FUNDAMENTAL PROPERTIES OF MATTER. 37 



by this surface-action are extremely manifold, and some are of suffi- 

 cient interest to be taken into consideration. 



Adhesion. Most solid substances, when immersed in water, 

 alcohol, or many other liquids, become moist ; immersed in mercury, 

 they remain dry. We explain this fact by saying that the 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 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. While 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 



