RESIDUO-CAPILLARY ATTRACTION. 207 



at their two extremities, while the fluids meet and mix in the 

 interior. 



6. Dutrochet argues that capillary attraction cannot be the cause 

 of endosmose, because it can only raise a fluid to a small height in a 

 capillary tube, and is utterly incapable of drawing it beyond the limits 

 of the tube. 



In stating these objections, he perhaps does not consider that the 

 height at which a fluid may be sustained in a capillary tube is inversely 

 as its diameter, and consequently in a tube of so extremely small a 

 diameter as those of which it is necessary to suppose the membrane to 

 consist, that height might be almost indefinitely great. It is true that 

 in the case of a single fluid, this effect would require for its production 

 that the tubes themselves should be coextensive with the fluid raised ; 

 but this is no longer necessary when the two ends of the tube are 

 immersed in different fluids. The reason why a homogeneous fluid 

 cannot be drawn beyond the limits of the tube, is, that, were it to 

 be so, the tube, acting equally at its two ends, would produce no 

 effect whatever upon the fluid. But the circumstances are very different 

 when the extremities communicate with different fluids. In that case the 

 full residual effect, consisting of the difference of effects, which the same 

 tube indefinitely extended, is capable of impressing separately upon the 

 two fluids, might be produced by an extremely small length of tube, 

 not exceeding a small multiple of the sphere of attraction of the par- 

 ticles of the tube, and there is no doubt that the thickness of the 

 finest membrane is a considerable multiple of this magnitude. In fact, 

 if we cut off" from the ends of the tube a distance greater than the 

 tube's sphere of sensible attraction, it is plain that the fluids which 

 occupy the intermediate part, in whatever way they may communicate 

 there, will suffer no effective attraction from the tube, since every 

 elementary portion will be drawn by it equally in both directions. The 

 only effective attractions will therefore be those exerted by an insensible 

 portion at each extremity ; we may therefore imagine these two por- 

 tions to be brought together as near as we please without any diminution 

 of effect. 



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