POROSITY. 33 



surface-attraction by which liquids are caused to adhere to solids is 

 \ called adhesion^ 



This adhesion may be noticed also between two plates having plane 

 surfaces. A drop of water pressed between these plates will cause 

 them to adhere to each other. The application and use of glue and 

 mucilage, our methods of writing and painting, the welding together 

 of pieces of metal, etc., depend on this kind of surface-action. 



Capillary attraction. Whilst it is the general rule that liquids 

 in vessels present a horizontal surface, this rule does not hold good 

 near the sides of the vessel. When the liquids wet the vessel, as in 

 the case of water in a glass vessel, the surface is somewhat concave 

 in consequence of the attraction of the glass surface for the particles 

 of water ; on the contrary, when the liquids do not wet the vessel, as 

 in the case of mercury in a glass vessel, the surface is somewhat 

 convex. The smaller the diameter of the vessel holding the liquids, 

 the more concave or convex will the surface be. If a narrow tube is 

 placed in a liquid, this surface-action will be more striking, and it 

 will be found that a liquid wetting the tube will not only have a 

 completely concave surface, but the level of the liquid stands per- 

 ceptibly higher in the tube than the level of the liquid outside. 

 Substances not wetting the tube will show the reverse action, namely, 

 the surface inside of the tube will be convex, and will be below the 

 level of the liquid outside. 



The attraction of the surface of tubes for liquids, manifesting 

 itself in the concave shape of the surface and in the elevation of the 

 liquid near the tube, is known as capillary attraction. Capillary 

 elevations and depressions depend upon the diameter of the tube, 

 temperature, and the nature of the liquid. The narrower the tube, 

 the higher the elevation or the lower the depression ; both are 

 diminished by increased temperature. /Capillary elevations and 

 depressions, all other circumstances being equal, are inversely pro- 

 portional to the diameters of the tubes.) 



Defining the phenomena of capillary attraction more scientifically, 

 we may say that the adhesive force of glass, wood, etc., for water and 

 most other liquids exceeds the cohesive force acting between the 

 molecules of these liquids, while in mercury the cohesive force pre- 

 dominates over the adhesive. 



Surface-attraction of solids for gases. Any dry solid sub- 

 stance, carefully weighed, will, after having been exposed to a higher 



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