Movements of Fluids Throvgh Soils. 125 



is generally true that the larger the surface of the soil 

 grains the more water the soil will retain. 



If a marble is lifted out of water it retains a film sur- 

 rounding it and its surface is wet; so if rains fall upon a 

 sand or soil surface until percolation takes place, there is 

 held back upon the grains a certain amount of water which 

 is characteristic of or peculiar to each type. It is clear 

 that a soil whose internal surface is 4 acres per cubic foot 

 may contain a large amount of water even though the film 

 is extremely thin. In an acre there are 43,560 sq. ft. and 

 in four acres 174,240 sq. ft. The thickness of a water 

 film on this surface sufficient to equal 4 inches on the level 

 per square foot of soil would be 



or one-half the thickness of the film of a soap bubble when 

 it becomes yellow just before appearing black and breaking, 

 from thinning out. This thickness is also about -J the di- 

 ameter of the soil grain itself. 



In the case of a fine sand having grains .08188 m. m., 

 which retains, after complete drainage 8 feet above stand- 

 ing water, 3.44 per cent, of water, the film would have to 

 have a thickness of only about *V of the diameter of the 

 grain, and when containing 20 per cent, of its dry weight 

 then the film need have a thickness of only about T*T of the 

 diameter of the sand grains, that is, .0072 m. m. 



It is clear, therefore, from these considerations that the 

 surface of soil grains has much to do in determining the 

 water-holding power of a soil and that the films may be 

 very thin and yet on account of their great extent represent 

 a high per cent, of the soil itself. 



147. Movement of Air Through Soil. There is perhaps 

 nothing which shows how physically different the fine and 

 the coarse grained soils are as clearly as the rates at which 

 air will pass through them when dry, and in the next table 

 some of these are given. 



