68 INFLUENCE OF THE ATMOSPHERIC PRESSURE 



One chief condition of the efficiency of a bladder, 

 in regard to the rise of a column of liquid, is, that 

 it is kept constantly in contact with the liquid, for 

 without this contact the absorbent power cannot 

 manifest itself. 



By the evaporation a continual efflux of water, in 

 the form of vapour, towards the side on which the 

 air lies, is produced ; and by the capillary action of 

 the bladder on the other side water is absorbed and 

 retained with a force which counterpoises 12 or 

 more inches of mercury, according to the thickness 

 of the bladder, 

 influence of Now, since the rise of the mercury is an effect of 



the state 



of the the atmospheric pressure, it is plain, that the height 



barometer. 



to which the mercury rises, must depend to a certain 

 degree on the state of the barometer. 



In a tube, filled with water, and closed with 

 bladder, the absorbent force of which is equal to 

 the pressure of a column of 12 inches of mercury, 

 the mercury rises by evaporation to the height of 

 12 inches, as long as a column of 12 inches of mer- 

 cury can be sustained by the external atmospheric 

 pressure. If this external pressure fall below that 

 limit, the mercury in the evaporation tube falls to 

 the same extent, and if there be water above the 

 mercury, this water separates from the bladder. 



This property of bladder, therefore, would appear 

 unaltered at an elevation at which the barometer 

 should stand at 12 inches ; at a still greater eleva- 

 tion, on the contrary, the liquid would separate 

 from the bladder. 



