PHYSICAL COMPOSITION AND CLASSES OF SOILS 83 



The mixed sediments, containing the sand and all the silt 

 except the finest, is placed in an upright glass cylinder, containing 

 a rotating fan or churn, run by a suitable motor. This breaks up 

 the compound particles. A current of water is run in so that it 

 moves through the cylinder at the rate of 0.25 mm. per second, 

 until the water becomes clear. The particles carried over are 

 allowed to settle, dried and weighed. The current of water is 

 increased to 0.5 mm. per second, so as to remove another grade 



Fig. 26. Hilgard's apparatus for mechanical analysis by means 

 of a current of water. 



of particles, and these operations are repeated until all the grades 

 of particles are separated. 



Relation of Grain Size to Soil Texture. This relation is studied 

 by ascertaining the mechanical analysis of soils of known prop- 

 erties, and also by investigating the properties of the various 

 groups of particles separated in the analysis. 



In sands the coarser particles of the soil predominate. Such 

 soils are open or porous, easy to cultivate, not very retentive of 

 moisture, and warm up early in the spring. Coarse sands are 

 least retentive of moisture and most porous. Soils containing 

 quantities of very fine sand are much more retentive of moisture. 



