36 THE MECHANICAL ANALYSIS OF SOILS [chap. 



beam of light is passed through the liquid, as is not the 

 case with bodies truly dissolved. From this point of view 

 the "colloid clay" would only represent the limiting 

 state of fineness, differing in degree, but not in kind, 

 from the other clay particles. 



The question still remains whether we shall give 

 to clay a physical or a chemical definition ; in the first 

 place, does the fineness of the material alone confer the 

 characteristic clay properties of plasticity, impermeability 

 to water, and shrinkage and tenacity on drying, or do 

 these properties depend on the chemical composition of 

 the substance making up the clay. 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 composition 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 : — 



Of these fractions the first contained all the mica, 

 the others were practically pure kaolinite, yet the second 

 fraction showed none, and the third very little of the 

 characteristic properties of clay ; when dried they fell 



