<o THE MECHANICAL ANALYSIS OF SOILS [chap. 



the water, as exhibited in such matters as the surface 

 tension between their material and water and the 

 nature of the electric charge they carry. While truly 

 suspended particles, however fine, may be pictured as 

 exhibiting a sharp boundary to the water, colloids may 

 be conceived as surrounded by successive shells of less 

 and less hydrated material shading off into the water. 

 They thus consist of particles too fine to settle down in 

 water, or to be arrested by a filter even of porous 

 porcelain, but which are still sufficiently coarse to show 

 their presence when a strong beam of light is passed 

 through the liquid, as is not the case with bodies truly 

 dissolved. We thus see that clay cannot be given 

 a purely chemical definition ; the characteristic clay 

 properties of plasticity, impermeability to water, and 

 shrinkage and tenacity on drying depend in the first 

 place upon the fineness of grain of the particles com- 

 posing it. It is easy to show that fineness of division 

 is a necessary factor in the existence of clay, because 

 we can obtain material possessing the chemical com- 

 position of typical clays which yet behave physically 

 as if they were sand. A sample of crude kaolinite rock, 

 as dug in Cornwall from the surface of granite, was 

 powdered and passed through a sieve retaining all 

 particles above o-2 mm. in diameter; the remainder, 

 which consisted mainly of kaolinite with a little mica, 

 was further separated by sedimentation from water into 

 four fractions : 



