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CHAPTER XXIL 



CAPILLARITY, DIFFUSION OF LIQUIDS, AND OSMOSIS, 

 THEIR APPLICATION TO THE PHYSIOLOGY OF AB- 

 SORPTION AND SECRETION. 



WE have had under consideration certain elementary 

 laws applicable to masses of liquids. There are, how- 

 ever, phenomena exhibited by liquids which are 

 capable of explanation only by supposing that the 

 ultimate molecules of all liquids exert forces on one 

 another, and on solid bodies with which they may be 

 in contact ; in the one case the force is that of cohesion, 

 in the other that of adhesion. Or, to put the terms in 

 a more general way, for they are equally applied to 

 solid and liquid bodies, attraction between the mole- 

 cules of the same body is cohesion, and between 

 different bodies in contact, adhesion. 



Cohesion. The molecules of a body mutually 

 attract one another with a certain intensity in all 

 directions. In liquids the intensity of the cohesion 

 is not great, the molecules are readily displaced, 

 and hence the ease with which a mass of liquid 

 suits itself to the vessel which contains it. Still, 

 the cohesion of liquids is manifested in various 

 common phenomena. Thus, a drop of water falling 

 freely assumes the spherical form, and this is due to 

 the mutual attraction of all its molecules. Again, a 

 globule of mercury on a plate of glass or wood main- 

 tains a more or less spherical form ; and it does this 

 against the force of gravity, which tends to flatten out 

 the globule, to destroy the sphere. If, however, the 

 drop becomes very large, then the form is generally 

 altered ; it becomes flattened. This is because the 



