[.] SURFACE TENSION 79 



cert pressure on the liquid beneath, depressing it 

 :low the general surface of the liquid in proportion 

 1 the degree of convexity. These instances will help 

 ; to realise that*' the surface of .a-liquid may eyert ^ 

 th er a , pull o r a pressure on the liquid within, 

 :cording to the curvature of the surface, and the 

 eater the curvature the greater will be the force 

 certed.) It is this tension of the surface film which 

 Luses movements of water in soil, other than those due 

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

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

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



seen to work outwards through the soil, the water 

 Ivancing from particle to particle as it wets them, just 



the same manner as it rises up the capillary tubes. 

 ; hen a soil is saturated, the whole pore space is filled ^_ 

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

 ater is pulled away by gravity, but much remains 

 inging round the particles in the stretched film con- 

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

 avity. Perhaps the best illustration of the state of 

 fairs in a wet but drained soil may be obtained by 

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

 lotograph (Fig. 5), and then dipping the whole into oil. 

 r hen the oil has ceased to drip it will be seen that 

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

 ;tween the balls there is a Jayer of oil much thicker in 

 e lower than in the upper layers. The whole surface 

 m is equally stretched, but the stretching in the upper 

 yers is largely due to the pull from the oil below, 

 hile in the lowest layer of all the whole tension 

 certed by the stretched film is devoted to holding up 

 s own thick film of oil. If oil be taken away at any 

 Dint, the curvature of the film, and therefore the tension 

 : the surface in that region, is increased : a readjust- 



