PHYSICAL PROCESSES IN CELLS. 



41 



the water will rise within the tube considerably above the level of 

 the water outside, and the surface of the water in the tube will be 

 concave (concave meniscus); while the mercury will be depressed in 

 the glass tube below the level of the mercury on the outside and the 

 surface of the mercury within the tube will be convex (convex meniscus) 

 (Figs.- 29 and 30). If any -two bodies, such as two glass plates, are 

 immersed sufficiently near to -each other in a liquid, the liquid will 

 rise or be depressed between them, according as the liquid has or 

 has not any adhesion to the plates (Fig. 31), the degree of elevation or 

 depression being one-half what it would be if a tube of glass whose 

 diameter equals the distance between the two plates were immersed in the 

 same liquids. If a drop of water be placed in a conical glass tube of 

 small angle and horizontal axis, each end of the drop will have a concave 

 meniscus and it will move from the large to the small end of the tube : 

 if the liquid be mercury, each end will have a convex meniscus and it 

 will move in the reverse direction (Figs. 32 and 33). 



In the explanation of capillary phenomena 

 two causes deserve attention : first, the cause of- 

 the curvature of the surface, and, second, the cause 

 of the ascent or depression of the liquid within 

 the tube. 



Fig. 31. 



Fig. 32. 



Fie. 33. 



The form of the surface of a liquid in contact with a solid depends 

 on the relation between the attraction exerted by the solid on the liquid 

 and the mutual attractions of the molecules of the liquid. Any molecule 

 of a liquid in which a solid is immersed is acted on by three forces: 1st, 

 gravity; 2d, the attractive force of the solid for the liquid molecule ; and 

 3d, the cohesive attractions of the other molecules of the fluid. 



According to the relative intensities of these forces their resultant 

 may occupy one of three positions :— 



First.— If the attraction of the solid balances the cohesive attrac- 

 tion of the fluid, the resultant of these two forces will coincide with the 

 force of gravity and the surface of the fluid will be horizontal; for to 

 be in equilibrium the surface of a liquid must be at right angles to the 

 direction of the resultant of all the forces acting on that liquid 



(Fig. 34). 



Second.— If the attractive force of the solid for the fluid increases, 

 or if the cohesive force of the liquid diminishes, the resultant will fall 

 outside of the line of 'gravity or between the line of attraction of the 



