76 THE WATER-SUPPLYING POWER OF THE SOIL 



of absorption that must needs be attained before the convection current 

 necessary for the action of the instrument becomes operative. Or, 

 they may be related to maximum sugar concentration in the membrane 

 itself, which might control the intake of water through its action in 

 clogging the water ways. 



The fact that the high initial rate and rapid subsequent decrease do 

 frequently occur when the instruments operate against water clearly 

 shows that soil is not essential to this occurrence. It is clear, however, 

 that the phenomenon in question is not nearly so pronounced in the 

 water tests as in most of those with soil. Furthermore, long-continued 

 fall in absorption rate appears to occur only with soils having a moisture 

 content about or above the critical optimum, as has been remarked. 

 This suggests that failure to show a maintained rate (which is accom- 

 panied by the exhibition of a very high initial rate) is mainly due to some 

 adjustment which gradually takes place in the wetter soils, but which 

 does not occur either in water or in the drier soils. If we cast about 

 for some possible condition which may be supposed to control this 

 important distinction between soils above their optimum water content, 

 on the one hand, and both water and soils below their optimum, on the 

 other hand, we discover that there is one pronounced feature of wet 

 soils which is not marked in drier ones and not found at all in water 

 itself. This feature is ready variability in the thickness of the water 

 films which surround and lie between the soil particles. There are 

 no such films in water itself and the films of drier soils are not very 

 easily altered in thickness, and the possible amount of such alteration is 

 greatly restricted. 



If we consider a wet soil, such as the 25 per cent mixtures dealt with 

 in our experiments, we must suppose that such a soil is, roughly, a 

 three-phase system. The solid particles are each to be conceived as 

 surrounded by a layer of water, these layers completely closing the 

 spaces between the particles where the latter are small, but not com- 

 pletely closing them where they are large. The central portions of the 

 larger spaces are filled by gas bubbles. If such a soil is in static 

 equilibrium, as far as water adjustment is concerned, the water layers 

 and air bubbles must remain constant in form and size. The water 

 is held in place through (1) adhesion of water to the solid particles 

 and (2) cohesion of the water itself. This latter force is here mainly 

 manifested as the familiar force of gas-liquid surface tension, which 

 tends to keep the gas bubbles small and thus resists removal of water. 

 It is of much lower magnitude than the force of adhesion. These two 

 forces together make up what is frequently termed capillary force, or 

 simply capillarity. 



If, now, we suppose a vertical plane to be passed through our hypo- 

 thetical soil mass, the portion of the soil lying on one side of this plane 

 being removed, and if we let a water-absorbing membrane (such as 



