H. A. Baker — Loose Arenaceous Sedir)ie7its. 325 



If F < ^■- — g<^'^ it will fall again, and so will not be 

 9 /<■ 



washed off. 



If F > ^ • — — - • ga^ the particle will be washed off. 



Thus, the critical point at which washing off just ceases will 

 be when 



For a spherical grain of a specified mineral, falling through a 

 specified liquid at a specified temperature, 



— • — • gr = a constant, 



9 /<- ^ 



consequently we have 



Y = kB\ 



where F is the upward velocity of the water current, 



yt is a constant, and 



D is the diameter of the grain. 



This is known as the law of Viscous Resistance. R. H. Richards,^ 

 experimenting on ordinary quartz grains, considered this law to 

 hold approximately for grains not exceeding about "2 mm. diameter. 

 The character of Richards' results suggests, however, that he made 

 no allowance for temperature changes in the water in which the 

 grains were subsiding. (See plot of Richards' results, Fig. 1.) In 

 the light of later research work by another investigator it is now 

 known that the law of viscous resistance does not hold closely for 

 even small grains. 



For larger particles and greater velocities it has been suggested 

 that the rate of subsidence might conform to the law 



R = kV^ 

 where R is the resistance to motion and k is a constant. 



This was suggested by Newton, and hence the law is sometimes 

 referred to as Newton's Law of Eddying Resistance. In the case 

 where dynamic equilibrium exists between the particle and the 

 liquid resisting its subsidence, R is just equal to the effective weight 

 of the particle in the liquid. Rittinger considered that the formula 



where (7 is a constant, 



D is diameter of mineral grain, and 



?> is specific gravity of mineral, 

 was applicable to the case of mineral grains settling in water. 

 Richards found that Rittinger's formula held with tolerable accuracy 

 for grains exceeding r55 mm. diameter, but below this limit the 

 value of Rittinger's C decreased rapidly. 



^ Eichards, Textbook of Ore Dressing, 1909, pp. 262-76. 



