12 PHYSICAL CONSTITUTION OF SOIL 



difficulty arises when soil particles are separated by currents 

 of water of definite velocity, the specific gravity of the sub- 

 stance affecting the size of the particle which is removed by 

 the current. 



Osborne takes 30 grams of soil which has passed through 

 a 3-0 mm. sieve. By the aid of water the soil is passed 

 successively through sieves having circular holes of i-o, 0-5, 

 and 0-25 mm. diameter. By fractional subsidence in water 

 the still finer particles are then divided into four groups of 

 diameters -0525 mm. (fine sand) ; -o 105 mm. (silt) ; below 

 01 mm. (dust) ; matter not deposited in twenty- four hours 

 (clay). The clay may be precipitated by adding ammonium 

 nitrate to the water. Osborne's method answers well for the 

 amount of separation just described, but is not convenient 

 when a more detailed analysis is required. 



The methods making use of currents of water of different 

 velocity to separate the variously sized particles are numerous ; 

 the most satisfactory is that of Hilgard (American Jour. 

 Science and Art, 1873, 288, 333; California Exp. Station 

 Report, 1891-92, 248; Wiley, op. cit., 225). He removes the 

 coarser constituents of the soil by sieves, the finest sieve 

 having meshes 0-5 mm. in diameter. Of the sifted soil, 10 

 or 15 grams are boiled in a narrow-necked sloping flask for 

 815 hours, the contents transferred to a beaker, diluted to 

 about ij litre, well mixed, and allowed to stand a short time 

 till the particles of 0-25 mm. per second hydraulic value, with 

 all heavier particles, have deposited. The turbid water is then 

 decanted, and the sediment again stirred up with water, and 

 again allowed to settle ; and this treatment is repeated till 

 the water comes off clear. The united turbid water is well 

 mixed, and any deposit occurring in a short time is added 



