DENSITY AND SPECIFIC GRAVITY 



31 



B 



FIG. 26. 



A, Capillary Rise of Water; B, Capillary 

 Depression of Mercury. 



action. "Capillary" means "hairlike." The phenome- 

 non is so called because it is commonly studied in tubes 

 of small diameter (Fig. 26). 



Capillary action in water is due to adhesion between the 

 water and the capillary tube, and to the cohesion of the 

 water, which produces an 

 elastic surface. If the tube 

 is a large one, the water is 

 raised, but only at its edges, 

 as in the case of water in a 

 dish. The elastic surface 

 cannot exert force enough 

 to lift all the water in the 

 tube. But if the tube is of 

 small diameter a " capillary " tube the weight of the 

 water in the tube is small, and the elastic liquid surface 

 lifts the whole column of water up the tube. 



Water rises in a capillary tube until the weight of the water is just 

 equal to the force exerted by the elastic surface. The upper surface of 

 the water column in a capillary tube is still concave. 



If a glass capillary tube is placed in mercury (Fig. 26, ), the effect 

 is the opposite of that in water, and we have a capillary depression 

 instead of elevation. 



Water rises up between two glass plates held close together just as 

 it does in capillary tubes. In the case of the blotter, cloth, string, etc.. 

 the fibers of the material are so near one another that the spaces be- 

 tween them act like a multitude of fine tubes. 



33. Density and Specific Gravity. If you were 

 "hefting" a piece of wood and a piece of lead, you would 

 say that wood is a light substance and lead a heavy one. 

 In saying this you would not mean that a large board of 



