hi.] SURFACE TENSION 73 



be less than atmospheric, or the water would not 

 stand higher within than without the tube ; further, 

 the pressure beneath a y the most convex and therefore 

 most stretched surface film, is lower than the pressure 

 beneath b> and still lower than that beneath c. Per 

 contra^ the mercury surfaces are convex outwards, and 

 exert pressure on the liquid beneath, depressing it 

 below the general surface of the liquid in proportion 

 to the degree of convexity. These instances will help 

 us to realise that the surface of a liquid may exert 

 either a pull or a pressure on the liquid within, 

 according to the curvature of the surface, and the 

 greater the curvature the greater will be the force 

 exerted. It is this tension of the surface film which 

 causes movements of water in soil, other than those due 

 to gravity; for example, if a flowerpot stands in a 

 shallow dish of water, the whole of the soil within the 

 pot is kept moist ; or if water is poured on to dry soil, it 

 is seen to work outwards through the soil, the water 

 advancing from particle to particle as it wets them, just 

 in the same manner as it rises up the capillary tubes. 

 When a soil is saturated, the whole pore space is filled 

 with water ; if this soil be allowed to drain, some of the 

 water is pulled away by gravity, but much remains 

 clinging round the particles in the stretched film con- 

 dition, the tension in the film balancing the pull due to 

 gravity. Perhaps the best illustration of the state of 

 affairs in a wet but drained soil may be obtained by 

 linking a series of toy balls together, as shown in the 

 photograph (Fig. 4), and then dipping the whole into oil. 

 When the oil has ceased to drip it will be seen that 

 every ball is covered by a thin film of oil, and that 

 between the balls there is a layer of oil much thicker in 

 the lower than in the upper layers. The whole surface 

 film is equally stretched, but the stretching in the upper 



