water from 

 within, in virtue of the attraction of the substance 

 of the pores for water. The volume of the water 

 in the tube diminishes, and thus a vacuum arises, in 

 which the mercury is forced to rise by the atmo- 

 spheric pressure. The space formerly occupied by 

 the water which has evaporated is now filled with 

 mercury. 



When the mercury has reached a permanent 

 level, the external pressure, which acts on the 

 water in the pores of the bladder (and which tends 

 to displace the particles of water) is obviously equal, 

 before air enters, to the attraction which the sub- 

 stance of the bladder has for the particles of water, 

 and these last to each other. Were the attraction 

 less, air w r ould enter, and the particles of water 

 could not maintain their position. 



The rise of the mercury, or its motion towards 

 the surface of the bladder, that is, towards the 

 point where evaporation is going on, is the result of 

 a difference of atmospheric pressure, determined by 

 the evaporation of the water, or of the liquid which 

 penetrates through the bladder, and by the ab- 

 sorbent power of the bladder for that liquid. 



F2 



