THE SOIL PARTICLE 89 



formation of compound particles. A screen placed just 

 above the stirrer serves to prevent the whirling motion 

 from being communicated to the ascending column of 

 water in which the separation occurs. The various grades 

 in the separation are regulated by the rate of water flow. 

 With this apparatus it is necessary to remove the finer 

 particles below .01 mm. in diameter by subsidence 

 previous to the determination. 



While this method is very nearly accurate and will 

 give a separation of the various grades such as is impossible 

 with most other methods, it is impracticable in ordinary 

 soil work. The large quantity of water which is used 

 in carrying over each grade, and which of course must 

 be evaporated before the sample can be weighed, is the 

 first objection. The length of time necessary for the 

 separation, and the cost of the apparatus, are two addi- 

 tional objections urged against it. As mechanical analysis 

 is used largely in determining soil texture, rapidity and 

 ease of operation are of more importance than the ex- 

 tremely accurate separation of the particles. 



67. Yoder's centrifugal elutriator. One of the ob- 

 jections to the methods already described is the length 

 of time necessary for a determination. This is due to 

 the fact that very fine particles subside in water very 

 slowly. In order to hasten the separation, Yoder l 

 devised a machine in which hydraulic force may be 

 supplemented by a centrifugal pull. This ingenious 

 apparatus consists of an elutriator bottle (see Fig. 13) 

 mounted in a centrifuge. The muddy water is introduced 

 into the bottle at the center of the centrifuge. It then 

 passes to the bottom of the bottle and back again to the 



1 Yoder, P. A. A New Centrifugal Soil Elutriator. Utah 

 Agr. Exp. Sta., Bui. 89. 1904. 



