308 Michdnlral A/iali/s!s of Soils ttiid otiter DispevslouK 



type of curve will be given and it will diispense with any assumption as 

 to the size, shape and density of the settling particles. 



In this connexion, the probable effect of the gel coating which has 

 been postulated for the finer particles of a soil or clay on the sedimen- 

 tation of such material is worthy of consideration. It has been suggested 

 that such coatings are not to be considered as uniform and discrete but 

 as concentric shells of increasing degree of hydration. As an approximate 

 basis of calculation let us assume that the emulsoid coating has the 

 same density as the water in which the particles are suspended. What 

 vnW be the effect of this coating on the settling velocity? If a be the 

 radius of the falling particle and d the thickness of the gel coating or 

 shell, then the velocity of fall of the particle in the absence of any coating 

 is given by 



using the usual notation and assuming the density of water = 1, and 

 the velocity of the coated particle by 



_2 gja + dy- 



where p^ is the mean density of the coated particle. 



weight of particle + shell 

 volume 



|7ra»p + |7r[(a + df - a'] 



weight of particle + shell 



Now pi= — - -.. 



'^ volume 



fn{a+df 

 a^p + [(a + df - o»] 



{a + df 



_ 2 g{a + df \a?p + [(a + rf)3-a'] _ ] 

 lli.n ^^__. _ .^ - \a + df j 



^2 flr a ^(p-l) 

 9'ij' a+d ' 



v^a + d 



" Vi a ' 



a 

 or i"o = V, 



a+d' 



It will be seen that the presence of a coating of appro.vimatcly the 

 same density as the suspension medium will have a marked effect on 

 the velocity of fall. The low velocities corresponding to the finer fractions 



