ATTRACTION OF LIQUIDS FOR MEMBRANES. 27 



that in addition to the chemfcal attraction which the dissimilar particles of the 

 liquids have for each other, a new cause, namely, the strong attraction of one of 

 them for the suhstance of the partition, is introduced, which accelerates their motion 

 or passage, and must have this effect, that one of them flows out in larger quantity, 

 in the same time, than the other. 



The experiments (Fig. 3) elucidate this process, and show besides, that the 

 exchange of the two liquids on both sides of the bladder is essentially determined 

 bf their unequal specific gravities.* As long as the difference in their composition 

 (which may here be measured by the specific gravity) is very great, the change of 

 volume (increase of one and decre.ase of the other) takes place rapidly ; but at 

 last, when this difference becomes very small, the liquids mix without further 

 visible change of volume, obviously, because the attraction of the bladder to h; 

 mixtures on the opposite sides does not perceptibly differ, although the specific 

 gravities are still somewhat unequal. 



In the ultimate result, the action of dissimilar liquids on the substance of animal 

 tissues, in consequence of which their mixture is attended with a. change of volume, 

 appears to be equivalent to a mechanical pressure, which is stronger from one side 

 than from the other.t p- g 



J If the tube (Fig. 9,) which is closed with bladder at its 

 wide opening, be filled with brine to the mark a, if so 

 much mercury be then poured into the narrow vertical 

 part as by its pressure to cause brine to begin to flow out 

 in fine drops from the pores of the bladder, and if now, 

 after removing so much of the mercury that the efflux is 

 no longer visible, we place the apparatus in a vessel with 

 pure water, colored blue, as in the figure, the mercury does 

 not change its level ; and when, after one or two hours, 

 we carefully remove the tube from the water, we find that 

 in the upper part of the wide end of the tube, which con- 

 tained colorless brine, a dark blue stratum has been formed, 

 which floats on a colorless liquid. After a longer time, the 

 blue color spreads gradually downwards, till at last the 

 brine acquires a uniform blue tint. 



It will readily be perceived, that the two liquids here mix, 

 as if no pressure had been applied to the brine, for a 

 mechanical pressure exerts no influence on the mixture ; 

 but, in consequence of the pressure, the mixture takes 

 place without change of volume. The mechanical pres- 

 sure which the water, in virtue of its stronger affinity for 

 the bladder, exerts on the brine in the pores of the bladder, is held in equilibrium 

 by the column of mercury, and the result is that exactly as much brine flows 

 out as water flows in. 



Let us suppose the column of mercury to be removed, and the rise of the 

 brine in the narrow tube is explained at once. If we close a short tube, filled 

 with alcohol or brine, with bladder at both ends (an arrangement which may 

 represent a cell,) and suspend it in a vessel of pure water, both surfaces of the 

 bladder become convex outwards ; they swell, but without bursting. As soon as 

 the pressure, gradually increasing by the influx of water into the interior of the 

 tube, is sufficient to keep in equilibrium the affinity of the water for the bladder, 

 and consequently its further influx, the exchange goes on, for the future, without 

 change of volume. 



Most porous bodies exhibit the phenomena described in the preceding pages, if 

 their pores are so minute that a feeble hydrostatic pressure is not propagated 



* Mixture is essentially determined by the unequal density of the liquids. 



o ositeSS? 1 f tw ll( J ulds on amm al tissues equivalent to a mechanical pressure, unequal on 

 t Experiment to show that an external pressure prevents change of volume. 



