io8 CLAYS AND CLAY INDUSTRY. 



particles have been removed. The water is drawn off from the 

 top of the tube by a suitable pipe and led into a settling jar, the 

 different sizes of course being carried into separate receptacles. 

 This method requires a long time and much water, but is more 

 accurate than the preceding. 



The most satisfactory method is that known as the centrifugal 

 method. The apparatus used consists of a fan motor 1 placed 

 with the armature shaft in a vertical position. This carries a 

 framework with eight test-tube holders, trunioned so that they 

 can swing outward and upwards as the frame revolves. 



The disintegrated sample in suspension in water is placed 

 in these tubes, and twirled at a high speed for several minutes. 

 As a result of this all particles except the finest clay grains 

 are thrown to the bottom of the tube by centrifugal force. These 

 are then decanted off, the tubes refilled with water and the 

 sediment again stirred up. A second twirling of the tubes either 

 at a lower speed or for a shorter period precipitates everything 

 except the fine silt, which is then also decanted off. The sub- 

 sequent sizes are then separated from each other partly by settling 

 and partly by sieves. 



The different sizes which can be so separated and their dimen- 

 sions are shown in the table below : 



Table showing size of grains of sandj silt and clay. 



Conventional Size of Diameters. 



name. Inches. Millimeters. 



1 Gravel, V12-V25 2-1 



2 Coarse sand. 1 /ss- ± /m 1-.5 



3 Medium sand, Vso-Vioo 0-25 



4 Fine sand, V100-V250 



5 Very fine sand V230-V500 



6 Silt Vaoo-V^oo 



7 Fine silt V2500-V5000 



8 Clay. 7,000-72; 



5000 



25-. I 

 I -.05 

 05-.OI 

 OI-.005 

 OO5-.OOO I 



In making" the mechanical analyses mentioned under tensile 

 strength, the centrifugal method was used, but only 5 sizes were 

 determined, or rather 5 groups of sizes; numbers 8, 7 and 6 



1 For complete description see Bulletin No. 64, Bureau of Soils, Dept. of 

 Agriculture, Washington, 1900. 



