34 MOTION OF THE JUICES OF THE ANIMAL BODY. 



The sinking of 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. 



To displace the aqueous particles in the pores of the bladder by other aque- 

 ous particles, requires obviously a much smaller pressure than is necessary to dis- 

 place them by particles of air.* In the one case, where both surfaces of the 

 bladder are in contact with the liquid, the attractive force (that of the bladder for 

 the water and of the water for the bladder) 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 pres- 

 sure ; the experiment shows, that the absorbent power and permeability 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 evapora- 

 tion to a heighth of 12 inches, less than 12 inches of mercury are required, in the 

 apparatus, Fig. 1, to cause water to pass through the bladder. 



py jo t When the tube, (Fig. 13,) is filled with water, closed with blad- 



der at both ends, and exposed to evaporation, the bladders in a 

 short time become concave, that is, they are pressed inwards. As 

 the evaporation of the water through the moist surfaces of the 

 bladder proceeds, there is formed in the upper part of the tube a 

 vacuum, which is filled with aqueous vapor, and which continues to 

 increase. The place of the water which evaporates is, as in the 

 experiments previously described, 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 



. 14. filled with water, and closed at both ends with bladder, is placed 

 in a vessel containing brine, and exposed to evaporate in the air, 

 as in Fig. 14, it is plain, that when the atmospheric pressure, 

 increasing in consequence of the evaporation of the water on both 

 the surfaces of the bladder, reaches the point at which the brine 

 flows through the pores of the bladder, then the place of the 

 water which evaporates is occupied by brine. 



In fact, when the brine is colored blue, we observe, after a few 

 hours, that a blue stratum forms within the tube, which constantly 

 increases, till at last the vessel of brine is emptied, and the tube 

 is entirely filled with brine. 



If the longer limb be immersed in bile instead of brine, the 

 tube fills with bile, and if we employ, for closing one end, a 

 ^^^^^ membrane rather thinner than we use for the other, from which 



the evaporation takes place, and then place the end with the thinner membrane in 

 oil (oil of marrow,) the tube gradually fills with oil. 



In all these cases, no air enters the tube, which continues full of liquid, as it 

 was at first. 



J If we connect the evaporation tube by collars of caoutchouc with short bits 

 of tube (Fig. 15,) full of water, and tied with bladder at both ends ; and if we 

 immerse the last bit of tube in brine, urine, oil, &c., all these cells, and at last the 



* Water passes through moist bladder more easily than air does. 



t Experiments with a tube closed at both ends with bladder : with one end in brine, the tube 

 bmg filled with water, with one end in bile, and in oil. 

 J Effect of a s ries of short tubes, closed at both ends with bladder. 



