16 



MOTION OF THE JUICES OF THE ANIMAL BODY. 



Fig. 2. If the bent tube a b (Fig. 2,) one end of which is tied over with 



bladder, and the other open, be filled with brine colored blue^ 1 ) and 

 if pure water be placed in the tube c, there is soon perceived under 

 the bladder a colorless or nearly colorless stratum of liquid, which 

 continues for hours to float in the same place. If the bent tube a b 

 be filled with colorless brine, while c is filled with pure water colored 

 blue, there is found, after a time, above the bladder, a colorless or 

 nearly colorless stratum of liquid. 



It appears from this, that an exchange of both liquids goes on 

 through the substance of the bladder ; in the first experiment color- 

 less water passes from the tube c to the colored brine in the tube 

 a b ; in the second, colorless brine passes from the tube a b to the 

 colored water in the tube c. 



It is obvious that the brine in the tube a #, which is in contact 

 with the bladder, becomes diluted by the addition of water from the 

 tube c ; but this diluted brine is specifically lighter than the original brine which 

 is below it, and remains therefore floating at its surface. 



On the other hand, the water in the tube c, when mixed with brine from the 

 tube a b, becomes heavier than the pure water, and rests, therefore, on the upper 

 surface of the bladder, or that which is turned towards the water. 



Hence it follows, that from the moment when these two strata have been formed 

 above and below the bladder, neither concentrated brine nor pure water comes any 

 longer in contact with the bladder. 



From the bladder downwards, in the tube a b are strata of liquid, containing 

 successively more salt ; from the bladder upwards in the tube c are strata containing 

 successively more water. 



In the beginning of this experiment we observe,, that the volume of the water 

 and of the brine changes in both tubes ; the liquid in the limb b rises from 1 to 2 

 lines ; but as soon as the strata above mentioned have been distinctly formed above 

 and below the bladder, hardly any further rise is perceptible, although the mixture 

 of the liquid proceeds, and the water in c becomes constantly more charged with 

 salt, while the brine in a b loses salt. 



If we reverse the positions of the two liquids in the apparatus 

 fig. o. Yig. 2, or what is simpler, if we close with bladder a tube 



1 centimetre ( T Vhs f an inch) wide, fill it with brine, and 

 immerse the end closed with the bladder in a wider vessel 

 filled with pure water, giving to the tube an inclination of 

 about 45, we may observe (most distinctly when both 

 liquids contain some fine particles of indigo suspended) in 

 both liquids a continual motion.* We see in the tube (Fig. 

 a current of liquid rising from the bladder in the direction 

 the arrow, and flowing down again on the opposite side. 

 A similar circulation is observable in the vessel of water. 



If the tube a, with brine, is about 2 centimetres (fths of an 

 inch) wide, and if we support it vertically in the vessel b of 

 water, the motion proceeds from the middle, and in both the 

 tube and the vessel we perceive currents in opposite directions. 

 (Fig. 4.) 



These currents hardly require explanation. To the brine in 

 the tube , pure water passes through the bladder ; there is formed 

 above the bladder a mixture containing less salt, and therefore 

 specifically lighter than the brine ; this mixture rises, and the 

 denser brine descends to occupy its place. 



On the other hand, the pure water receives through the bladder 

 salt, and becomes thereby specifically heavier; while it sinks to 

 the bottom of the vessel, its place is supplied by water containing 



( ') For this purpose it is best to take a solution of indigo in sulphuric acid, diluted, 

 * When the heavier liquid is above the membrane. 



