in.] WATER IN SATURATED SOIL 67 



extent of surface exposed by the soil particles is im- 

 portant because it is their active part ; other conditions 

 being equal, the amount dissolved from a solid body in 

 a given time by any solvent will be proportional to 

 the surface exposed. 



Again, the water in a soil usually exists as a film, 

 coating the surface of the soil particles, and the amount 

 of water that can be held under particular conditions 

 becomes a function of the extent of surface ; even the 

 power of a soil to remove certain substances from solution 

 is likewise dependent on the surface. 



Capacity of the Soil for Water. 



So far, the structure of the soil in a dry state has 

 only been considered, it is now necessary to consider 

 its behaviour when fully saturated with water, before 

 passing on to the more usual state when the soil con- 

 tains both air and water. 



The amount of water which a soil will hold when com- 

 pletely saturated will depend upon the pore space, will, 

 in fact, be the pore space together with whatever water 

 the material of the particles can imbibe without causing 

 any swelling. Perhaps the best method for determining 

 the water capacity of a soil is one devised by Hilgard. 

 A small cylindrical brass box. is constructed, 1 cm. 

 deep and 6 cm. in diameter. The bottom is a sheet of 

 perforated brass, and the whole is supported on three 

 legs ; the capacity of the box is about 30 c.c The 

 exact capacity is determined by waxing up the holes, 

 weighing, filling with water, and reweighing. A circle 

 of thin filter paper cut to fit the box is laid inside 

 and wetted, any superfluous water that comes through 

 being wiped away. The box is then weighed, care- 



