86 SOILS: PROPERTIES AND MANAGEMENT 



in question. Sieves of all kinds have the one great dis- 

 advantage that their meshes cannot be made small enough 

 to separate the finer grades of soil. When one considers 

 that many soil particles are less than .005 millimeter in 

 diameter, the inadequacy of sieve separation becomes 

 apparent. However, sieves may be used in connection 

 with other methods as an easy way of dealing with the 

 larger soil particles. Air in motion 1 is inadequate, as 

 it can be used only for very fine particles. Even with 

 these the separation is slow and inaccurate, because of 

 the tendency of the dry particles to cohere. These two 

 methods have therefore been largely abandoned as dis- 

 tinct methods, and water is used as the medium of separa- 

 tion in all the modern systems of mechanical analysis. 



The principle involved in the subsidence of soil particles 

 in water, whether the force of gravity or centrifugal 

 force is utilized, is recognized by every one. When 

 fragments of rock or soil are suspended in water, they 

 tend to sink slowly, and it is a well-recognized fact that 

 other things being equal, the rate of settling depends 

 on the size of the particle. As the particle is decreased 

 in size, its weight decreases faster than the surface ex- 

 posed to the buoyant force of the water. As a conse- 

 quence, the rapidity with which the soil particles settle 

 is proportional to their size. The suspension of a sample 

 of soil would therefore be the first step in mechanical 

 separation by water ; the second step would be subsidence 

 and the withdrawal of each successive grade of particles 

 as it slowly settled ; the third step would be determina- 

 tion of the percentage of each grade, or group, of particles 



1 Cushman, A. S., and Hubbard, P. Air Elutriation of 

 Fine Powders. Jour. Amer. Chem. Soc., Vol. 29, No. 4, pp. 

 589-597. 1907. 



