32 INTRODUCTION. 



surfaces of most solid substances exert an attraction for the 

 particles of such liquids as water and alcohol to such an extent 

 that these particles adhere to the surface of the solids. Such 

 an attraction, however, does not manifest itself for the particles 

 of mercury. This form of surface-attraction by which liquids 

 are caused to adhere to solids is called adhesion. 



This adhesion may also be noticed between two plates of 

 even surface. A drop of water pressed between these plates 

 will cause them to adhere to each other. The application and 

 use of glue and mucilage depend on this kind of surface-action. 



Capillary attraction. Whilst it is the general rule, that liquids 

 in a vessel 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 com- 

 pletely 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. Capil- 

 lary 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 ele- 

 vations and depressions, all other circumstances being equal, 

 are inversely proportional to the diameters of the tubes. 



Surface-attraction of solids for gases. Any dry solid substance, 

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



