76 



THE TEXTURE OF THE SOIL 



[chap. 



mean size of the channels formed between the soil 

 grains. King made some experiments with sands 

 graded by sieves and formed into columns 14 inches 

 long and 1 square foot in section, above which the water 

 was maintained at a head of 2 inches. He obtained 

 the following results expressed in inches of water 

 passing in twenty-four hours ; the second column gives 

 the number of meshes to the inch of the sieves which 

 respectively passed and retained the sand : 



It will be noticed that there is a great diminution 

 in the rate of flow as soon as a soil containing small 

 clay particles is introduced ; of course, one of the 

 characteristic properties of clay is that it will not 

 allow any flow of water through it when it has been 

 puddled. In the puddled condition, the particles 

 constituting the clay are no longer aggregated, and their 

 colloid " gel " coating absorbs water and swells, whereby 

 pore spaces between them must become extremely 

 small. Not only is the flow diminished by the increase 

 of friction in the narrow channels, but in the case of 

 clay their dimensions have become so small that prob- 

 ably the contained water is wholly within the range 

 of the molecular forces to be described later; it is thus 

 prevented from flowing at all, and only moves by 

 diffusion. If we assume for clay particles a mean 

 diameter of 0-0002 mm., and a structure similar to A in 

 Kig. 3, p. 67, it is easy to show that no molecule in the 



