Internal Surface of Soils. 119 



It will be seen from this table that the internal surface 

 of an ideal soil'increases in the same ratio that the diam- 

 eter of the grains decreases, that is, reducing the diameter 

 one-half doubles the surface to which water may adhere 

 and upon which it may act 



141. Difficulties in Determining the Surface of a Soil Accu- 

 rately. While it is possible to determine accurately the 

 surface in a given weight of spheres of known dimensions 

 the case is quite different with true soils. Indeed, it is 

 not practicable to determine with much accuracy the sur- 

 face in a soil. This will be clear from a consideration 

 of a simple problem. 



Take a soil composed of grains, (a) .009 and (b) .00015 

 m. m. in diameter and let these be mixed in the propor- 

 tions of 



A. 90 per cent, of (a) with 10 per cent, of (b). 



B. 10 per cent, of (a) with 90 per cent, of (b). 



C. 50 per cent, of (a) with 50 per cent, of (b). 



Under these conditions the surface of one gram of such 

 mixtures of soil having a specific gravity of 2.65 is 



For A. 



Surface. 



90 per cent, of grains fa) .009m. m. diameter 2,264 sq, cm. 



10 per cent, of grains (b) .00015 m. m. diameter 15,034 sq. cm. 



Total surf aco... 17,358sq. cm 



For B. 



10 per cent, of grains (a) .009 m. m. diameter 251. 6' sq. cm. 



90 per cent, of grains (b) 00015 m. m. diameter 13p,848.9 sq. cm. 



Total surface 136,100.5 sq. cm. 



ForC. 



60 per cent, of grains (a) .009 m. m. diameter l,258.0sq. 



60 per cent, of grains (b) .00015 m. m. diameter 75, 481 .7 sq. cm 



l,258.0sq. cm. 

 5,4S1.7sq. cm. 



Total surf ace... 76,739 7 sq. cm. 



