172 SOILS: PEOPEBTms AND MARAGEMBNT 



already in use. For all practical purposes in soil dis- 

 cussions, general descriptive terms may be employed. 



113. The cause of plasticity. — Exactly what may be 

 the cause of plasticity has long been under discussion. 

 The various theories advanced may be grouped under the 

 following heads : ^ — 



A, Structure of clay particles 



1. Fineness of grains 



2. Plate structure 



3. Interlocking particles 



4. Sponge structure 



B, Presence of hydrous aluminium silicates 

 C Molecular attraction between particles 

 B, Presence of colloidal matter 



Of these theories accounting for the plasticity of cer- 

 tain bodies, that of colloid content seems the most rea- 

 sonable.^ The presence of gelatinous colloidal matter, 

 with a certain optimum amount of water, seems to facili- 

 tate the ready movement of the particles while at the 

 same time exerting sufiScient force to prevent the body 

 from splittingapart at the time of movement, or when 

 the pressure is removed or the material dried. Thus, 

 in general, other conditions remaining equal, materials 

 become more plastic the greater the content of colloidal 

 matter. In general the colloids function as a measure 

 of plasticity. The consideration of shrinkage, hygro- 



^ Davis, N. B., The Plasticity of Clay. Trans. Amer. 

 Cer. Soe., Vol. 16, pp. 65-79. 1914. 



2 Cushman, A. S. The Colloid Theory of Plasticity. 

 Trans. Amer. Cer. Soe., Vol. 6, pp. 65-78. 1904. Also, Ashley, 

 H. E. The Colloid Matter of Clay and Its Measurement. 

 U. S. Geol. Sm-vey, BuL 388. 1909. 



