82 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



three minutes, the time depending on the speed of the machine 

 and the quantity of suspended material present. The material 

 in suspension is then examined with a microscope carrying a 

 micrometer eye-piece, to see if any particles larger than clay (.005 

 mm. in diameter) remain in suspension. If larger particles are 

 found, the centrifugal is run until they settle. When the water 

 contains only clay particles, it is decanted. The residue is stirred 

 up with water, the machine started, and the separation made as 

 before. The operation is repeated until the clay has been all 

 removed, as shown by a microscopic examination of the residue. 

 The water is evaporated to dryness, and the clay weighed. 



The residue left in the tubes is brought in suspension with 

 water, and allowed to settle, until microscopic examination shows 

 only silt in suspension. This requires but a short time. The silt 

 water is decanted, and the operation repeated until all the silt 

 (less than .05 mm. in diameter) has been washed out. The silt, 

 after being allowed to settle, is collected, dried and weighed. The 

 sand is then collected, dried, and separated into five groups by 

 means of a nest of four sieves, two of brass with circular 

 perforations I mm. and 0.5 mm. in diameter, and two of silk 

 bolting cloth with openings 0.25 and o.i mm. wide. The various 

 separations are then weighed. 



Hilgard's Method. The following is an outline of Hilgard's 

 method. (For full details see Wiley's "Agricultural Analysis.") 

 The two grades of grit are removed by sieves. Ten or 15 grams 

 of the sifted soil are boiled with water to break up the compound 

 particles, transferred to a beaker, mixed with water, and allowed 

 to stand a short time until only the finest silt and clay remain 

 in suspension. The treatment with water is repeated until the 

 water has removed all the finest silt and clay. The mixture 

 of clay and silt-water is allowed to stand 24 to 60 hours, until all 

 silt is deposited. The sediment is rubbed with a rubber pestle, 

 mixed with water, and allowed to settle again until free from 

 clay. It is then dried and weighed. An aliquot of the clay-water 

 may be evaporated to dryness and the residue weighed, or the 

 clay may be precipitated by coagulation with salt. 



