VI.] SURPACM TENSION 105 



run. It has often been thought difficult to explain how 

 drains come to have free water in them at all, being 

 themselves composed of porous material which clings to 

 water like the fine-grained soil with which it is in 

 contact ; but when any of these fine particles gets so 

 overcharged with water that the elastic skin is stretched 

 beyond its holding power, the skin will rupture and 

 detach a drop of free water into any open space that 

 may be present. Returning now to our original 

 illustration of the set of beads dipped in oil, what will 

 happen when liquid is taken away from the top ? The 

 skin is reduced in thickness, its stretching and therefore 

 its pressure on the liquid inside is reduced, so that a 

 rise of liquid takes place from below until the tension is 

 once more equalised all over. Thus water can rise in a 

 soil from the lower wet layers in order to replace that 

 which has been taken away by evaporation at the 

 surface, but it is necessary that there shall be a con- 

 tinuous film of water from the evaporating surface to 

 the wet layer below. If the evaporation continues until 

 the films become thinner and thinner and more and 

 more stretched ; at last the stretching becomes too much 

 for the elasticity of the material, the film ruptures and 

 shrinks up to a smaller surface with a relieved internal 

 strain. Similarly, if liquid is taken away at the side 

 there will be thinner films created at that point with 

 reduced stretching, which exerts a pull on the liquid 

 until the tension of the film is once more equalised all 

 over. Water will move horizontally from "a wetter 

 to a drier area, just as it can move upwards against 

 gravity, or still more readily downwards with gravity ; 

 as long as the film of water round the particles is 

 continuous, this motion in order to restore equilibrium 

 can take place. This property of water and other 

 liquids to generate a kind of stretched skin on the 



