DETERMINATION OF PORE-SPACE 75 



stirred or shaken to remove any occluded air, and the 

 volume again noted. From these observations the 

 percentage of air-spaces in the sand may be readily 

 calculated. 



54. Pore-space of Soils by Calculation. 



The pore-space of a soil may be readily calculated 

 from a knowledge of its true and apparent densities. 

 The experiment should be carried out with samples, 

 taken in situ, of {a) a clay soil, {U) a light sandy soil, 

 and {c) a calcareous soil. 



The apparent density may be determined as above 

 (52), or may be obtained by weighing the samples 

 taken in the field by the cubic sampler (see p. 84), of 

 known dimensions. The true specific gravity is found 

 on the soil dried at 100° C, as described in 51. 



If D =real specific gravity, 

 Dj^ = apparent specific gravity. 



Then the weight of soil equal to the pore-space of 

 unit volume = D — D^. 



.'. Volume of pore-space in unit volume = — p- — ^ 



. ., 100 (D- DO 

 t.e.^ Percentage of pore-space in soil = ^ ~ 



Absorption of Water by Soils. 



The retention and rate of percolation of water through a soil is 

 very largely dependent on its texture, and the amount of organic 

 matter which is present. 



55. Hygroscopic Moisture. 



The percentage of water present in air-dried peat, 

 sand, and clay is determined by drying weighed 

 quantities of material (about 10 grams) to constant 



