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PRINCIPLES OF AGRICULTURAL CHEMISTRY 



It is possible, by combining some of the groups of Hilgard, to 

 compare the results with analyses of the Bureau of Soils. As 

 Hilgard 1 remarks, a subdivision of six or seven classes, as is 

 made by the Bureau of Soils, is sufficient for a great many cases. 

 There is, however, considerable difference in the properties of the 

 grades of silt which are separated by Hilgard but grouped 

 together by the Bureau of Soils. 



Methods of Analysis. For the separation of the finer particles, 

 all methods of mechanical analysis take advantage of the different 

 rates of subsidence of particles of different diameters when sus- 

 pended in water. Methods such as that of Osborne depend upon 

 subsidence under the influence of gravity in stationary columns 

 of water. Hilgard's method depends upon the difference between 

 the action of gravity and the carrying power of a current of 

 water. The Bureau of Soils throws down all except the clay 

 particles by centrifugal force. Sieves are used for separation of 

 the coarser particles ; compound particles are broken down by shak- 

 ing with water, or boiling. Since clay is liable to form compound 

 particles, and otherwise interfere with the separations, it is 

 removed first. 

 1 The Soil. 



