THE COLLOIDAL PROPERTIES OF SOIL 167 



on the assumption that these two kinds of soil water existed. 

 Shull's results (see p. 224) presented more difficulty and led him 

 to regard the state of the soil moisture as unbroken and con- 

 tinuous. Keen (146^) furnished rigid proof: he found that the 

 relationship of water to soil differed fundamentally from its re- 

 lationship to sand. The evaporation of water from sand, silt, 

 china clay, and ignited soil proved to be relatively simple and 

 could be explained by the known laws of evaporation and 

 diffusion. But the evaporation of water from soil could not : 

 it was more complex. Instead of the simple proportionality 

 between water content and rate of evaporation observed in the 

 case of sand, the curves for soil were more exponential in type. 

 The difference was traced to the soil colloids and it disappeared 

 when the soil was ignited and the colloidal properties lost : the 

 curve then became identical with that obtained for sand. 

 The influence of the colloids has so far only been expressed 

 empirically, but it is probably connected with the relation 

 between vapour pressure and moisture content. But there is 

 clearly something else at work, for the curve is not of a simple 

 exponential type. It is necessary to allow for another factor : 

 the effect on the rate of evaporation of the decreasing water 

 surface in the soil, the surface obviously diminishing in area 

 as evaporation continues. 



The equation finally developed by Keen is : 



A ~ = V + I [ 2 '33 logioO + K) - log.K], 



dw 

 where -^- = rate of evaporation. 



w percentage of water present by weight. 

 s = specific gravity of the soil. 

 A and K = constants. 



This relationship holds without any break, proving that the 

 water in a normally moist soil is all held in the same way 

 without any break in physical state (Fig. 21). At one end of 

 the curve the water is more easily given up than at the other, 

 and in the competition for water between soil colloids and 



