SOILS. 



449 



by shaking or boiling for half a day or more. The soil is then rinsed into the nar- 

 rower of a series of gravity elutriators (of the Schone type), while a slow stream of 

 water flows through to prevent the sediment from choking the tube. After all the 

 soil has been transferred the stream is turned on to a velocity of nearly 37.09 cc. per 

 minute. When the larger part of the fine material has been removed the current is 

 turned on to the full strength of 37.09 cc. per minute required to separate particles 

 of 0.0316 to 0.1 mm. in diameter. The overflow from the narrower elutriator passes 



Fig. 6. — Yoder's centrifugal elutriator; 



vertical section. (From U. S. Dept. of Agr., Bureau of 



Soils Bui. '24.) 



into the wider elutriator and thence into a beaker for further use in the centrifugal 

 elutriator. 



The first 300 to 600 cc. of the muddy water from the gravity elutriator containing 

 nearly all the fine clay is collected, and while the gravity elutriation is being com- 

 pleted this water is run through the centrifugal elutriator at the speeds required for 

 the separation of the finest grade (0.001 mm. particles), namely, with a current of 

 43.75 cc. per minute and a centrifuge speed of 1,888 revolutions per minute. The 

 muddy water thus obtained contains most of the fine clay, a small portion being car- 

 ried down to the bottom of the bottle with the coarser sediment. This sediment is 



Fig. 7. — Centrifugal elutriator bottle; longitudinal section. 



Soils Bui. 24.) 



(From U. S. Dept. of Agr., Bureau of 



thoroughly shaken up with more of the muddy water from the gravity elutriator 

 and all is put through the machine at such a speed of centrifuge and water flow as 

 will give a separation of the 0.01 mm. particles, namely, 91.46 cc. and 404 revolu- 

 tions per minute. 



When the muddy water has all passed through, followed by a little rinsing water, 

 the machine is stopped. The sediment will be found to be so firmly imbedded in 

 the bottom of the elutriator bottle that the muddy water can be poured off from it. 

 The sediment is then shaken up in 150 to 200 cc. of clear water and run through the 



