26 MOTION OF THE JUICES OF THE ANIMAL BODY. 



If the moistening of solid bodies by liquids be the effect of a chemical attraction 

 the force of which is different in dissimilar liquids, it follows that, when a porous 

 body is saturated with a liquid, and brought in contact with a second liquid, which 

 has a stronger attraction for its substance than the first has, then the liquid must be 

 displaced from the pores by the second, even in the absense of hydrostatic pressure, 

 and this, whether the two liquids be miscible or not.* 



We may suppose that the attraction of the second liquid, of more powerful affi- 

 nity, which displaces the other, is equal to the pressure of the column of mercury 

 required to force the latter through Hie porous substance. 



If we tie over one end of a cylindrical tube with a very thin membrane, saturated 

 with concentrated brine by steeping 24 hours, and if we dry the outer surface of 

 the membrane carefully with bibulous paper, and now pour a few drops of pure 

 water into the tube so as just to cover the inner surface of the membrane, the outer 

 surface is seen in a few moments to be covered with minute drops of brine ; that is, 

 brine flows out of the pores of the bladder. 



A thick ox-bladder, saturated with oil, exhibits the same phenomenon in contact 

 with water. The oil is expelled from the pores of the bladder by the water, which 

 occupies its place. 



When the bladder is brought in contact with pure water, it takes up a certain 

 quantity of that liquid. If its pores are previously filled with brine, and if we cover 

 one side of it with pure water, the water mixes with the brine in the pores of the 

 bladder ; and on the side next the water there is formed a diluted brine, which, 

 being in contact with a stratum of pure water, mixes with it, and in this way the 

 successive strata of water receive, from the bladder outwards, a certain quantity of salt. 



In the interior of the bladder, there are formed in like manner, towards the outer 

 surface, mixtures of unequal saline strength. If we suppose the bladder to consist 

 of several strata, all these strata receive, from the surface in contact with the water, a 

 certain quantity of water ; the outer stratum, in contact with the air, receives least, 

 and is the most highly charged with salt. 



The cause of mixture is the cheminal affinity of the salt for the newly-added 

 particles of water ; this affinity is equal on both sides, but the attraction of the sub- 

 stance of the bladder is stronger for the more aqueous or less saline liquid, than for 

 the more concentrated. In consequence of this difference in the attraction of the 

 liquids for the substance of the bladder, a part of the mixture is displaced from the 

 bladder ; the less saline liquid te-kes the place of the more saline ; a part of the 

 latter is expelled, and, with it, a part of that water which has been added to the 

 outer stratum by mixture. Brine and water flow out in the direction of least resist- 

 ance. The efflux towards the side on which the pure water was poured is prevented 

 by the more watery liquid for the substance of the bladder. 



If we remove from the outer surface of the bladder the displaced saline liquid 

 (which has been mixed with some water,) and put stronger brine in its place, and 

 if on the opposite side we remove the very diluted solution, replacing it by a still 

 more diluted one, the same process is repeated. There arises a permanent differ- 

 ence, and a state of mixture and efflux continues till the liquids on the opposite 

 surfaces of the bladder have the same, or very nearly the same, composition. 



If we suppose, that the two liquids moisten the bladder unequally, it follows, 



* One liquid expels another from a membrane. 



15 atoms of water must have moved past each other; but this is impossible, siace 1 

 atom of salt requires 18 atoms of water for solution, (10 parts of salt to 27 of water.) 

 The weight of the pure water in the outer vessel was 19'26 grammes ; consequently, 

 the weight of the brine was to that of the pure water as 1 : 2'22. In another experi- 

 ment, in which the weight of the brine in the tube was to that of the water outside, as 

 1 : 7'98 ; the tube gained 0*822 grammes in weight ; the liquid in the tube contained 

 at first 0-947 grammes of salt ; and 24 hours after, 0'148 grammes: hence, 1'621 grammes 

 of water had entered, while 0'799 grammes of salt had passed out. For 1 atom of salt, 

 which passed from the tube with brine to the vessel with water, there passed from the 

 latter to the former rather more than 13 atoms of water ; (for 58'6 parts, or 1 atom of 

 alt, 118 parts of water.) 



