128 



SOIL PHYSICS AND MANAGEMENT 



The water containing the silt and clay is put in test tubes and 

 whirled at a speed of about 1000 revolutions per minute. The time 

 necessary to throw down the silt is determined by microscopic exami- 

 nation of the material in suspension. After decanting the clay re- 

 maining in suspension, the test tube is filled with water, the sediment 

 is stirred and the operation repeated until the clay is all removed. 

 By running the centrifuge at a slower rate or shorter time another 

 grade may be left in suspension and decanted. 



(f) Voder's Centrifugal Elutriator. 5 One of the best ma- 



chines for physical analysis is 

 Yoder's, in which he has com- 

 bined the principles of the cen- 

 trifuge and the elutriator, as 

 shown in figure 70. The par- 

 ticles are subjected to two forces, 

 the centrifugal tending to throw 

 them down and the hydraulic 

 carrying them upward. The 

 centrifugal effect is exerted to a 

 greater extent upon the -coarser 

 particles and the hydraulic upon 

 the finer. By this combination 

 a more rapid separation may be 

 accomplished. The apparatus 

 consists of an elutriating bottle, 

 B, into which the suspended soil 



FIG. 7i. King's aspirator for the determi- is placed after the sands are re- 

 nation of the effective diameter of soil parti- move d. Water enters at F, and 



the overflow with the sediment 



is collected in the tube T. While it does its work very thoroughly 

 and quickly, it is a very expensive and a rather delicate piece of 

 apparatus. 



(g) King's Aspirator Method. 6 King was of the opinion that 

 ordinary mechanical analyses do not furnish a basis for determining 

 any very important data for soils. The arrangement of the particles 

 into groups is of much consequence in physical phenomena, but 

 mechanical analysis does not indicate the structure. In 'order to 

 overcome this difficulty he worked out the method for finding the 

 " effective diameter " of soil particles. The grouping of particles 

 upon which the percolation of air and water and other phenomena 

 depends is taken into account. The rate at which air passes through 

 a column of air-dried soil of a given cross section and length under 



o 



