HYDRAULIC CURRENTS. 55 



placed between the glasses, it will be perceived to make its way rapidly to the central 

 spot, certain optical changes, depending on its superior refractive power as compared 

 with atmospheric air, accompanying its progress ; and hence we infer, that if a chink 

 or cleft, not exceeding the half of a millionth of an inch, occurred in an animal tissue, 

 water would find its way into it. 



210. In vessels of large diameter, fluids readily adjust themselves hydrostatically, and 

 currents set in any direction without obstruction. When the dimensions of the con- 

 taining vessels become very small, a new order of things is set up, and the particles 

 have, as it were, to obey newly-created forces. In large masses, the action of gravity 

 produces the leading phenomena, and the effects of all the molecular forces vanish. 

 When minute quantities are operated on, the action of gravity diminishes, and fric- 

 tion, cohesion, capillary action, and other molecular forces, become obvious. The me- 

 chanical relations, therefore, of small and large quantities are totally distinct : hydro- 

 static equilibrium, which is effected so readily in larger vessels, is accomplished with 

 more difficulty through pores, and as these decrease in their dimensions, the forces of 

 resistance rapidly increase. Water, at all pressures, will adjust itself hydrostatically 

 with great readiness, when it is obstructed by a porous medium, provided the pores are 

 of sensible size ; but if that size diminishes, the resisting forces continually increasing, 

 the conditions of hydrostatic equilibrium are fulfilled with more difficulty, and at last 

 cease to be fulfilled at all. 



211. The foregoing remarks enable us to come to a decision in reference to the char- 

 acter of endosmosis, as indicated in (183). We perceive that this force, far from 

 being the attribute of organized matter, exhibits its phenomena when substances whose 

 inorganic character is unquestionable are made use of. A variety of porous minerals may 

 be employed in lieu of organic tissues with success; and, if the endosmosis of gases be 

 allowed to be a phenomenon of the same kind, then we know that such liquids as 

 water may be employed as barriers ; a peculiar degree of porosity is required, a structure 

 dense enough to obstruct readily hydraulic currents, but open enough to allow very 

 small columns of fluid material to traverse it. A crack in glass, because of its width, 

 allows too great a freedom of motion ; but bladder, peritoneum, or condensed cellular 

 tissue, fulfil, at ordinary pressures, the required condition. It is necessary, too, that 

 the liquids under trial shall wet the surface of the solid; for want of this action, water 

 fails to pass through the narrow interstice between mercury and glass. The degree 

 of pressure generated either during the action, or existing at the commencement of the 

 experiment, is an important element, for upon it depends the appearance or non-appear- 

 ance of the phenomenon. Thus, at ordinary pressures, bladder will exhibit the change 

 of level when water passes into alcohol ; but if pressure on one side of the membrane 

 be increased, a hydraulic current sets through it, and the experiment fails, because the 

 success of the result depends on the excess of the molecular over the hydraulic current, 

 and in this case the latter predominates. The relation which the liquids bear to each 

 other is also important ; the facility with which they unite with each other, and therefore 

 remove each other on transuding through the barrier, may sometimes make up for an 

 increased size of the pores. 



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