72 NATURE AND PROPERTIES OF SOILS 



drops of ammonia are added to dissolve tlie organic matter 

 and to make defloecnlation easier. Tiie sample is then agi- 

 tated in the bottle until disintegration is complete. This 

 period ranges from five to twenty hours, depending on the 

 sample. (See Pig. 15,) 



The separation of the silt and the clay from the sands is 

 made in the shaker bottle by simple subsidence, the time for 

 decantation being determined by a microscopic examination 

 of a drop of the suspension. The silt and the clay are de- 

 canted directly into a test-tube fitted into a centrifuge. "Whirl- 

 ing 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 several times ; 

 therefore the processes of gravity subsidence and centrifugal 

 subsidence are carried on side by side, material being con- 

 stantly 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 separa- 

 tion of several duplicate samples at once. The various sep- 

 arates made by this method are dried and weighed. The 

 sands, which are obtained in bulk, are further separated by 

 sieves into the grades desired. When a large quantity of 

 organic matter is present it must be determined and included 

 in the final report on the sample. 



This method of mechanical analysis as perfected by the 

 Bureau of Soils has been very commonly adopted by soil work- 

 ers. It has many advantages over other methods.^ In the 

 first place, it is rapid, often requiring only hours where other 



^ Classification of the Various Methods of Mechanical Analysis 



i to sepa; 

 methods. 



1 <?-.*^,^ J r^-r Used to separate sands in practically all 



[Dry 



