ENERGY AND FORCE 



49 



(Figure 42) . In which tube does the water rise the highest ? Try 

 mercury instead of water. What happens ? 



Touch a lump of sugar to the surface of a glass of water and 

 watch the rise of the water. Mention other examples of capil- 

 larity. 



Capillary action is due to two forces, cohesion in the 

 water or liquid and adhesion between the water and the 

 tube. If a glass tube of large diameter is used, the water 

 is raised only at the edges, since the elastic surface of the 

 water cannot exert enough force to raise all the water 

 of the tube ; but if the 

 tube is small, the weight 

 of the water in the tube 

 is small and the whole 

 column of water is carried 

 up the tube by the force 

 of the contraction of the 

 elastic surface. We no- 

 ticed that the height of 

 the water was different in 



FIG. 43. An Example of Capillarity. 



tubes of different diam- 



' eter. The water is raised in each case until its weight is 

 just equal to the force of the elastic surface of the water. 

 Will the water ever run out of the top of the tube ? Why ? 



If a wick is wet and allowed to hang over the side of 

 a dish of water (Figure 43), the water will rise in the wick 

 and flow over the side of the dish. The flame of the 

 kerosene lamp is fed by the oil that is carried to it by 

 capillarity. 



Mention three uses of capillarity. 



Diffusion. Diffusion is the close intermingling of 

 gases or liquids which takes place independently of 

 gravity or of any currents in the substances themselves. 



