94 



SOILS: PROPERTIES AND MANAGEMENT 



the clay are decanted directly into a test tube fitted into 

 a centrifuge (see Fig. 15). Whirling at the rate of 800 to 

 1000 revolutions a minute will cause the subsidence of 

 the silt to the bottom of the test tube in a few minutes. 

 The clay is then decanted. The microscope is necessary 

 here, in order to determine when the settling of the silt 

 is complete. As small particles tend to cling to the 

 larger particles, the entire operation must be repeated 



FIG. 15. Apparatus for centrifugal mechanical analysis of soil, show- 

 ing shaker bottle, shaker, centrifuge, and test tube. 



several times; therefore the processes of gravity sub- 

 sidence and centrifugal subsidence are carried on side by 

 side, material being constantly poured from the shaker 

 bottle into the centrifuge tubes and from the test tubes 

 into the receptacles for the clay. 



The centrifuge is usually large enough to allow the 

 separation of several duplicate samples at once. The 

 various separates made by this method are dried and 



